ENGINEERING AFFINITY LIGANDS FOR MACROMOLECULES

Abstract: A method is disclosed for obtaining highly specific and tailored ligands suitable for purifying a particular product target or for eliminating particular target impurities in a feed stream. Engineered affinity ligands according to the invention will bind a target with high specificity at a preselected binding condition and release the target at a preselected elution condition. The ligands are isolated by contacting a target with a multiplicity of polypeptides derived through variegation of the structure of a candidate binding domain, the variants (or analogues) including polypeptides favoring binding to the target under desired binding conditions and release from the target under elution conditions, where the binding and elution conditions differ according to one or more parameters, such as pH, temperature, concentration of salt or volume % of an organic solvent.

Description: Engineering Affinity Ligands for Macromolecules Field ofthe Invention The present invention relates to the field of purification of biomolecules. Specifically, the present invention relates to discovery of and isolation of affinity ligands for particular target biomolecules. Such affinity ligands will be useful for purification of target biomolecules from solutions under desirable separation conditions. Background ofthe Invention Biomolecules of medical or industrial importance, which may be proteins, glycoproteins, lipoproteins, polysaccharides, lipids or nucleic acids, are produced by a wide variety of methods including chemical synthesis; secretion into culture medium by naturally occurring or recombinantly transformed bacteria, yeasts, fungi, insect cells, and mammalian cells; accumulation in cultured cells (e.g., in inclusion bodies); secretion from genetically engineered organisms (e.g., in the milk of transgenic mammals); and recovery from biological sources such as urine, blood, milk, plant infusions, fungal extracts, and the like. Most biomolecules thus produced are of little use, however, without purification away from other elements (i.e., "impurities") present in the solutions in which they are produced or without concentration ofthe biomolecule so that it comprises a much larger fraction ofthe solution. Chromatography is a dominant purification and concentration technique used in the large- scale isolation of biomolecular targets. With a properly designed series of chromatographic steps, a single biomolecular component can be isolated from a complex mixture with contaminants that range from inorganic salts to incorrectly folded or partially degraded forms ofthe target macromolecule itself. At the same time, chromatographic purification is a major (and often the largest single) cost in the manufacture of a biomolecule product. Typical biotherapeutic purifications require from two to six or more chromatographic steps, utilizing size-exclusion, ion-exchange, hydrophobic interaction, and affinity chromatographic modes. While size-exclusion is often used for buffer exchange or as a final step to remove aggregated material, and ion-exchange and hydrophobic interaction are useful to concentrate the produα and remove major impurities, none of these chromatographies can approach the dramatic single-step increases in purity achieved using affinity chromatography. Narayanan (1994), for instance, reported a 3000-fold increase in purity through a single affinity chromatography step. Affinity chromatography is not, however, a commonly used technique in large-scale production of biomolecules. The ideal affinity chromatography ligand must, at acceptable cost, (1) capture the target biomolecule with high affinity, high capacity, high specificity, and high selectivity, (2) either not capture or allow differential elution of other species (impunties), (3) allow controlled release ofthe target under conditions that preserve (l e , do not degrade or denature) the target, (4) permit samtization and reuse ofthe chromatography matnx, and (5) permit elimination or inactivation of any pathogens However, findmg high-affinity ligands of acceptable cost that can tolerate the cleaning and samtization protocols required in pharmaceutical manufacturing has proved difficult (see, Knight, 1990) Although far from ideal, dyes (such as αbachron blue) and proteins of known affinity (such as Protem A) have been employed extensively in affinity chromatography These matenals, however, cannot be adapted to new targets and therefore lack the flexibility to be more widely used Murine monoclonal antibodies (MAbs) also have been used as affinity ligands MAbs can be readily generated, and new MAb ligands specific for a new target molecule can be obtained, giving MAb technology a degree of flexibility to meet the individual requirements of a particular manufacturer Monoclonal antibodies, on the other hand, are not without drawbacks in the field of affinity chromatography MAbs are expensive to produce, and they are prone to leaching and degradation under the cleaning and samtization procedures assoαated with punfication of biomolecules, leading MAb-based affinity matnces to lose activity quickly (see, Narayanan, 1994, Boschetti, 1994) In addition, although MAbs can be highly specific for a target, the specificity is often not sufficient to avoid capture of impunties that are closely related to the target Moreover, the bmdmg diaractenstics of MAbs are determined by the immunoglobulm repertoire ofthe immunized animal, and therefore practitioners must settle for the bmdmg diaractenstics they are dealt by the animal's immune system, 1 e , there is little opportumty to optimize or select for particular binding or elution diaractenstics using only MAb technology Finally, the molecular mass per bmdmg site (25 kDa to 75 kDa) of MAbs and even MAb fragments is quite high Thus, there is a continuing need to develop less expensive, more serviceable and more tailored affinity ligands for particular biomolecular targets Specifically, there is a need for affinity ligands that more closdy approach the diaractenstics ofthe ideal affinity ligand descnbed above, that not only bund to a given target molecule with high affinity but also rdease the target under desirable or selected conditions, that are able to discnminate between the target and other components ofthe solution in which the target is presented, and/or that are able to endure cleaning and samtization procedures to provide regenerable, reusable chromatographic matnces Such affinity ligands and methods for obtaining them are provided herein Summarv ofthe Invention The present invention provides methods for obtaining affinity ligands for a particular biomolecular target of interest which exhibit desirable or selected binding properties and release properties. In its broadest aspects, the present invention provides a method for obtaining affinity ligands useful for isolating virtually any taiget biomolecule from a solution containing it, which ligands may be engineered to exhibit not only favorable binding characteristics for affinity chromatography but also desired release (elution) characteristics and other desired properties such as stability, resistance to degradation, durability, reusability, ease of manufacture, etc. The present invention also provides a method for obtaining affinity ligands that bind particular impurities (or closely-related groups of impurities) with very high affinity to remove essentially all of that impurity (or impurities) from a solution containing a target. In this case, releas ofthe impurity intact is not important (with respect to purification ofthe target), but it will be economically advantageous if the affinity matrix containing the ligands can be regenerated and reused The present invention also provides a method for isolating affinity ligands capable of binding a particular taiget under a first specific set of solution conditions and releasing the target under a second specific set of solution conditions. The present invention thus relates to a method for isolating an affinity ligand suitable for separating a target molecule from a solution containing it without denaturing the target molecule, the method comprising: (a) selecting, with respect to the target molecule, a first solution condition (i.e., the binding conditions) at which it is desired that an affinity ligand should bind to said target molecule; (b) sdecting, with respect to the target molecule, a second solution condition (i.e., the release conditions) at which it is desired that an affinity complex between said target and said affinit ligand will dissociate and wherein said second solution condition is different from said first solution condition; (c) selecting a polypeptide candidate binding domain for said taiget molecule that is stable under both said first and said second solution conditions; (d) sdecting amino add positions in said candidate binding domain (preferably positions on the surface ofthe domain, and most preferably close together in the domain) as variable positions; (e) providing a library of analogues of said candidate binding domain, wherein each analogue differs from said candidate binding domain in the substitution of a different amino acid at one or more ofthe selected variable amino add positions; (f) contacting said library of analogues with the solution containing said taiget molecule at the first solution condition, for suffident time to permit analogue target binding complexes to form; (g) removing analogues that do not bind under the first solution condition; (h) altering the conditions ofthe solution of contacting step (f) to the second solution condition; and (i) recovering the candidate binding analogues released under the second solution condition, wherein the recovered analogues identify isolated affinity ligands. In certain circumstances, e.g., where the nature ofthe target molecule is obscure or unknown to the practitioner, or where features involved in the purification ofthe target (such as the content of the solution in which the target is produced, nature of persistent impurities in the solution, biological activity ofthe target, production source ofthe target, etc.) leave doubt as to the range of solution conditions under which the target molecule will remain stable, a preferred method according to the invention for obtaining an affinity ligand suitable for separating the target molecule from the solution containing it will comprise the steps: (a') ascertaining a range of stability for the target molecule with respect to two or more parameters selected from temperature, pH, ionic strength, dielectric constant, concentration of solutes (e.g., concentration of chaotropes, concentration of organic solvents (e.g., methanol, ethanol, acetonitrile, DMSO, DMF, methyl acetate, acetone, methylethylketone, and the like)), and presence or absence of metal ions (e.g., Zn* 4 , Ca**, Mg**) or chelating agents (e.g., EDTA), thereby defining a stability envelope for said target molecule; (a) selecting, with respect to the target molecule, desired binding conditions within the stability envelope for the target molecule; (b) selecting, with respect to the target molecule, desired rdease conditions within the stability envelope for said target molecule, wherein said release conditions are different with resped to at least one parameter from said binding conditions; (c) selecting a polypeptide candidate binding domain for said target molecule that is stable under both said binding and rdease conditions; (d) selecting amino add positions on the surface of said candidate binding domain as variable positions; (e) providing a library of analogues of said candidate binding domain (e.g., a phage display library in which a variegated set of polypeptide binding domain analogues is displayed on the surface of baderiophage), wherein each analogue differs from said candidate binding domain in the substitution of a different amino add at one or more ofthe variable amino add positions designated in step (d); (f) contacting said library of analogues with the solution containing said target molecule at the binding conditions, for suffident time to permit analogue target binding complexes to form; (g) removing analogues that do not bind the target under the first solution condition; (h) altering the conditions ofthe contacting step (f) to the release conditions; and (i) recovering the analogues rdeased under the release condition, wherein the recovered analogues identify isolated affinity ligands. From the analogues recovered, protein binding domains having the best combination of properties can be selected. Alternatively, additional rounds of selection from additional libranes (steps (e)-(i), above) can be used to enrich the recovery of ligands with those having desirable properties. The practitioner's knowledge of die taiget, its production and the fadors involved in its separation from solution will guide the selection of candidate binding domains, the designation of variable amino adds and the sdection of amino adds for substitution at variable positions, to favor the properties desired in the final affinity ligand. Alternatively, if a variegated candidate binding domain library has previously been prepared, the selections steps (c) and (d) need not be repeated, an the practitioner can move directly from sdecting binding and release conditions to contacting the library and the target. It is particularly preferred that the creation of polypeptide analogue libraries for the presentation of a multiplidty of possible affinity binding domains, some of which will bind to the target under binding conditions and release the target under release conditions, make use of technique for display of analogue proteins on the surface of a genetic package such as a bacteriophage. Such techniques are set forth in U.S. 5,223,409 (Ladner et al.), incoφorated herein by reference. By using these powerful techniques, a great number (e.g., IO 7 ) of polypeptides of similar strudure can be readily produced for use in the methods of this invention. The affinity ligands prepared according to this invention will be spedfically tailored to the separation of a particular target biomolecule. Affinity ligands will thus be obtained in accordance with this invention that have the optimal balance of properties to make them almost ideal affinity ligands for the separation of that particular target, in terms of affinity for binding the target, spedfidty for the target, release ofthe target under spedfied conditions, durability under sanitization conditions, and any other desirable property sdeded for. Bπef Descnption ofthe Drawings Figure 1 shows a chromatogram of tissue plasminogen activator or "tPA" (25 μL of lmg/mL tPA) over an affinity chromatography column havmg an immobilized tPA affinity hgand (CMTI deπvative #109, descnbed m Example 16), with elution over a pH 7 - pH 3 gradient The peak at 15 mmutes is estimated to contain approximately 90% ofthe injected tPA Figure 2 shows a chromatogram of Coagulation Standard (diluted 10X) over a tPA affinity column (CMTI deπvative #109) with dution as descπbed above The small peak at 15 6 mmutes was shown to be a gradient artifad Figure 3 shows a chromatogram of a mixture consisting of 25 μL of Coagulation Standard (Diluted 10X) spiked with 25 μL of tPA, with elution over a pH 7 - pH 3 gradient The peak at 1 minute is the collection of plasma proteins and the peak at 15 4 mmutes is tire tPA Figure 4 shows the same chromatogram as shown in Figure 3 with vertical scale expanded Figure 5 shows the results of an ELISA testing the bmdmg of the CMTI phage isolates C3 , C21, C22, C23, and a TN-6/I phage isolate T49 (see Example 18 infra) agamst an immobilized human monodonal antibody targd at pH 7, an immobilized human monodonal antibody target at pH 2, immobilized goat IgG at pH 7, immobilized goat IgG at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 The ELISA identifies affinity ligands useful for separation ofthe humanized antibody from a feed stream including goat antibodies Figure 6 shows the results of an ELISA testing the bmdmg ofthe TN-6/I phage isolates T41, T42, T48, T52 and T74 (see Example 18 infra) against an immobilized human monoclonal antibody targd at pH 7, an immobilized human monoclonal antibody targd at pH 2, immobilized goat IgG at pH 7, immobilized goat IgG at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 The ELISA identifies affinity ligands useful for separation ofthe humanized antibody from a feed stream including goat antibodies Figure 7 shows the results of an ELIS A testing the bmdmg of the TN- 10/V phage isolates T61, T64, T66, T70, T72 and T75 (see Example 18 infra) agamst an immobilized human monoclonal antibody targd at pH 7, an immobilized human monoclonal antibody targd at pH 2, immobilized goat IgG at pH 7, immobilized goat IgG at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 Figure 8 shows the results of an ELISA testing the bmdmg ofthe TN-6/I phage isolates havmg urlαnase bmdmg affinity from four rounds of screening TU33, TU34, TU36, TU37, TU39 and TU42 (see Example 19 infra) agamst immobilized urokinase at pH 7, immobilized urokinase at pH 2, immobilized tPA at pH 7, immobilized tPA at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 The ELISA identifies affinity ligands useful for separation of urokinase from a feed stream such as unne Figure 9 shows the results of an ELISA testing the bmdmg of some of the TN- 10/VUJa phag isolates having urkinase bmdmg affinity from four rounds of screenmg TU51 , TU53, TU56, TU58, TTJ60 and TU62 (see Example 19 infra) against immobilized urokinase at pH 7, immobilized urokinase at pH 2, immobilized tPA at pH 7, immobilized tPA at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 The ELISA identifies affinity ligands useful for separation of urokinase from a feed stream such as unne Figure 1 OA shows the results of an ELISA testing the bmdmg of some ofthe CMTI phage isolates havmg urkinase bmdmg affinity from four rounds of screenmg CU22, CU29, and CU32 (see Example 1 infra) agamst immobilized urokinase at pH 7, immobilized urokinase at pH 2, immobilized tPA at pH 7, immobilized tPA at pH 2, immobilized BSA at pH 7, and immobilized BS at pH 2 Figure 10B shows the results of an ELISA testing the bmdmg of some ofthe CMTI phage isolates havmg urkinase bmdmg affinity from four rounds of screenmg CU25, CU27, CU28, CU 31 and CU32 (see Example 19 infra) agamst immobilized urokinase at pH 7, immobilized urokinase at pH 2, immobilized tPA at pH 7, immobilized tPA at pH 2, immobilized BSA at pH 7, and immobilized BSA at pH 2 None of these isolates were suitable as an affinity hgand accordmg to the cπtena ofthetest Figure 11 A shows the results of an ELISA testing the bmdmg of some ofthe LACI F phage isolates having urkinase bmdmg affinity from four rounds of screenmg LU2, LU5, LU9 and LU12 (see Example 19 infra) agamst immobilized urokinase at pH 7 and immobilized BSA at pH 7 Figure 1 IB shows the results of an ELISA testing the bmdmg of some ofthe LACI F phage isolates havmg urkinase bmdmg affinity from four rounds of screenmg LU2, LU4, LU10 and LU12 (see Example 1 infra) against immobilized urokinase at pH 7 and immobilized BSA at pH 7 None of these isolates were suitable as an affinity hgand accordmg to the cπtena ofthe test Detailed Descnption ofthe Preferred Embodiments The present mvention makes possible the efficient punfication of targd biomolecules by affinity chromatography As used herein, the term "targd" will indicate a particular prote , glycoprotein, lipoprotem, polysacchaπde, lipid, lipopolysacchande, nudeic aαds or combmations of molecules from these classes for which a specific ligand is sought Targets may be active fragments of proteins or may be complexes or aggregates of more than one protem Targds are not limited to naturally occumng biomolecules, they may be synthetic organic molecules and especially molecules containing one or more chiral centers A targd may exist as part of a larger structure, e g , as a surface molecule on an organism such as a virus Indeed, the target may be a cell or a virus particle Targd biomolecules may be produced in any known way, including chemical synthesis, production m transformed host cells, secretion mto culture medium by naturally occurπng or recombinantly transformed or transfected badeπa, yeasts, fungi, insect cells, and mammalian cells, secretion from genetically engineered organisms (e g , transgenic mammals), or in biological fluids or tissues such as unne, blood, milk, dc The solution that contains the crude targd as it is initially produced (I e , the production solution) will sometimes be referred to as the "feed stream" Although the mvention is pπmaπly directed toward recovenng targd molecules from the solutions in which the target was produced, the mvention is not limited to recovery from production feed streams, and affinity ligands devdoped accordmg to the mvention can be used to recover a targd from any solution Each method of producing a targd biomolecule yields the target m a feed stream that contains the target and a number of impunties (with resped to the targd) One purpose ofthe present mvention is to produce affinity ligands and preparations (such as chromatography media) compπsing such ligands that allow rapid and highly specific punfication of targds Affimty ligands obtained herein bind the targd (preferably to the virtual exclusion of any other molecule the feed stream) with high affinity Further, the affinity ligands rdease the target intact and m active form when the solvent conditions are changed It is important that the release conditions do not adversely affect the targd Preferably, the affinity ligand can b d the targd even when it is present at low concentration in the feed stream and release it at high concentration so that one punfication step both removes many impunties and concentrates the targd Targets that are to be bound and rdeased in active form will sometimes be referred to as "product targds" Another aspect ofthe present invention is to provide affinity ligands that bmd particular impunties (or dosely-rdated groups of impunties) with very high affinity to sequester essentially all of that lmpuπty (or those impunties) and thus allow removal of other molecule(s) from it (them) In this case, release ofthe impunty lntad is not important, but it will be economically advantageous if the ligand can be recycled Targds that are to be captured with maximal affinity but for which rdease is unimportant are termed "impunty targds" Charadenzation ofthe Tared In the field of punfication, almost any fad known or discovered about the targd may be used to improve the effectiveness of a punfication scheme aimed at isolating it In pra icmg the methods ofthe present mvention for obtaining engineered affinity ligands for a targd, it is cπtical to know or to determine solution conditions under which the targd is stable, I e , conditions under which the biomolecule maintains its activity and does not denature The first steps ofthe method accordmg to the mvention mvolve selecting a sd of conditions in the targd punfication process at which it is desired that bmdmg to an affinity ligand will take place, and selectmg a different sd of conditions at whidi it is desired that bmdmg to the affinity ligand will cease, resulting in release and elution ofthe targd Both of these conditions must be conditions under which the targd is stable, m order that the ligands ultimately obtained have affinity for the intact, active targd In addition to understanding fadors affecting the stability ofthe targd, it is also useful to know other diaractenstics ofthe target, such as the concentration ofthe targd in the solution from which it is to be separated, bmdmg or enzymatic activities ofthe target, the aggregation state ofthe targd (1 e , whether it is a monomer or a multimer), the approximate size (molecular weight) ofthe targd, and the composition ofthe solution or feed stream containing the taiget Such factors will be helpful to the practitioner in selectmg (a) bmdmg conditions under which one or more protem ligands would be expected to bmd the target, (b) possible changes m conditions that would be likely to make at least some ofthe protem ligands release the targd without denaturing or harming it, and (c) the composition of one or more branes of potential bmdmg proteins or domains from which suitable ligands can be isolated Stability Conditions for the Tared Any biomolecule is stable under some solution conditions, but stability of a target in solution is affected by the conditions ofthe solution including its temperature, pH, ionic strength, dielectπc constant and the concentrations of other solutes, such as [H*], [Na*], [Cl " ], [urea], [guamdimum*], [SO,"], [HS0 4 -] 5 [P0 4 -], HPO [H 2 P0 4 -], [CIO " ], [NFL*],[OH], B 2 O t ], [Mg ** ], [K*],[Zn* * ], [Ca**], [Li*], [NO3 ], [detergent, eg , Tnton X-100], [dodecylsulfate], [glucose], [citrate], [benzoate], [ethanol], [methanol], [acdone], [acetate], [chloroacetate], [didiloroacetate], [tπchloroacetate], [formate], [N-methylfromamide], [formamide], [dimethylformamide], [dimethylsulfoxide], [methylethylkdone], [methylacetate], EDTA, dc For each of these paramders and any others known or dderrruned for a particular feed stream, the targd will be stable within a range, 1 e , within a temperature range, within a pH range, dc Outside this stability range, the targd denatures Collectivdy, the ranges of all parameters affecting stability of a particular targd determine a "stability envdope" for that feed stream, and the targd is expeded to remain stable so long as conditions ofthe solution are maintained within the stability envelope It is expected that different targds will have different stability envdopes, even m the same feed stream The first steps in obtaining affinity ligands suitable for affinity separation of a given targd are to seled bmdmg conditions and desired rdease conditions under which the targd is stable If insufficient information regarding stability conditions for a targd molecule is known to the practitioner to make a selection of bmdmg conditions, stability conditions can be ascertained either by referring to previous determinations or by determining the stability range with resped to one or more parameters empiπcally It should be noted that it is not necessary to test the stability ofthe target or determine the full extent ofthe target's stability envelope in order to select desired binding conditions The practitioner, based on experience, may simply guess at stability conditions and select a sd of bmdmg conditions appropriately For example, most proteins that have any stability are stable at pH 7, 25° C, and 150 mM NaCI (i.e., pH 7, 25° C, and 150 mM NaCI is usually within the envelope of stability for a given protem). Binding conditions may be selected at pH 7, 25° C, and 150 mM NaCI, and release conditions may be sdected in rdation to this, by varying one or more ofthe parameters (e.g., to pH 4, 25° C, and 150 mM NaCI) If dther of these conditions is outside the stability envelope of he target, the method will fail to identify any affinity ligands capable of separating the targd from a solution, and the method may be repeated, either after dderm ing the stability envelope more accuratdy or making another guess at where the boundanes ofthe envelope may lie As a broader example, very many proteins are stable a temperature range from 0°C to 35°C, at a range of pH from pH 8-9 down to pH 4-5, and for [NaCI] ranging between 0 M and 1 M Thus, binding conditions of pH 8, 0° C, 10 mM NaCI and elution conditions of pH 6, 30° C, and 0.5 M NaCI might be selected for most protem targds. Other pairs of bmdmg and rdease conditions based on this estimated stability envdope would include (1) Binding: pH 6, 25° C, 0.5 M NaCI and Elution: pH 8, 0° C, 10 mM NaCI; (2) Binding: pH 7, 0° C, 10 mM NaCI and Elution: pH 7, 35° C, 150 mM NaCI, 4 M urea, and (3) Binding. pH 8.0, 25° C, and 150 mM NaCI and Elution pH 6, 25° C, and 150 mM NaCI. If additional information concerning the stability of a particular targd in a feed stream must be ascertained to make suitable selections of bmdmg and release conditions, stability over a range of one or more paramders may be determined easily Illustrative tests to establish a useful stability envelope for a target are descπbed bdow. Many modifications of such tests are possible that will also yield useful stability data, and such modifications will suggest themselves to those skilled in this art. It is not important to determine the boundaries of stability with high accuracy, rather it is only necessary to define a range of conditions within which the targd is stable and within which two distind conditions (for binding and release) can be sdected. If stability conditions for a particular target are extremely narrow, it should be kept m mind that the target must not denature over the time penod necessary to perform the affinity punfication: After recovery ofthe targd even under conditions of borderline stability, the produd may be rdumed to a solution where it is highly stable before further processmg Illustrative Tests to Establish a Stability Envelope for a Tared Temperature Stability First, the temperature, T m , at which the targd molecule denatures or shows a substantial loss of activity may be measured in a solution at pH 7, 150 mM salt This T m could be ddermined, for example, using a scanning calonmder It is to be understood that T m indicates a fairly shaφ transition but that prolonged mcubation at slightly lower temperatures (at the upper end of a temperature stability range) could result m loss of activity Thus, it is preferred that the termperatu boundary ofthe stability envdope be drawn a few degrees inside the limit defined by T ro Alternatively, temperature stability may be determined by measuring the activity of samples of a targd after mcubation for a time (e g , from 15 mmutes up to 24 hours) at a sd of temperatures, su as 25° C, 37° C, 45° C, 55° C, and 65° C If the targd shows no loss of activity at one temperatur and significant loss of activity at the next temperature, the boundary ofthe temperature range may ddermmed to the degree of accuracy desired by measuring activity at one or more intermediate temperatures Next, it can be determined whether the targd disaggregates or precipitates at low temperature, down to the freezing point of 150 mM saline pH Stability The range of pH for which the targd retains adivity at several temperatures (e g , 4° C, 25 C, 37° C, and 50° C) may be ddermmed This can be done in a scanning caloπmeter by measuring T,,, for a vaπety of pH values Note that T m defines an upper boundary for stability at a given pH a that the envdope of stability should be drawn a few degrees lower at each pH For example, one could measure T m for pH 7, 6, 8, 5, 9, 4, 10, dc until a boundary is ddermmed Altematively, a sample ofthe target may be incubated for an appropnate time (such as 20 mmutes) at a particular temperature (25° C, 37° C, and 50° C) at pH 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , and 12 and the activity measured after adjusting the pH to one suitable for ddermination ofthe activity If the range is strongly temperature dependent, the range of pH may be determined over which the target retains activity at additional temperatures (eg , 15° C, 30° C, 43° C, and 60° C ) Range of Ionic Strength for which Targd is Soluble and Stable This is advantageously measured at two or three temperatures For example, useful temperatures might be 4° C (or the lowest at which the targd does not precipitate or disaggregate), 25° C, and 5° C bdow T m (or the highest temperature at which targd shows no significant loss of activity) T m might be measured in a scanning calonmder for a senes of concentrations of NaCI, such as 1 mM, 5 mM, 50 mM, 150 mM, 500 mM, 1 M and 3 M In each case, an appropnate buff should be added to maintain the pH at a specific value, for example, pH 7 or pH 8 Most protems are soluble water with some salt Proteins often precipitate or crystallize a very low salt or at very high salt Salts used for preαpitation of protems mclude NaCI, (NF ^SO^ ammonium acetate, KCl, potassium phosphate (KH 2 P0 4 and K 2 HP0 4 ), sodium phosphate (NaH 2 P0 and Na 2 HP0 4 ), and the like Preferably, the pomts (high salt and low salt) at which solubility ofthe targd drops shaφly are ddermmed for two or three salts, such as NaCI, ammonium sulfate, and ammonium acetate Stabilitv and Solubility ofthe Targd with Respect to Chaotropes Urea, guanidinium chloride, sodium thiocyanate, guanidinium thiocyanate, and the like are known to disrupt the interactions of proteins with other molecules. These agents can also cause proteins to unfold. Preferably, the stability ofthe targd is measured with respect to increasing concentrations of one or more ofthe following: urea, guanidinium chloride, sodium thiocyanate, sodium isothiocyanate, N-methylurea, and the like. Typically, the stability of a protein to denaturation by heat is parallel to its stability against denaturation by urea and other chaotropes. Whether the Tared Requires Speάfic Ions for Stability Some proteins bind metal ions (such as Mg**, Zn**, Ca**). In some cases, the proteins denature if the metal ion is removed, for example, by exposure to chelating agents such as EDTA. Whether the target contains metals may be ddermined by an elemental analysis. If the targd does contain metal ions, it may be ddermined whether exposure to EDTA or dialysis against distilled water causes the target to lose activity. Stability and Solubility ofthe Targd in Organic Solvents and Organic Solutes Preferably, the stability and solubility ofthe targd is determined in organic solvents, in mixtures of water and organic solvents, and in water with added organic solutes. For example, the stability and solubility ofthe target is measured in one or more ofthe following: acdonitrile (ACN) and water/ ACN mixtures, methanol (MeOH) and water/MeOH mixtures, ethanol (EtOH) and water/EtOH mixtures, isopropyl alcohol (IP A) and water/TPA mixtures, dimethylformamide (DMF) and water DMF mixtures, dimethylsulfoxide (DMSO) and water/DMSO mixtures, secondary butyl alcohol (SBA) and water/SBA mixtures, acetone and water/acetone mixtures, and the like. In addition, the stability and solubility ofthe target is tested in water with various added amounts of organic solutes such as formamide, N-methylformamide, acetamide, N-methylacetarnide, and the like. Phage Display and Organic Solvents A preferred method of obtaining selective ligands that bind targets is to select from a library of proteins displayed on phage, such as Ml 3. Thus, if phage display libraries are used, it is of interest to ddermine whether organic solvents disrupt the phage. ACN, MeOH, and DMSO are particularly preferred because these solvents do not ruinously affect Ml 3 phage. In particular, the following conditions allow substantial survival of Ml 3. Table 1 Stability of M13 Vaπous Organic Solvents Solvent % solvent by volume, Survival after Survival after in Luna broth 20 mm (%) 60 mm (%) ACN 10 84 74 20 80 91 40 92 89 80 30, 37 8, 3 DMSO 2 84 89 5 68 86 10 96 86 20 73 82 80 1 >1 MeOH 10 110 80 20 93 84 40 92 84 80 80, 21 70, 6 Incubation for 20 mmutes with 80% EtOH, IPA, or DMF left no surviving phage These solvents may be used with phage at lower concentrations Knowledge ofthe target may suggest other manipulable solution conditions that could be useful For example, the stability of a sugar bmdmg protem may be influenced by the concentration of a sugar that the protem bmds Isolation of Affinity Ligands Selecting Bmdmg and Rdease Conditions Usmg a proposed or an empincally ddermmed envelope of stability for the targd molecule, two solution conditions are sdeded, i e , bmdmg conditions and release conditions The bmdmg conditions are a sd of solution conditions under which it is desired that a discovered affinity ligand will bmd the targd, the rdease conditions are a set of solutions conditions under which it is desired that a discovered affinity ligand will not bmd the taiget The two conditions may be seleded to satisfy any cπteπon ofthe practitioner, such as ease of attainmg the conditions, compatability with other punfication steps, lowered cost of switching between conditions compared to other affinity media, dc Preferably the two solution conditions are (a) well within the boundanes ofthe stability envdope for the targd and (b) far apart with resped to at least one solution paramder For example, lf the targd is stable over a wide pH range, then favorable bmdmg conditions might be pH 11, 150 mM salt, 25° C and favorable release conditions might be pH 3, 150 mM salt, 25° C For a different targd havmg a narrow range of pH stability (for example, pH 6 2 to 7 8) but being stable over a wide range of salinity, two useful conditions might be bmdmg conditions pH 7 2, 3 M NaCI, 25° C and release conditions pH 7 2, 2 mM NaCI, 25° C A third target might have a narrow range of pH stability but a wide range of stability versus acetonitnle For this third hypothetical targd, useful conditions might mclude bmdmg conditions pH 7 0, 100 mM NaCI, 100 mM K 2 HP0 4 , 25° C and release conditions pH 7 0, 5 nM NaCI, 50% (v/v) acetonitnle Selection of a Candidate Bmdmg Domam i conjunction with selecting specific solution conditions for the desired bmdmg and rdease ofthe targd, a candidate bmdmg domam must be selected to serve as a structural template for the engineered affinity ligands that will exhibit the desired bmdmg and release capabilities The bmdmg domam may be a naturally occurπng or synthetic protem, or a region or domam of a protem The candidate bmdmg domam may be selected based on knowledge of a known interaction between the candidate bmdmg domam and the targd, but this is not cπtical In fact, it is not essential that the candidate bmdmg domam have any affinity for the targd Its puφose is to provide a structure from which a multiphαty (library) of analogues can be generated, which multipliαty of analogues will mclude one or more analogues that exhibit the desired bmdmg and release properties (and any other properties sdected for) Thus, the bmdmg conditions and the release conditions discussed infra may be seleded with knowledge ofthe exad polypeptide that will serve as the candidate bmdmg domam, or with knowledge of a class of protems or domains to which the candidate bmdmg domam belongs, or completely independently ofthe choice ofthe candidate bmdmg domam Similarly, the bmdmg and or release conditions may be sdeded with regard to known interactions between a bmdmg domam and the targd, e g , to favor the interaction under one or both ofthe solution conditions, or they may be selected without regard to such known interactions Likewise, the candidate bmdmg domam can be sdected taking into account the bmdmg and or release conditions or not, although it must be recognized that if the bmdmg domam analogues are unstable under the binding or release conditions no useful affinity ligands will be obtained In selecting a candidate bmdmg domam, the objed is to provide a template or parental structure from which a library of similarly strudured analogue domains can be generated The analogue library will preferably be a biased library (as opposed to a randomly generated library), m that vaπegation ofthe basic domam to create the library will be earned out m such a way as to favor the properties desired for the affinity ligands The nature ofthe candidate bmdmg domain greatly influences the properties ofthe deπved protems (analogues) that will be tested agamst the targd molecule In selectmg the candidate bmdmg domain, the most important consideration is how the analogue domains will be presented to the taiget, i.e., in what conformation the target and the analogues will come into contad. In preferred embodiments, for example, the analogues will be generated by insertion of synthetic DNA encoding the analogue into a replicable genetic package, resulting in display ofthe domain on the surface of a microorganism such as M13 phage, using techniques as described, e.g., in U.S. 5,403,484 (Ladner d al.) and U.S. 5,223,409 (Ladner d al.), incoφorated herein by reference. Structured polypeptides offer many advantages as candidate binding domains over unstrudured peptides. Mutation of surface residues in a protein will usually have little effect on the overall strudure or general properties (such as size, stability, temperature of denaturation) ofthe protein; while at the same time mutation of surface residues may profoundly affect the binding properties ofthe protein. This has been fully documented, for example, for BPTϊ-homologous Kunitz domains (see Ladner, 1995). Mutating surface residues on proteins or structured domains can lead to greater diversity of properties for the analogues than is obtained by mutating unstructured peptides because the protdn framework or the structure ofthe domain holds the mutated residues in conformations that differ from residue to residue and from framework to framework. This is espedally important for hydrophobic side groups that would become buried unless constrained in a strudure. The more tightly a peptide segment (domain) is constrained, the less likely it is to bind to any particular targd. If it does bind, however, the binding is likdy to be tighter and more spedfic. Thus, it is preferred to sdect a candidate binding domain and, in turn, a structure for the peptide analogues, that is constrained within a framework having some degree of rigidity. Alternatively, more than one candidate binding domain strudure can be selected, in order to increase the size ofthe library of analogues and to introduce additional variegated structures for presentation to the target. As the size ofthe library is increased, and higher numbers and diversely structured analogues are prepared, the probability of including a useful framework and displayed functional groups increases to the point where high-affinity ligands can be found for almost any targd. The size ofthe candidate binding domain is also an important consideration. Small proteins or polypeptides offer several advantages over large proteins, such as monoclonal antibodies (see Ladner, 1995). First, the mass per binding site is reduced. Highly-stable protein domains having low molecular wdghts, e.g., Kunitz domains (~7 kDa), Kazal domains (~7 kDa), Cucurbida maxima trypsin inhibitor (CMTI) domains (~3.5 kDa), and endothdin (~2 kDa), can show much higher binding per gram than do antibodies (150 kDa) or single-chain antibodies (30 kDa). Second, the possibility of non-speαfic binding is reduced because there is less surface available. Third, small proteins or polypeptides can be engineered to have unique tethering sites in a way that is impracticable for antibodies. For example, small proteins can be engineered to have lysmes only at sites suitable for tethering (e g , to a chromatography matnx), but this is not feasible for antibodies It is often found that only a small fraction of immobilized antibodies are active, possibly due to inappropnate linkage to the support Most small protems or polypeptides that are stabilized by disulfides do not contain cysteines that are not mvolved in disulfides This is because the oxidizing conditions that cause disulfide formation for stabilizing the protem also lead to disulfide formation by otherwise unpaired cysteines Thus, small proteins havmg stabilizing disulfides and an odd number of cysteines tend to form disulfide linked dimers (e g , homodimers or hderodimers) The disulfides between domams are more easily reduced than are the stabilizing lntradomain disulfides Thus, by selective reduction it is possible to obtam monomenc disulfide- stabilized domams havmg a smgle free thiol Such thiols can be used for highly stable immobilization of these domams by formation of a thioether with iodoacetamide, lodoacdic aαd, or similar α-iodo carboxylic aαd groups Small protem or polypeptide domams also can be chemically synthesized, which permits speαal immobilizing groups to be incoφorated a way that does not interfere with bmdmg to the targd For mstance, if small disulfide-contammg protems are chemically synthesized, the ammo aαd sequence can be altered by adding an extra cysteine residue with the thiol blocked m a different manner from other cysteines elsewhere in the sequence The sdected thiols can be deblocked and disulfides allowed to form, then the added cysteine can be deblocked and the molecule can be immobilized by reaction with a suitable material such as a substrate containing immobilized NH 2 -CO- CH 2 I Fourth, a constrained polypeptide strudure is more likely to retain its functionality when transferred with the structural domam intact from one framework to another Fon mstance, the bmdmg domam strudure is likely to be transferable from the framework used for presentation m a library (e g , displayed on a phage) to an isolated protem removed from the presentation framework or immobilized on a chromatographic substrate There are many small, stable protein domams suitable for use as candidate bmdmg domams and for which the following useful information is available (1) ammo aαd sequence, (2) sequences of several homologous domams, (3) 3-dιmensιonal strudure, and (4) stability data over a range of pH, temperature, salinity, organic solvent, oxidant concentration Some examples are Kunitz domams (58 ammo aαdes, 3 disulfide bonds), Cucurbida maxima trypsm inhibitor domams (31 ammo aαds, 3 disulfide bonds), domams related to guanylin (14 ammo aαds, 2 disulfide bonds), domams related to heat-stable enterotoxm IA from gram negative bactena (18 ammo aαds, 3 disulfide bonds), EGF domams (50 ammo aαds, 3 disulfide bonds), krtngle domams (60 ammo aαds, 3 disulfide bonds), fungal carbohydrate-binding domains (35 ammo aαds, 2 disulfide bonds), endothdm domains (18 ammo aαds, 2 disulfide bonds), and Streptococcal G IgG-binding domam (35 ammo aαds, no disulfide bonds) Most, but not all of these contain disulfide bonds that ngidify and stabilize the structure Libraπes based on each of these domams, preferably displayed on phage or other genetic packages, can be readily constructed and used for the selection of bmding analogues Providing a Library of Candidate Bmdmg Domam Analogues Once a candidate bmdmg domam has been seleded, a library of potential affinity ligands is created for screening agamst the targd at the bmdmg and dution (rdease) conditions The library is created by making a seπes of analogues, each analogue corresponding to the candidate bmding domam except havmg one or more ammo aαd substitutions m the sequence ofthe domam The amm aαd substitutions are expected to alter the bmdmg properties ofthe domam without significantly altenng its strudure, at least for most substitutions It is preferred that the ammo aαd positions that are selected for vaπation (vanable ammo aαd positions) will be surface ammo aαd positions, that is, positions m the ammo aαd sequence ofthe domams which, when the domam is m its most stable conformation, appear on the outer surface ofthe domam (l e , the surface exposed to solution) Most preferably the ammo aαd positions to be vaned will be adjacent or close together, so as to maximize the effect of substitutions In addition, extra ammo aαds can be added into the strudure ofthe candidate bmdmg domam In preferred embodiments, especially where a great deal of information is available concerning the 3-dιmensιonal structure ofthe targd or mteractions ofthe targd with other molecules, particularly the candidate bmdmg domam, those ammo aαd positions that are essential to bmding mteractions will be determmed and conserved m the process of buildmg the analogue library (1 e , the ammo aαds essential for bmdmg will not be vaned) The object of creating the analogue library is to provide a great number of potential affinity ligands for reaction with the targd molecule, and m general the greater the number of analogues the library, the greater the likelihood that a member ofthe library will bmd to the target and release under the presdected conditions desired for rdease On the other hand, random substitution at only six positions m an ammo aαd sequence provides over 60 million analogues, which is a library size that begins to present practical limitations even when utilizing screenmg techniques as powerful as phage display It is therefore preferred to create a biased library, in which the ammo aαd positions designated for vaπation are considered so as to maximize the effed of substitution on the bmdmg diaractenstics ofthe analogue, and the ammo aαd residues allowed or planned for use m substitutions are limited to those that are likely to cause the analogue to be responsive to the change m solution conditions from the bmdmg conditions to the rdease conditions As indicated previously, the techniques discussed m U S 5,223,409 are particularly useful preparmg a library of analogues corresponding to a selected candidate bmdmg domam, which analogues will be presented m a form suitable for large-scale screenmg of large numbers of analogues with respect to a targd molecule The use of replicable genetic packages, and most preferably phage display, is a powerful method of generating novel polypeptide bmdmg entities that involves introducing a novel DNA segment into the genome of a bacteπophage (or other amphfiable genetic package) so that the polypeptide encoded by the novel DNA appears on the surface ofthe phage When the novd DNA contains sequence diversity, then each reαpient phage displays one vaπant of the initial (or "parental") ammo aαd sequence encoded by the DNA, and the phage population (library) displays a vast number of different but related amino aαd sequences A phage library is contacted with and allowed to bmd the targd molecule, and non-bmders are separated from bmders In vaπous ways, the bound phage are liberated from the target and amplified Smce the phage can be amplified through infection of bactenal cells, even a few bmdmg phage are suffiαent to reveal the gene sequence that encodes a bmding entity Usmg these techniques it is possible to recover a bmdmg phage that is about 1 in 20 million in the population One or more hbraπes, displaying 10-20 million or more potential bmdmg polypeptides each, can be rapidly screened to find high-affinity ligands When the selection process works, the diversity ofthe population falls with each round until only good bmders remain, I e , the process converges Typically, a phage display library will contain several closely rdated bmders (10 to 50 bmders out of 10 million) Indications of convergence mclude increased bmdmg (measured by phage titers) and recovery of closely rdated sequences After a first sd of bmdmg polypeptides is identified, the sequence information can be used to design other hbraπes biased for members havmg additional desired properties, e g , discnmination between two highly similar molecules Such techniques make it possible not only to screen a large number of analogues but make it practical to repeat the binding/elution cycles and to build secondary, biased hbranes for screening analog-displaying packages that med initial catena Thus, it is most preferred in the practice ofthe present mvention (1) that a library of bmding domam analogues is made so as to be displayed on replicable genetic packages, such as phage, (2) that the library is screened for genetic packages bmdmg to the targd molecule wherem the bmdmg conditions ofthe screenmg procedure are the same as the bmding conditions preselected for the desired affinity ligand, (3) that genetic packages are obtained by elution under the release conditions presdeded for the affinity ligand and are propagated, (4) that additional genetic packages are obtained by elution under highly disruptive conditions (such as, e g , pH 2 or lower, 8 M urea, or saturated guamdmium thiocyanate, to overcome extremely high affinity assoαations between some displayed bmding domain analogues and the targd) and are propagated, (5) that the propagated genetic packages obtained m (3) or (4) are separately or m combination cycled through steps (2) and (3) or (4) for one or more (e g , one to five) additional cycles, and (6) a consensus sequence of high-affinity binders is determmed for analogues expressed in genetic packages recovered from such cycles, (7) that an additional biased library is constructed based on the original framework (candidate binding domain) and allowing the high-affinity consensus at each variable amino add position, and in addition allowing other amino add types seleded to include amino adds believed to be particularly sensitive to the change between the binding conditions and the release conditions; (8) that this biased library is screened for members that (a) bind tightly (i.e., with high affinity) under the binding conditions and (b) release cleanly (i.e., readily dissodate from the target) under the release conditions. Use ofthe Affinity Ligands in Chromatography After members of one or more libraries are isolated that bind to a target with desired affinity under binding conditions and release from the targd as desired under rdease conditions, isolation of the affinity ligands can be accomplished in known ways. If, for example, the analogue library is composed of prospective affinity ligands expressed on phage, rdeased phage can be recovered, propagated, the synthetic DNA insert encoding the analogue isolated and amplified, the DNA sequence analyzed and any desired quantity ofthe ligand prepared, e.g., by direct synthesis ofthe polypeptide or recombinant expression ofthe isolated DNA or an equivalent coding sequence. Additional desired properties for the ligand can be engineered into an analogue ligand in the same way release properties were engineered into the ligand, by following similar steps as described herein. The affinity ligands thus isolated will be extremely useful for isolation ofthe targd molecule by affinity chromatography methods. Any conventional method of chromatography may be employed. Preferably, an affinity ligand ofthe invention will be immobilized on a solid support suitable, e.g., for packing a diromatography column. The immobilized affinity ligand can then be loaded or contacted with a feed stream under conditions favorable to formation of ligand targd complexes, non-binding materials can be washed away, then the target can be eluted under conditions favoring release ofthe targd molecule from a ligand target complex. Alternatively, bulk chromatography can be carried out by adding a feed stream and an appropriately tagged affinity ligand together in a reaction vessel, then isolating complexes ofthe targd and ligand by making use o the tag (e.g., a polyHis affinity tag, which can by used to bind the ligand after complexes have formed), and finally releasing the targd from the complex after unbound materials have been eliminated. It should be noted that although pre se binding and release properties are engineered into the affinity ligands, subsequent use in affinity purification may reveal more optimal binding and release conditions under which the same isolated affinity ligand will operate. Thus, it is not critical that the affinity ligand, after isolation according to this invention, be always employed only at the binding and release conditions that led to its separation from the library. Affinitv Constants Assume that an affinity ligand is isolated having a molecular weight of approximately 3,000 Daltons (~3 kDaXCf, e.g., CMTI derivatives infra). Also assume that the ligand can be loaded onto a suitable chromatographic support at an effective concentration of 3 mM (i.e., —10 g L). Assume a targd with molecular wdght of 50 kDa and produced at 10 mg/L from cell culture medium. The concentration of protein is (0.01 g/L) ÷ (5 χ 10 4 g/mole) = 0.2 μM. If the ligand is present at 3 mM, then 1 L of affinity material could capture 3 millimoles (167 g) of targd. Assume that the ligand binds the targd according to the simple mass-action equation (1): K D [targd] 1 10 (1) = ≡ [ligand] [complex] X The fraction of targd that can be captured is given in equation (2): 1 (2) Fraction-of-Targd-Bound 1 + (1 X) Thus, the Fraction-of-Targd-Bound can be controlled by the dissodation constant K D and the amount of ligand loaded on the support. K D is a function ofthe solution conditions but is inherent in the affinity ligand molecule. The value of K D under Binding Conditions is K D BC and the value of K D under Elution Conditions is K D EC . Fraction of Binding (in one equilibrium stage) X = [ligand]/ K D Fraction-of-Targd-Bound K D o+σ/x))" 1 (Assumed [ligand] = 3 mM) 0.001 0.001 3 M 0.010 0.0099 300 mM 0.020 0.0196 150 mM 0.040 0.0385 75 mM 0.100 0.0909 30 mM 0.500 0.03333 6mM 1.000 0.50 3 mM 2.000 0.667 1.5 mM 5.000 0.8333 600 μM 10.000 0.90909 300 μM 100.000 0.990099 30 μM 1000.000 0.999000999 3 μM 10000.000 0.999900009999 300 nM The foregoing table shows how the Fraction-of-Targd-Bound is expected to vary with the ratio [ϋgandjYK D . Acceptable capture is assumed to be greater than 0.9, but the invention is not limited to any particular capture level. If [ligand] is approximately 3 mM, then acceptable capture can be obtained with K D BC at 300 μM or lower. More effective capture is obtained with [ligand]/ K D BC ≥IOO, and these higher values are preferred. The higher the value of [ligand]/ K D BC the better with th proviso that [ligand]/ K D ^ must be small enough to allow recovery ofthe targd. If more affinity ligand than 3 mM can be loaded on a support, then ligands having higher values of K D can be used. Acceptable dution involves [ligand]/ K D EC < 10; lower values of [ligand]/ K D EC are preferred. In particular, [ligand]/ K D 60 < 0.1 is preferred and [ligand]/ K D BC < 0.01 is more preferred. The lower [ligand]/ K D EC is the better without limit. After the ratio [ligand] K D EC is bdow 0.001, there is little to be gained in equilibrium release, but ligands having very high K D may have higher off rates and so may be superior in practice by releasing targd very rapidly. Thus, to have effident capture and elution, theory indicates that (K D EC / K D BC ) should be 1000 or greater; the higher this ratio, the better. The process is satisfactory if (K D EC / K D BC ) > 10 if at least 10% of bound material can be liberated under elution conditions. Nevertheless, the invention is not limited to any particular value of (K D EC / K D BC ) Ligands havmg very high affimty for a target could give higher speαfiαty, especially if the affinity matenal is loaded with a slight excess of targd so that impunties havmg less affinity are crowded out m favor of targd For very gh-affinity ligands, the amount of excess needed to crowd out impunties is small K D ^ IS as important as K D BC Preferably, K D EC is no greater than 0 1 and more preferably no greater than 0 01 Nevertheless, affinity punfication is functional even if K D ^ = 1 , which theoretically allows rdease of 50% ofthe bound matenal Four equi bπum washes would lead to 94% recovery ofthe matenal captured under the bmdmg conditions (l e , 50% + 25% + 12 5% + 6 25%) Hypothetically, 1 L of an immobilized hgand havmg K D BC = 1 μM and attached to a support at 3 mM could capture 99% of a targd from about 15,000 L of a 0 2 μM solution (10 mg/L of a 50 kDa targd molecule) It will be understood that the affinity ligand attached to a suitable support may be used many different ways to capture the targd, including chromatographic embodiments with many equilibnum stages For example, the hgand-support may be loaded mto columns and the solution containing the targd caused to flow through the column and (optionally) reαrculated until substantially all ofthe targd is bound or the ligand support is saturated with target Alternatively, the hgand-support may be mixed (shaken or stirred) with the target-containing solution until bmding is complete If K D EC is 3 mM and the loading is 3mM, then four 1 L equilibrated washes could recover up to 2 82 millimoles (94% of 3 mM) m 4 L to give a 670 μM solution of targd, I e , a 3350-fold concentration ofthe targd as compared to the feed stream Furthermore, it is expected that most impunties would be removed If K D ^ IS 30 mM and the loading is 3mM, then two 1 L equilibrated washes could recover up to 2 98 millimoles (99% of 3 mM) in 2 L to give a 1 49 mM solution of targd, l e , a 7450-fold concentration ofthe targd as compared to the feed stream If, on the other hand, K D EC is 0 3 mM and the loading is 3mM, then twenty-four 1 L equilibrated washes could recover up to 2 70 millimoles (90% of 3 mM) m 24 L to give a 112 μM solution of targd, I e , a 560-fold concentration ofthe targd as compared to the feed stream, but this would not be very desirable Thus, a pracUcal limit for ligand on a support is [ligand]/ K D EC = 10 with lower values being preferred The level of loading of support with ligand governs three fadors (a) the fraction of targd that can be captured from a feed stream, (b) the absolute amount of target that can be captured, and (c) the amount of impunties that will be captured Theoretical behavior ofthe first two factors have been given Smce the impunties in a feed stream are kdy to be complex and idiosyncratic, the mteractions between [ligand] and the bmding and elution of impunties is best worked out empiπcall Given a particular affinity ligand, an appropnate loadmg for optimal separation can be calculated Assuming an affinity ligand that has K D BC of 3 mM and K D EC of 3 μM, if loaded onto a support at 3 mM, capture of targd would be complde However, under dution conditions only > 1 could be eluted If the ligand were loaded at 3 μM, targd could be captured and released with X = 1 Altemativdy, vanants ofthe selected affinity ligand could be prepared m which ammo aαds m or near the bmdmg region would be changed so that one or more having a more suitable (I e , lower) affinity would be obtained The foregoing discussion has been in terms of batch bmdmg and elution An alternative approach is isocratic chromatorgraphy with many equi bnum stages Affinity ligands that have hig on and off rates may be more effective in chromatographic processes For chromatographic procedures, the most important attributes ofthe ligand are moderate affinity (1 μM to 1 mM) for the target and much less affinity for any other component ofthe feed stream To mcrease the likelihood of recovenng bmdmg analogues havmg lower affinities, the removal of unbound analogues (see step (g), above), e g , by washing, may be very bπef, especially the early rounds of screening For instance, a bmdmg domam havmg a K D = 3 μM could have a K M , = 10 3 /molar/sec and K„ff = 10 "3 /sec This corresponds to τ κ = 694 sec » 12 mm If the library oπgmally contains IO 7 different analogues, then a sample of IO 11 analogues will have 10,000 of each member of the library If 5,000 of a particular analogue bmd to the target, and washes of three half times (τ Vl ) are employed (-35 nun ), then 625 analogues would be captured Even though curtailed washing may give a higher background of non-specific analogues, it may be preferred to ensure that fast-on/fast-off members ofthe library are retained mto the next rounds From the foregoing, it can be seen that the highest affimty ligand is not necessaπly the best for controllable or cost effective recovery of a target molecule The method of the mvention permits sdection of ligands that have a vaπdy of desirable charadeπstics important to the practitioner seeking isolation of a particular target, such as speαfic bmdmg ofthe targd, coupled with predictabl and controlled, dean release of the targd, useful loadmg capaαty, acceptably complete elution, re- usabihty/recyclability, dc Elimination of Closely Related Impunties The present mvention also allows sdection of affinity ligands that can discnminate between a targd molecule and other molecules that are quite similar Here, the "taiget" is the molecule to which the affimty ligand should bmd and the "speαfic impunty" is the molecule to which binding is desired to be minimized Using the method ofthe invention, an affinity ligand can be found that is able to distinguish between a protein that contains Asn residues but not the deamidated derivative in which Asn has been chemically converted to Asp. Similarly, affinity ligands can be found that are useful to separate the R form of an isomeric organic compound from the S form. There are at least three ways to achieve this kind of spedfidty: First, a number of affinity ligands that bind the targd could be obtained and tested as clonal isolates for binding to the spedfic impurity. Second, the spedfic impurity could be immobolized and those members of a library that have high affinity for the spedfic impurity could be removed. This "deplded library" could then be screened for binders to the targd. Third, while screening a library of potential affinity ligands for binding to the immobilized targd, one could include the spedfic impurity (or spedfic impurities) in the binding buffer at a suffident concentration that any member ofthe library having affinity for both the targd and the spedfic impurity is much more likdy to bind the spedfic impurity than the immobilized targd. In this manner the members binding to the target are "engineered" to exhibit a lack of affinity for the spedfic impurity. The third method is preferred. For all three methods, it is important that the spedfic impurity be substantially free ofthe targd. It is preferred that the concentration of targd in spedfic impurity be as low as possible. In the third method, the spedfic impurity would be added at a concentration such that the concentration of targd free in solution would not be expected to bind a significant portion of any target-binding component ofthe library. Thus, as the library is screened through several rounds of bmding/washing/dution/amplification, the concentration of spedfic impurity may be increased, since the concentration of each remaining member ofthe library increases while the number of members decreases. As long as any targd contained in the preparation including the spedfic impurity cannot tie up all of a potentially useful affinity ligand, then some ofthe useful affinity ligand will be captured by the immobilized targd and will be carried forward to the next round. The following calculation would suggest an appropriate amount of spedfic impurity to use, but the invention is not limited to the use of any particular level of spedfic impurity or to any particular theory of binding and competition. In a phage display library of complexity 10 6 (i.e., IO 6 different potential affinity ligand peptides displayed), suppose the titer ofthe library is IO 13 phage/mL, i.e., IO 7 phage of each type/mL. If the targd is present at effectively 50 μM and the spedfic impurity at 0.5% of targd, then spedfic impurity would be added up to 1 ,000 μM and have free target be at 5 μM, which is unlikely to completely inhibit capture of affinity ligands that can bind targd in the presence ofthe spedfic impurity. When a library is screened without amplification between rounds, it may be suffiαent to add the speαfic impunty m one ofthe bmdmg steps It should be expenmentally determmed whether early bmdmg steps or late bmding steps are the optimal stage at which the speαfic impunty is to be added Isolation of affinity ligands accordance with this mvention will be further illustrated bdo The specific parameters mcluded m the following examples are intended to illustrate the practice of the mvention, and they are not presented to m any way limit the scope ofthe mvention Examples 1-15 It is often found that proteins that bmd to other protems have one or more hydrophobic side groups that are exposed m the free protem and that become buπed when the complex forms The ammo aαds Phe, Tyr, Leu, De, Val, Md, Tφ, Cys, and His have hydrophobic side groups Phe, Le De, Val, Met, and Cys (when part of a disulfide) are always hydrophobic Tyr and Cys (when not part of a disulfide) can be ionized at high pH His can become ionized at neutral or aαdic pH Protems that are likely to have exposed hydrophobic regions mclude protem hormones, receptors for protem hormones, receptors, bmding molecules, antibodies, and the like Enzymes and highly abundant storage and transport protems (such as trypsm, myoglobin, and hemoglobin) may not have any large hydrophobic regions on the surface If a targd is known to have exposed hydrophobic regions or is likdy to have exposed hydrophobic regions, then a preferred approach is to select for a ligand that bmds at high salt (which favors hydrophobic interactions) and 37° C and that releases at 4° C (which disfavors hydrophobic mteractions) and/or m acetonitnle (which disfavors hydrophobic mteractions) at 50% (v/v) and 10 mM NaCI (low salt) If a target protein has a high number of ionic groups (Asp, Glu, Lys, Arg, and His, as can be ddermmed from the ammo aαd composition or ammo aαd sequence), then a preferred approach is to sde for a ligand that bmds at one pH and rdeases at a second pH For example, for a targd that is stable between pH 3 0 and pH 9 5, it is advantageous to sded for ligands that bmd at pH 8 5, 20 m NaCI, and 10° C, and that release at pH 4 0, 1 M NaCI, and 30° C Low temperature and low salt favor ionic mteractions, while higher salt and temperature reduce the bmdmg force of ionic groups Altematively, one could sded for ligands that bmd at pH 8 5, 1 M NaCI, and 30° C and that rdease at pH 4 0, 10 mM NaCI, 10° C, and 10% (v/v) MeOH If a water-soluble targd has a very low number of ionic groups (Asp, Glu, Lys, Arg, and His for proteins), then it is likely to have many neutral polar groups on the surface (viz., Ser, Thr, Asn, Gin, and Tyr for protems) Many sugars will fall mto this class Such neutral polar groups (hydroxyl, amide, ether, ester, and the like) are capable of forming hydrogen bonds Thus, a library havmg hydrogen bond-forming ammo aαd types allowed at many positions is likely to contain members that will bind spedfically to molecules of this class with useful affinity. Amino adds having side groups that can form hydrogen bonds include Asp (D), Glu (E), His (H), Lys (K), Asn (N), Gin (Q), Arg (R), Ser (S), Thr (T), Tφ (W), and Tyr 0 Hydrogen bonding is strongly affected by pH, ionic strength, dielectric constant ofthe solvent, and by chaotropes such as urea and guanidinium ions. Hydrogen bonds lose strength at higher temperature. Thus, appropriate binding conditions would have low salt and low dielectric constant, while elution conditions would have high salt, high dielectric constant, and a pH different for the pH ofthe binding condition, preferably by at least 1 pH unit. For example, binding conditions could be pH 8, 10 mM NaCI, and 4° C, while elution conditions could be pH 6, 500 mM NaCI, and 35° C. Alternatively, one could seled for ligands that bind at pH 8.5, 1 M NaCI, and 30° C and that rdease at pH 4.0, 10 mM NaCI, 10° C, and 10% (v/v) MeOH. The solution condition most easily and economically altered is pH. It is wdl known that a change in the ionization of an amino add within an intermolecular interface can greatly disrupt the binding ofthe molecules. Thus, to increase the likelihood that the binding of an analogue to the target will be highly sensitive to changes inpH, the library can be designed to allow a t least His and preferably Glu, Asp, Tyr and Lys at most or all varied positions. It is to be understood that other amino add types may also be allowed. For example, a library in which each varied position allows the sd {His, Asp, Glu, Tyr, Lys, Ala, Ser, Asn, Leu and Phe} would be highly likely to contain ligands that bind to a targd in a highly pH-sensitive manner. Libraries that allow only the sd {His, Glu, Asp and Tyr} are also likely to function, but allowing additional amino add types increases the likelihood of obtaining binding under the first solution condition. One or two groups that change ionization state within the interface is suffident to disrupt the binding under the second set of solvent conditions. As shown in Table 2, the pK,s of His, Glu, Asp, Tyr and Lys are in the range 3.5 to ~11, a range of pH over which many proteins are stable. In proteins, however, the positioning of each side group can influence its adual pK,. For example, a Lys held in close proximity to an Arg can show a lower pK, than it otherwise would. Using the type of infoπnation in Tables 2 and 3 relating to the reactivity of amino add side chains and the properties amino add groups are likely to confer on a protein in solution, it is possible to select the amino add substitutions necessary for the design of libraries that will have increased likelihood of containing candidate binding domains that are sensitive to the particular kinds of changes in solvent conditions (moving from binding to elution conditions) where it is desired that the affinity separation process will operate. Table 2: Ionizable Groups of Proteins Group Amino Add Type pK a range α-Amino any except Pro 6.8-8.0 α-Carboxyl any 3.5-4.3 β-Carboxyl Asp 3.9-4.0 γ-Carboxyl Glu 4.3-4.5 δ-Guanido Arg 12.0 ε- Amino Lys 10.4-11.1 Imidazole His 6.0-7.0 Thiol Cys (not in disulfide) 9.0-9.5 Phenolic hydroxyl Tyr 10.0-10.3 (From Cneghton, Proteins: Strudures and Molecular Properties. 2nd Ed. (W. H. Freeman and Co., New York, 1993), p. 6.) Table 3 gives classes of amino add types that are likdy confer on a protein binding that is sensitive to the listed change in solvent conditions. Preferred candidate binding domain analogues of the present mvention contain disulfide bonds that stabilize the strudure and thus allow the domains to exhibit high affinity, spedfidty, and stability. Example 1 illustrates proposed binding and release conditions for a library of binding domain analogues in which histidine is allowed at many or all variable positions. At pH > 7, His is uncharged and the side group is hydrophobic, although it can donate and recdve hyrdogen bonds. High salt and high temperature reduce the contribution to binding energy of ionic interactions. Thus, binding at high salt and high temperature decreases the chance of obtaining a binder having strong ionic interactions and increases the chance of obtaining a ligand that has hydrophobic interactions. The release conditions of low salt and low temperature increase the importance of ionic interactions, particularly the effect of incorred ionic interactions. Thus, a complex that has one or more His residues in the binding interface is likely to be destabilized at a dic pH and low salt. Example 2 illustrates sdection from a library having His allowed at many or all variable positions. The binding and release conditions are selected to foster isolation of one or more binding domains that bind the targd in a manner that is strong at pH 8.0 but is very sensitive to lowering the pH. Histidine is not the only amino add that can foπn pH-sensitive interactions. The amino adds shown in Example 3 are all capable of being ionized in the pH range in which many proteins are stable. Often, the pH at which ionization takes place is highly context-dependent Thus, an aspartic add side group within a protein-targd interface may not have a pK a within the range shown in Table 2. When a group within a protein-targd interface changes ionization state, it is quite likely that the stability ofthe complex will be strongly altered. Thus, if a complex is stable at one pH, it is unlikely to be stable at a pH suffidently different that a group within the interface has changed ionization state. The pH at which such a transition takes place is difficult to calculate and need not be one ofthe pK a s shown in Table 2. Example 4 illustrates that a library of analogues having many Asp, Glu, Lys, Arg, Tyr, and unpaired Cys residues is likely to contain members which would bind to a target at pH 3.5, 4° C, 50 mM NaCI and rdease at pH 7.5. Example 5 illustrates that low salt could be used to foster binding through ionic interactions and that elution with high salt and a different pH will disrupt these interactions. Example 6 illustrates that binding at pH 8.2, 10 mM NaCI, 25° C and elution at pH 5.7, 1 M NaCI is likely to elute binding domains that are sensitive to increased ionic strength and lower pH. Ionic strength can also strongly affect protdn-targd binding. It is understood that different complexes have different sensitivity to salt. In general, complexes held together by hydrophobic interaction are more stable in high salt, while those held together with ionic interactions are less stable m high salt than they are m low salt In Example 7, this is exploited by using a library πdi in ionic groups, low salt for bmdmg and high salt for elution Example 8 illustrates that this effect could be seen by bmdmg the library at 5 mM KCl and eluting with saturated KCl Alternatively, if the library also allows at least some hydrophobic groups, the bmdmg and elution conditions can be reversed, as shown m Example 9 From the same library, it is expected that the bmding and dution conditions of Examples 8 and 9 would yield different bmdmg domams for the same targd Example 10 illustrates that a library allowing hydrophobic groups at many or all ofthe vanable positions ofthe candidate bmdmg domam is likely to have members that bmd tightly in aqueous 1 M NaCI but that release m conditions of 50% by volume ACN/water and no salt This is because hydrophobic mteractions can be disrupted by organic solvent Example 11 teaches that domams havmg several ofthe ammo aαds from the group listed tn the bmdmg interface would be expected to bmd m water and release m 30% MeOH with some salt Protem complexes that are pπmaπly held together by hydrophobic interactions are known to "cold denature" Thus, a library that allows the listed hydrophobic residues of Example 12 at many ofthe vaned positions is likely to contam members that will bmd a targd at 33° C and high salt and to release at low temperature (e g , 0° C) and low salt Chaotropes and other solutes can also cause the bmdmg of a stable domam to a target to be very sensitive Urea typically breaks up mteractions between groups that interact through hydrogen bonds Gin, Ser, Thr, Asn, Tyr, and His are particularly preferred while Asp, Glu, Lys, and Arg are also likely to make mteractions that are susceptible to disruption by urea (see Example 13) The side groups of Asp and Glu are capable of interacting with other aαdic groups through chelation of di- or tπvalent metal ions, especially Ca , Mg , Zn ++ , Cu 4-1" , and the like Carbonyl oxygens m the protem mam chain and the side groups of Asn and Gin and the amide nitrogens of a proton mam diam and Asn and Gin can also from bonds to certain metal ions Metal ions can also be sequestered with chelating agents such as EDTA Thus, m a library that allows Asp and Glu at several surface positions, there are likely to be members that will bmd a targd that contains aαd groups (and does not depend on bmdmg multivalent metal ions for stability) only when a divalent or tπvalent metal ion is present m solution (see Example 14) Release of such members can be effected with EDTA or other chelating agents Basic side groups can interact with anions such as chloπde, acetate, phosphate, sulfate, αtrate, or lactate For multivalent anions, two or more side groups can mterad with one ion Even if such interaction does not greatly stabilize the bmdmg, a polyvalent ion can mask repulsion and allow formation of a complex that would otherwise be unstable Removal ofthe polyvalent anion could then cause the complex to dissoαate, as illustrated m Example 15 Table 4 summarizes suitable codons for generating, e.g., in a library providing phage display of binding domain analogues, groups of amino adds that would contribute to the analogues' responsiveness to changes in the listed solution parameters: Table 4: Variegation codons suited to particular separation conditions Codon Amino Acids Allowed Separation Conditions RNS IMTNKSRVADEG change pH, salt, urea, EDTA NAT YHND change pH, salt, urea, EDTA NAS YHQNKDE change pH, salt, urea, EDTA NRT YCHRNSDG change pH, salt, urea, EDTA NYT SPTAFLIV organic solvent, low temperature NTT FLIV organic solvent, low temperature TNS FLSYCW organic solvent, low temperature RRS NKSRDEG change pH, salt, urea, EDTA Example 16 The techniques described above were employed to isolate affinity ligands for recombinant human tissue-type plasminogen activator (tPA). The process of creating tPA affinity ligands involved three general steps: (1) screening of approximatdy 11 million variants of a stable parental protein domain for binding to tPA, (2) producing small quantities ofthe most interesting ligands, and (3) chromatographic testing of one ligand bound to activated beads for the affinity purification of tPA from a plasma spiked sample. For this work, tPA was purchased from CalBiochem (#612200) and immobilized on Reacti- Gel™ agarose beads from Pierce Chemical Company by methods described in Markland d al. (1996). Approximately 200 μg of tPA were coupled to 200 μL of Reacti-Gel™ slurry. Four libraries of phage-displayed protdns were picked for the screening process. Three were based on the first Kunitz domain of lipoprotein assodated coagulation inhibitor (LACI-K1), called Lib#l , Lib#3 and Lib#5, and one was based on Cucurbida maxima trypsin inhibitor I (CMTI-I). CMTI-I is a protein found in squash seeds and is able to withstand the addic and proteolytic conditions of the gut. These proteins each have three disulfide bridges, making them highly constrained and stable. Members of these protein families have been shown to have outstanding thermal stability (>80° C without loss of activity), outstanding pH stability (no loss of activity on overnight incubation at pH 2 and 37° C or on 1 hour exposure to pH 12 at 37° C), and outstanding stabihty to oxidation The number of potential ammo aαd sequences in each library is given in Tabl 5 below Table 5 Phage display library populations used m the tPA screenmg Library name parental domam Number of members Lιb#l LACI-K1 31,600 Lιb#3 LACI-K1 516,000 Lιb#5 LACI-K1 1,000,000 CMΠ CMTI-I 9,500,000 Total 11,000,000 The total diversity ofthe phage-display libraπes screened agamst tPA in this work is estimated to be around 11 million The Kunitz domam and the CMTI domam could display much greater diversity b varying other parts of their surfaces Two screenmg protocols were used "slow screen" and "quick screen" In a slow screen, phage from each round were amplified m E. coli before the next round In a quick screen, phage recovered from the targd m one round served as the input for the next round without amplification In a quick screen, both the input and recovered number of phage decreased rapidly over several rounds The input level can be kept constant in a slow screen The constant input m a slow screen allows compansons between rounds that can indicate selection or lack thereof, but compansons between rounds of quick screens are difficult to inteφret Quick screening increases the likelihood that phage will be sdeded for bmding rather than other irrelevant properties (e g , infectivity or growth rates) The phage libraπes descπbed were screened for bmding to tPA through four rounds In the first round, the phage libraπes were mixed m separate reactions with tPA agarose beads at pH 7 m phosphate-buffered saline (PBS) Bovine serum albumin (BSA) was added at 0 1 % to reduce non- speαfic bmding Unbound phage were washed off at pH 7, and the bound phage eluted at pH 2 for the first screen only The subsequent three quick screens had a different elution protocol and used pooled outputs ofthe first screen Pool A consisted ofthe combined outputs from the CMTI and Lιb#l libraπes, and Pool B consisted ofthe combined ouφuts ofthe Lιb#3 and Lιb#5 libraπes The bmding of pooled branes was performed at pH 7, however, the first dution to remove bound phage was earned out at pH 5 and a subsequent elution at pH 2 to elute phage that are released in the pH 5 - pH 2 range This was repeated twice more for a total of 4 rounds of selection The phage titers from the final three rounds of screenmg are shown below m Table 6 The ouφut of one round was the input to the next round Table 6 Phage titers pπor to screening and after the last three rounds of screening libraπes agamst tP A Pool A Pool A Pool B Pool B pH 5 elution pH 2 dution pH 5 dution pH 2 elution Pπor to Quick 7 x 10" 7 x lO π 5 x l0 π 5 x lO 11 Screenmg After second round l x lO 8 3 x l0 7 7 x l0 6 3 x l0 6 After third round 2 x l0 5 2 x l0 6 2 x l0 3 7 x l0 3 After fourth round 3 x l0 4 2 x l0 5 150 90 From the phage titers, it is appears that Pool A converged and contams strong bmders, whereas Pool B had neither significant convergence nor strong bmders Forty phage clones were picked from the third round quick screen selectants of each pool for further analysis, 20 from the pH 5 pool and 20 from the pH 2 pool The phage DNA was amplified usmg PCR to determine whether CMTI- or LACI-deπved gene fragments were present CMTI-denved construds were found in 38 out of 40 phage isolates from the quick screen of pool A The remainmg isolates did not yield a PCR produd, indicating a deletion Only 10 of the 40 phage isolates from the quick screen of pool B contained the appropnate construd, another mdication that the search had not succeeded One sign that a particular phage-displayed protem has a high affinity for the targd molecule is that it is found repeatedly From the 18 CMTI-denved phage isolates that released at pH 2, one sequence was found five times, a second, four times, and two ofthe remaining occurred three times The 18 sequences formed a dosely-rdated family of selected molecules, a further sign that the search had successfully converged Table 7 shows the vanabihty ofthe observed sequences as a function ofthe permitted vanabihty and the selection pH The CMΗ library was constructed by mtroduαng combmatonal sequence diversity mto codons speαfymg a surface-exposed loop formed between cysteines 3 and 10 of the parental CMTI protein The cysteines were not vaned because they form an important part of the structure Table 7: Construction of CMΗ Library by Vanegat on of CMTI-I framework sequence CMTI-I = RVCPR ILMEC KKDSD CLAEC VCLEH GYCG (SEQ ID NO : 1) (see, Dung, L-N. et al., Int'lJ. Peptide Protem Res., 34: 492-497 (1989) ammo adds F Y S G A R LSWP C FSYC encoded (SEQ ID NO. 2) QRM LPHR TKVA ΓΓNV EG ADG codon position -5 -4 -3 -2 -1 1 2 3 4 codons TTC TAT TCC GGA GCC CGT NNG TGT NNT (SEQ ID NO: 3) restriction sites i — ACC Π — ι P3 P2 or position amino adds KRTI FSYC FSYC LSWP EKRG C K K D encoded LPHR LPHR QRM ΓΓNV ΓΓNV TKVA ADG ADG EG codon position 5 6 7 8 9 10 11 12 13 codons ANA NNT NNT NNG RRG TGT AAG AAG GAT restπction sites Pl Pl' P2' P3' or position amino adds S D C L A E C V C encoded codon position 14 15 16 17 18 19 20 21 22 codons TCT GAT T GC TTA GC A GAA TGC GTT TGC restriction sites 1 Fτnl , 1 or position Table 7 (continued): Construction of CMTI Library by Variegation of CMTI-I -, "'' , " " - ammo adds L E H G Y C G A G encoded codon position 23 24 25 26 27 28 29 100 101 codons CTC GAG CAT GGT TAT TGT GGC GCC GGT restriction sites I — Xhol ' " Kasl 1 or position ' ammo adds P S Y I E G R I V encoded codon position 102 103 104 105 106 107 108 109 110 codons CCT TCA TAC ATT GAA GGT CGT ATT GTC restriction sites or position amino adds G S A A E ...rest of mature HI encoded codon position 111 112 113 201 202 codons GGT AGC GCC GCT GAA ..rest of coding sequence for HI restriction sites or position This gives 9 13 x IO 6 prote sequent zes and \( 3.8 x lO 6 5NA seqi βnces Table 7 shows the DNA sequence ofthe CMTI library. Residues F. s and Y^ correspond to residues 14 and 15 in the signal sequence of M13mpl8 from which the reαpient phage was engineered Cleavage by Signal Peptidase I (SP-I) is assumed to occur between A. t and Ri . Residues designated 100-113 make up a linker between the CMTI vanants and mature HI, which begins with residue A 20 ι The ammo add sequence YioJEGRTV should allow spedfic deavage ofthe linker with bovine Factor X, between Rioβ and Iirø. The Ml 3-related phage m which this library was construded carries an ampidllm-resistance gene (Ap κ ) so that cells infeded by library phage become Ap resistant. At each vanable ammo aαd position, the wild-type ammo aαd residue is shown underscored The ammo add sequence shown in Table 7 is designated SEQ ID NO 2; the nucleotide sequence shown in Table 7 is designated SEQ ID NO 3 In those sequences, ammo aαds 1-29 of SEQ ID NO 3 represent the CMTI-denved polypeptide analogues, nucleotides 16-102 of SEQ ID NO 2 code for t CMTT-deπved polypeptide The isolates obtained from the pH 5 selection procedure exhibited greater sequence diversity than did the pH 2 selectants (Table 8) Despite the greater sequence vanabihty, pH 5 selectants compπsed a family of closely-rdated protem sequences Formmg all combinations ofthe ammo-aα types observed at each position gives only 13,400 (=2x4x3x4x7x5x4) which is 0 15% ofthe oπguial population In the 20 sequences ddermined, there were four sequences that occurred more than onc suggesting that the adual diversity is less than 13 ,400 Although the family of pH 5-sdected sequences is clearly related to the pH 2-seleded family, there was only one example of sequence identity between the two sequence populations Table 8 Reduction in vanabihty at lositions in CMTI upon selection for bmding to tPA Position 2 3 4 5 6 7 8 9 10 Permitted Vanabihty 13 C 15 4 15 15 13 4 C Vanabihty ofpH 5 Selectants 2 C 4 3 4 7 5 4 C Vanabihty ofpH 2 Selectants 1 c 1 2 3 3 1 2 C At positions 6 and 7, most (12 of 15) allowed ammo aαd types were rejeded in the pH 2 selectants From the sdected sequences, it is not clear whether the sdected ammo aαds at positions and 7 contπbuted to bmdmg or merely represent the elimmation of unacceptable possibilities at these positions The powerful convergence ofthe selection process is particularly evident for the pH 2 selectants at positions 2, 4 and 8, where, although many ammo aαd types could occur, only one ammo aαd type was found This is a strong mdication that this speαfic ammo aαd is cπtical to bmdmg At each of these positions, the uniquely sdeded type ofthe pH 2 population was also the most common type at that position in the pH 5 population Allowing all observed ammo aαd types a each position ofthe pH 2 pool gives only 36 sequences, 0 0004% ofthe initial population That several sequences appeared more than once suggests the number of different sequences present m the pH 2 pool is not larger than 36 Table 9 shows the ammo aαd sequences ofthe vanegated region (ammo aαd positions 1-12) for the 38 sequenced analogues of CMTI-I The appearance of methionine residues at a position not designed to be vaned (position 11) indicates a DNA synthesis error m formation ofthe library In Table 9, analogues having the sequences designated 101-120 were obtained by elution at pH 5 and analogues having the sequences 221-238 were obtained by fractionation at pH 2. The speαfiαty of phage-bound ligand candidates was tested by ddermining their affinity for other immobilized proteins The phage-bound proteins showed no affinity for the rdated human serum proteases plasmin and thrombin bound to beads (data not shown) Additionally, expenments were performed on the phage isolates to ddermme relative affinity for and release charadeπsϋcs fro immobilized tPA In the case ofthe pH 5-releasmg phage isolates, the majoπty ofthe phage are released at pH 5 and an order of magnitude fewer are released by further dropping the pH to 2 This indicates suitable affinity ligands with a relatively dean release upon lowering the pH to 5 In the case ofthe pH 2 rdeasmg isolates, only isolate #232 gave a truly sdective bmdmg at pH 5 and then release at p 2 Ligand Synthesis and Immobilization Next, free CMTI-denvative polypeptides were synthesized usmg the sequence information determmed from the DNA ofthe phage isolates Although the CMTI denvatives (analogues) could have been readily chemically synthesized, it was deαded to express the polypeptides m yeast One of the pH 5-rdeasmg isolates, #109 (Table 9, ref SEQ ID NO 12), and one ofthe pH 2-releasmg isolates, #232 (Table 9, ref SEQ ID NO 35), were sdected for expression Pichia pastons At positions 4 and 8, isolate #109 has the same ammo aαd types as seen in the pH 2 sdectants, at position 2, isolate #109 differs from the pH 2 sdectants The appropnate gene constructs were synthesized and mserted mto the Pichia pastons expression system (Vedvick et al , 1991 and Wagner et al , 1992) to create production strams Five- liter fermentations of each strain resulted high-levd expression It was estimated that the protems were secreted mto the fermentation broth in excess of 1 g L The crude fermentation suspensions were claπfied by centnfugation, and 0 2 μ microfiltration steps The 0 2 μ filtrates were punfied by ultrafiltration usmg PTTK cassettes with a 30 kDa NMWL cutoff The ligands were punfied from the ultrafiltration filtrates by cation exchange chromatography with Macro-Prep High S cation exchange support (BioRad), followed by two reversed-phase separations The reversed-phase separations used a lmear gradient starting with water containing 0 1 % TFA and havmg mcreasmg acdonitnle (containing 0 1 %TFA) which was increased to 50% at 90 mmutes The resulting protem was more than 95% pure as measured by PDA spectral analysis on a 5 μm reversed phase column The ligand candidate was immobilized on a bis-acrylamide/azlactone copolymer support with immobilized diaminopropylamine (Emphaze Ultralink™, Pierce Chemical Co ) accordmg to the manufadurer's instructions About 30 mg of CMΗ analogue #109 were coupled to 1 mL ofthe activated chromatography support Column Testing A Waters AP Minicolumn, 5 0 cm x 0 5 cm ID nominal dimensions was mo fied by the addition of a second flow adaptor which allowed the column length to be reduced to 2 5 cm Th s column was packed with #109-Emphaze Ultralink™ beads usmg the recommended protocol and washed using a seπes of mcreasmg NaCI concentration washes at pH 7 concluding with a 1 M wash In a first test ofthe #109 affinity column tissue-type plasminogen activator obtained from CalBiochem was made up to manufacturer's specifications to provide a lmg/mL solution of tPA Coagulation Standard (Coagulation Control Level 1 from Sigma Diagnostics, Catalog #C-7916) lyophilized human plasma, was reconstituted accordmg to the manufacturer's instructions, then ώluted 1 OX and thetPA added This sample was loaded onto the column and eluted m the presence of 1 M NaCI m all buffers, which was suffiαent to suppress non-speαfic protem bmdmg to the column and to permit the pH-controlled binding and release of the tPA There were two distind peaks The first contained the plasma protems (which were not retained on the column), the second, obtained after lowering the pH, contained the tPA, without any contaminating plasma components The results were confirmed by silver stamed gel (not shown) 90% ofthe tP A produd was recovered A second affinity column usmg the #109 CMTI analogue was prepared usmg EAH Sepharose 4B™ agarose beads (Pharmaαa, Upsala SE) as the chromatography support The separation was performed on an HPLC system manufactured by Waters Inc (Milford, MA) The system compπsed a Modd 718 Automjedor, a Model 600 solvent delivery system with pumpheads capable of delivering 20 mL/minute, and a Modd 996 photodiode array ddector All ofthe equipment was installed accordmg to manufadurer's specifications The system was controlled by a Pentium 133 IBM-compatible computer supplied by Dell Coφ The computer was furnished with a 1 gigabyte hard dnve, 16 megabytes of RAM, and a color monitor, onto which the Millenium software supplied by Waters Inc was loaded Spectral data m the range 200 nm to 300 nm were collected with 1 2 nm resolution Figures 1 , 2, 3, and 4 were collected at 280 nm The mobile phases for the chromatographic work were Buffer A and Buffer B Buffer A consisted of 25mM potassium phosphate, 50mM arginine, and 125mM NaCI, buffered to pH 7 with potassium hydroxide Buffer B consisted of 50mM potassium phosphate and 150 mM NaCI, buffered to pH 3 with phosphoπc aαd In all cases, samples were injected m 100% Buffer A, followed by washing with 100% Buffer A from t=0 to t=2 mm From t=2 mm to t=8 mm , dution was with 100% Buffer B After t=8 mm , elution was with 100% Buffer A The gradient delay volume ofthe HPLC system was approximately 4 mL and the flow rate was 0 5 mL/min tPA from another commercial source was made up to manufadurer's speαfications to provide a lmg/mL solution Coagulation Standard (Coagulation Control Level 1 from Sigma Diagnostics, Catalog #C-7916), lyophilized human plasma, was reconstituted according to the manufacturer's instructions. This solution was diluted 10:1 to obtain a solution having roughly 10 times the absorbance ofthe tPA solution. In the test shown in Figure 1, a sample of pure tPA (25 μL of 1 mg/mL tPA) was run over the #109 CMTI derivative-containing column and duted as described above. The chromatogram shows shaφ elution of about 90% ofthe tPA material after about 15 min. In the test shown in Figur 2, a sample of Coagulation Standard (10X dilution) was run over the #109 CMΗ derivative- containing column with elution conditions as described above. Virtually all the material eluted immediately from the column (was not retained). In the test separation shown in Figures 3 and 4, a sample containing tPA added to human plasma standard was loaded onto the column and eluted as described above. As can be seen in Figures 3 and 4, the tPA was retained and the plasma proteins eluted in the void volume. Bound tPA was released at about 15.4 minutes. It was estimated that the tPA was rdeased at about pH 4. The tPA peak was collected and examined using a silver-stained, reducing SDS-polyacrylamide gel, and, when compared with the starting material, was found to be > 95% pure. Example 17 The tPA affinity ligands isolated from the CMTI library were examined further in order to design additional candidate domains that might bind to tPA as wdl. As noted above, almost all o the variegation in amino add positions used in building the CMTI library occurred between two cyteines at positions 3 and 10 of CMTI-I (see Table 3). In the parental CMΗ protein, these cysteines form disulfide bonds with other cysteine residues elswhere in the protein (see SEQ ID NO: 1), however with the successful isolation of affinity ligands from the CMTI library, a secondary library was conceptualized which was based on variegating a truncated 15-amino add segment ofthe isolate #109 (see amino adds 1-15 of SEQ ID NO: 12). If the C 3 and Cio cysteines of these members formed a disulfide bond, then a constrained loop having tPA binding properties might be obtained. Initial studies with the 15-amino add segment derived from affinity gand isolate #109 bound to a chromatographic support indicated that the C 3 - Cι 0 loop formed and that the immobilized loop bound to tPA. The foregoing experiments point to two new families of tPA affinity ligands isolated in accordance with this invention, comprising polypeptides including the sequences: Arg-XpCys-Xi-Xs-Xi-Xs-Xe-X -Cys-Xg-Lys-Asp-Ser-Asp-Cys-Leu-Ala-Glu-Cys-Val-Cys-Leu-Glu- His-Gly-Tyr-Cys-Gly (SEQ ID NO: 42) and Arg-X 1 -Cys-X 2 -X 3 -X 4 -X 3 -X6-Xτ-Cys-X 8 (SEQ ID NO: 43), wherein Xi is Tφ or Leu; X 2 is Pro, Ser, Thr or De; X 3 is Arg, Lys or Thr; X 4 is Ser, Tyr, Thr or Ala; X 5 is Ser, Tyr, Asp, Val, Pro, Ala, His, Asn or Thr; X 6 is Leu, Md, Gin, Arg or Lys; X 7 is Glu, Gly or Arg; and Xg is at least Lys or Md. Since the presence of Md residues at position 11 in the sequence was not planned but turned out to be favored for binding to t A, it is likdy that other amino adds, for instance other non-polar amino adds such as Ala, Val, Leu, De, Phe, Pro or Tφ substituted at position 11 will provide additional tPA-binding analogues. Example 18 This example illustrates the isolation of affinity ligands useful in the separation of a targd humanized monoclonal antibody from a feed stream of clarified goat's milk. The target purified humanized monoclonal antibody of IgG isotype and unknown spedfidty was supplied by the producer and stored in 500 μg aliquots in 50 μL PBS at -70° C. Polyclonal goat IgG was purchased from Cappel. Varying amounts (i.e., 1 μg, 500 μg, 200 μg, 100 μg, 50 μg and 0 μg) ofthe humanized monoclonal antibody (hMAb), made to 100 μL with PBS, were each added to three wells of an Immulon 2 plate and incubated ovemight at 4° C. The wells were washed three times with 200 μL PBS with 0.1% non-ionic detergent (Tween 20), then blocked with 100 μL PBS with 1% BSA per well for 1 hour at 4° C. Plates were tested using goat anti-human IgG-HRP (CalBiochem) developed with OPD substrate solution (SigmaFast #9187, Sigma Chemical Co.), quenched with 4M HzSO*, and A 9 o read in a Bio-Tek microtiter plate reader. For the screen, polyclonal goat IgG was used as a competitive binder. The screening was carried out using three phage display libraries: CMTI, TN-6/I and TN-10 V. The construction of CMTI is given above (Table 7). The peptide construction of TN-6 I and TN-10/V is given bdow (Table 10 and Table 11 , respectively). In the following tables, the encoded amino adds ofthe variegated, phage-displayed polypeptide domain are shown. DNA encoding the polypeptides was inserted into Ml 3 gene iii in as similar manner as described above with resped to the CMTT library. Table 10 Construction of TN-6 I Library by Variegation of Microprotein-6 framework sequence. AEGX X C X ! X 2 X 2 XιC XiSY { SEQ ID NO : 44 ) Xi = ACDFGHILNPRSTVY X 2 = AEGKLMPQRSTVW ammo adds A E G ACDF C ACDF AEGK AEGK ACDF encoded (SEQ ID NO: 44) GFUL GFUL LMP LMP GFUL NPRS NPRS QRST QRST NPRS TVY TVY VW VW TVY amino add 1 2 3 4 5 6 7 8 9 position ; ammo adds C ACDF S Y encoded GFDL NPRS TVY amino add 10 11 12 13 position This library design gives 8.55 x 10 protein sequences and 17 x 10 DNA sequences. This library design gives 8.2 x 10" protein sequences and 1.1 x IO 9 DNA sequences. Four rounds of screening were completed. The binding screen was performed at neutral pH and approximately 80 mM NaCI with 1% polyclonal goat IgG in the binding solution as a competitive binder. Elution was performed with a pH switch. The binding conditions for all rounds were: incubation at room temperature (RT) for 2 hours in l/2x PBS (V_ concentration PBS), 0.1% BSA, 1% goat IgG. The wash and elution conditions for each round are summarized in Table 12. Table 12: Washes and Elution Conditions Step Screening Round # 1 2 3 4 Wash: 15 times rapidly at RT with yes yes yes yes l/2x PBS, 0.1% Tween 20 Wash: 10 min. at RT with no no yes yes l/2x PBS, 0.1% BSA Elute: 2 times 5 min. at RT with no yes yes yes 50 mM dtrate, 150 mM NaCI, 0.1% BSA, pH 5 Elute: 2 times 5 min. at RT with yes yes yes yes 50 mM dtrate, 150 mM NaCI, 0.1% BSA, pH 2 After each round, the phage eluted were counted and then amplified by transduction. pH 5 and pH 2 eluates were amplified separately after Round 2 and kept separate during successive screens, so that the pH 2 eluates selected candidates that did not release at pH 5. Table 13 below shows the convergence ofthe screen over the four rounds for TN-6/Ϊ at pH 2, for TN-10 V at pH 2, for CMTI at pH 2, and for CMTI at pH 5. From each ofthe four convergent screens, approximately 12 phage isolates were selected for preliminary sequencing. In all but the TN-10/V isolates, a significant homology amongst the selectants was seen. For example, the twelve CMTI pH5 isolates sdeded for sequencing collapsed to 3 DNA sequences and 2 amino add sequences. From the sequenced sdectants, 16 candidates were selected for charaderization ofthe relative binding affinity, spedfidty and pH-rdease diaraderistics of the phage-bound proteins for the targd hMAb, using pH 2 as the release test. The test mvolved immobilization ofthe targd hMAb, polyclonal goat IgG, and BSA to microtiter plates, followed by ddection of relative binding ofthe phage using a biotinylated sheep anti-M13 antibody ELISA ltit from 5 Prime → 3 Prime, Inc. (Boulder, Colorado US). The phage isolates tested had the following designations : • from TN-6/I, pH 2 release: T41 , T42, T48, T49, T52 • from TN-10 V, pH 2 release: T61, T64, T66, T70, T72, T74, T75 • from CMTI, pH 5 release: C21, C22, C23 • from CMTI, pH 2 rdease: C3 Individual isolate phage were tested for binding to immobilized hMAb, goat IgG, or BSA including parental phage controls for hMAb, goat IgG, BSA and anti-M13 binding. Binding to hMAb, goat IgG and BSA was carried out in the presence of 0.5% goat IgG in l/2x PBS and 0.1% BSA solution and under two pH conditions, pH 7 and pH 2. The biotinylated ddection anti-M13 antibody was visualized using streptavidin-conjugated alkaline phosphatase. The microtiter plates were read at 405 nm in a Bio-Tek microtiter plate reader. The results are shown in Figures 5, 6 and 7. In these figures, "CMTI", "MAEX" and "MKTN" are control phage exhibiting the "parental" candidate binding domain, i.e., the native CMTI-I sequence (SEQ ID NO: 1) for the CMTI control, and randomly sdeded non-binding members ofthe TN-6 I (MAEX) and TN-10 V (MKTN) libraries, respectively. A potential affinity ligand was identified by having ( 1 ) significantly higher binding affinity for the targd hMAb than the control phage, (2) a significantly higher binding affinity for the target under binding conditions (pH 7) than at elution conditions (pH 2), and (3) little or no binding to goat IgG. From the ELISA results, seven DNA isolates (encoding six binding polypeptides) were suitable for use as affinity ligands: C3, C21, C22, C23 (same amino add sequence as C22), T42, T49 and T52. The reduction in variability among the CMTI-I pH5 isolates and among the TN-6A pH 2 isolates is shown in Tables 14 and 15 below. Table 14: Reduction in variability at positions in CMTI-I isolates C21 , C22, C23, selected for binding to immobilized hMAB Position: 2 3 4 5 6 7 8 9 10 Permitted Variability 13 C 15 4 15 15 13 4 C Variability of pH 5 Selectants 2 C 2 1 1 1 2 1 C Table 15: Reduction in variability at positions in TN-6/I isolates T42, T49, T52, selected for binding to immobilized hMAB Position: 4 5 6 7 8 9 10 11 Permitted Variability 15 C 15 13 13 15 C 15 Variability of pH 2 Selectants 1 C 1 1 3 1 C 2 The unity of amino acid types at varied amino acid positions 5, 6, 7 and 9 in the CMTI analogues and at varied positions 4, 6, 7, 9 and 10 in the TN-6/I analogues is an indication of a powerful convergence on a consensus peptide from these libraries. Amplification of these analog- bearing phage, isolation and sequencing of the encoded insert DNA will reveal specific affinity ligand candidates for purification of the hMAb. Example 19 This example illustrates the isolation of affinity ligands useful in separating natural urokinase. High molecular wdght urokinase derived from human urine was immobilized on Reacti- Gel™ agarose beads (Pierce Chemical Co.) at 4° C essentially as described in Markland et al. (1996). The screening was caπied out using four phage display libraries: CMTI, TN-6/I, TN- 10/VIIIa, and LACI/F . The construction of CMΗ is given above (Table 7). The construction of TN- 6/1 is given above (Table 10). The peptide construction of TN-10 VHIa and LACI/F are sd forth bdow (Tables 16 and 17, respectively). In the following tables, the encoded amino adds ofthe variegated, phage-displayed polypeptide domain are shown. DNA encoding the polypeptides was inserted into Gene HI as described with respect to the CMTI library above (Example 16; Table 7 supra). Table 16. Construction of TN-10/VHIa Library by Vanegation of Microprotein-10 framework sequence: AEGAS CX 1 X 2 X 1 P X 1 X 3 X 4 X 5 C AGP (SEQ ID NO : 46 ) Xi = ACDEFGHIKLMNPQRSTVWY X 2 = AGPR X 3 = AEG PQRV X 4 = AEGKLMPQRSTVW X 5 = AGPRST ammo adds A E G A S C ACD AGP ACD encoded (SEQ ID NO. 46) EFGH R EFG IKLM ΠKL NPQ MNP RSTV QRS WY TVW Y amino add 1 2 3 4 5 6 7 8 9 position ammo adds P ACDE AEGL AEGK AGPR C A G P encoded FGHI PQRV LMP STW KLM QRST NPQR VW STV WY ammo add 10 11 12 13 14 15 16 17 18 position This library design gives 2.3 x 10 7 protein sequences and 1.3 x IO 8 DNA sequences This library design gives 3 12 x 10 proton sequences. Pπor to screenmg, the immobilized urokinase agarose beads were tested with clonally pure phage preps displaying each ofthe parental bindmg domam polypeptides, to ensure that under the screening conditions there was a low background level of phage recovered. For each ofthe parental polypeptides, the fraction of input phage recovered was less than or equeal to 1 x 10 * ', which was an acceptably low background levd of binding. Four rounds of screening were performed Each round consisted of a bindmg step, a wash procedure, and one or more elution steps. The bindmg conditions for all rounds were mcubation at 4° C for 20 hours in PBS, 0.1% BSA, 0.01% Tween 80 The wash and elution conditions for each round are summanzed m Table 18. Table 18: Washes and Elution Conditions Step Screening Round # 1 2 3 4 Wash: 5 times/5 min. at 4° C with yes no no no PBS, 0.1% BSA, 0.01% Tween 80 Wash: 5 times/5 min. at 4° C with no yes yes yes PBS, 0.05% Tween 80 Elute: 10 min. at RT with no yes yes yes 50 mM dtrate, 150 mM NaCI, 0.1% BSA, pH 5 Elute: 2 times/10 min. at RT with yes no no no 50 mM dtrate, 150 mM NaCI, 0.1% BSA, pH 2 Elute: 10 min. at RT with no yes yes yes 50 mM citrate, 150 mM NaCI, 0.1% BSA, pH 5 After each round, the phage eluted were counted and then amplified by transduction. pH 5 and pH 2 duates were amplified separately after Round 2 and kept separate during successive screens, so that the pH 2 duates selected candidates that still had a comparatively high affinity at pH 5. Table 18 below shows the convergence ofthe screen over the four rounds for each ofthe libraries. Table 18: Ave. fraction of input phage contained in each round of screening against urokinase Round # TN-6 TN-6 TN-10 TN-10 LACI/F LACI/F CMTI CMTI pH 5 pH 2 pH 5 pH 2 pH 5 pH 2 pH 5 pH 2 elution dution elution dution dution dution dution dution 1 N/A 3 10-* N/A lxlO '5 N/A 4 10-* N/A 6 10^ 2 lxlO "3 9x10-* 7x10^ 5x10 " * 4x10^ 3x10-* 4x10^ 5 10^ 3 1x10" lxlO '5 4x10-' 9x10^ lxlO "5 7 1 -* 2x l0 "5 2xl0 '5 4 3xl0 "3 1x10" 4xl0- 3 2xl0 "5 2xl0 "3 2x l0- 5 5xl0 "5 3xl0- 5 A convergent screen is one in which the fraction of input increases over successive rounds, indicating that the diversity ofthe phage library is being reduced. This is a desired result, because it indicates that a ligand candidate for the immobilized targd molecule is potentially being selected from the population The table above shows some convergence between Rounds 2 and 4 for all of the libranes, with the most pronounced results for the TN-6 I (pH 2) and the CMTI (pH 5) dutions From each of four convergent screens, approximately 12 phage isolates were selected for sequencing all of the sequenced isolates, some homology amongst the selectants was seen The greatest homology was seen m the TN-6/1 and CMTI sequences, which is consistent with the findmg that these libranes showed the greatest ennchment duπng screening For example, rune ofthe TN-6/I isolates were found to have the same DNA and ammo aαd sequence Candidates were selected for characteπzation ofthe relative bmdmg affinity, speαfiαty and pH-release diaractenstics ofthe phage-bound protems for the target, usmg pH 2 as the release test The test mvolved immobilization of urokinase and BSA on Immulon 2 microtiter plates, with ddection of relative bmdmg ofthe phage usmg a biotinylated sheep anti -Ml 3 antibody ELISA kit from 5 Pnme → 3 Pπme, Inc (Boulder, Colorado US) The phage isolates tested had the following designations • from TN-6/I, pH 2 release TU33, TU34, TU36, TU37, TU39, TU42 • from TN-10 VTfla, pH 2 release TU50, TU51, TU53, TU54, TU55, TU56, TU57, TU58, TU60, TU62, TU63 • from CMΗ, pH 5 release CU22, CU25, CU27, CU28, CU29, CU31 , CU32 • from LACI F, pH 2 release LU2, LU4, LU5, LU9, LU10, LU12 Individual isolate phage were tested for bmdmg to immobilized urokinase or BSA Because urokinase has a high sequence homology to tPA, the candidates also were tested for tPA bmdmg The results are shown m Figures 8, 9, 10 and 11 A potential affinity ligand was identified by havmg (1) significantly higher bmdmg affinity for the taigd urokinase than the control phage, (2) a significantly higher bmdmg affinity for the targd under bmdmg conditions (pH 7) than at elution conditions (pH 2), and (3) little or no bmdmg to BSA From the ELISA results, seven DNA isolates (four from TN-6/I and three from TN- 10/VTfla) were suitable for use as affinity ligands TU33, TU36, TU39, TU42, TU53, TU56 and TU58 All seven isolates exhibited high affinity at pH 7 for urokinase and reduced affinity at pH 2 No ligand candidates were discovered from the CMTI or LACI/F libraπes that bound with higher affinity to urokinase than the parental display phage The reduction vanabihty among the CMTI-I pH5 isolates and amonge the TN-6/I pH 2 isolates is shown m Tables 21 and 22 below Table 21: Reduction in variability at positions in TN-6/I isolates TU33, TU36, TU39 and TU42 selected for binding to immobilized urokinase Position: 4 5 6 7 8 9 10 11 Permitted Variability 15 C 15 13 13 15 c 15 Variability of pH 2 Selectants 3 C 2 1 1 2 c 1 Table 22: Reduction in variability at positions in TN-10/VIIIa TU53, TU56 and TU58 selected for binding to immobilized urokinase Position: 6 7 8 9 10 11 12 13 14 15 Permitted Variability C 20 4 20 P 20 8 13 7 C Variability of pH 2 Selectants C 3 2 3 P 2 3 2 1 C The unity of amino add types at varied amino add positions 7, 8 and 11 in the TN-6/I analogues and the shaφ reduction in amino add variability at varied positions 7, 9, 11 and 14 in the TN-lO Vma analogues is an indication of a powerful convergence on a consensus peptide from these libraries. Amplification of these analog-bearing phage, isolation and sequencing ofthe encoded insert DNA will reveal spedfic affinity ligand candidates for urokinase purification. Following the foregoing description, the charaderistic important for the separation of a targd from any feed stream can be engineered into the binding domains of a designed library, so that the method of this invention invariably leads to several affinity ligand candidates suitable for separation ofthe targd under desirable conditions of binding and rdease. High yidd ofthe targd without inactivation or disruption ofthe produd, with high purity, with the dimination of even closedly related impurities, at acceptable cost and with re-usable or recyclable materials all can be achieved according to the present invention. Additional embodiments ofthe invention and alternative methods adapted to a particular target will be evident from studying the foregoing description. All such embodiments and alternatives are intended to be within the scope of this invention, as defined by the claims that follow. References Boschetti, E., J. Chromatography, A 658: 207-236 (1994). Ladner, R. C, "Constrained peptides as binding entities," Trends in Biotechnology, 13(10): 426-430 (1995). Markland, W., Roberts, B.L., Ladner, RC, "Sdection for Protease Inhibitors Using Bacteriophage Display," Methods in Enzymology, 267: 28-51 (1996). Narayanan, S.R., "Preparative affinity diromatography of protdns," J. Chrom. A, 658: 237-258 (1994). Knight, P., Bio/Technology, 8: 200 (1990). Vedvick, T., Buckholtz, R.G., Engd, M., Urcam, M., Kinney, S., Provow, S., Siegd, R.S., and Thill, G.P., "High level secretion of biologically active aprotinin from the yeast Pichia pastoris", J. Industrial Microbiol., 7: 197-202 (1991). Wagner, S.L., Siegel, R.S., Vedvick, T.S., Raschke, W.C., and Van Nostrand, W.E., "High level expression, purification, and charaderization ofthe Kunitz-type protease inhibitor domain of protease Nexin-2/amyloid β-protein precursor," Biochem. Biphys. Res. Comm., 186: 1138-1145 (1992). SEQUENCE LISTING (1)GENERAL INFORMATION: (i)APPLICANT:DYAX CORP (i)APPLICANT AND INVENTOR:MACLENNAN, ohn M (i)APPLICANT AND INVENTOR:LADNER,Robert C (ii)TITLE OF INVENTIONEngineering Affinity Ligands for Macromolecules (iii) UMBER OF SEQUENCES:48 (iv) CORRESPONDENCE ADDRESS: (A)ADDRESSEE:Banner & itcoff, Ltd. (B)STREET:75 State Street (C)CITY:Boston (D)STATEMassachusetts (E) COUNTRY:United States of America (F)ZIP:02109 (v) COMPUTER READABLE FORM: (A)MEDIUM TYPE:Diskette, 3.50 inch, 1.44Mb storage (B)COMPUTER:IBM PC Compatible (C) OPERATING SYSTEM:MS-DOS (D)SOFTWAREMicrosoft Word 6.0 (vi) CURRENT APPLICATION DATA: (A)APPLICATION NUMBER: (B)FILING DATE:20-March-1997 (C)CLASSIFICATION: (vii) PRIOR APPLICATION DATA: (A)APPLICATION NUMBER:08/619,885 (B) FILING DATE:20-March-1996 (viii)ATTORNEY/AGENT INFORMATION: (A)NAME:YANKWICH, Leon R (B)REGISTRATION NUMBER:30,237 (C)REFERENCE/DOCKET NUMBER:DYX-1 PCT (ix)TELECOMMUNICATION INFORMATION: (A)TELEPHONE:617-3 5-9100 (B)TELEFAX:617-345-9111 (2) INFORMATION FOR SEQ ID NO:l: (i)SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:l Arg Val Cys Pro Arg He Leu Met Glu Cys Lys Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:2: (i)SEQUENCE CHARACTERISTICS (A) LENGTH:50 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: Phe Tyr Ser Gly Ala Arg Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Cys Lys -5 1 5 10 Lys Asp Ser Asp Cys Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr 15 20 25 Cys Gly Ala Gly Pro Ser Tyr He Glu Gly Arg He Val Gly Ser Ala 30 35 40 Ala Glu 45 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:150 bases (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE:other nucleic acid (A) DESCRIPTION:synthetic DNA fragment (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 3: TTCTATTCCG GAGCCCGTNN GTGTNNTANA NNTNNTNNGR RGTGTAAGAA 50 GGATTCTGAT TGCTTAGCAG AATGCGTTTG CCTCGAGCAT GGTTATTGTG 100 GCGCCGGTCC TTCATACATT GAAGGTCGTA TTGTCGGTAG CGCCGCTGAA 150 (2) INFORMATION FOR SEQ ID NO:4: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: Arg Trp Cys Pro Lys Thr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5 : Arg Leu Cys Pro Lys Thr Tyr Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:6: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6: Arg Trp Cys Ser Thr Tyr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:7: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7: Arg Trp Cys Ser Thr Tyr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:8: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8: Arg Leu Cys Pro Lys Thr Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:9: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9: Arg Trp Cys Ser Thr Tyr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 10: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 10: Arg Leu Cys Pro Lys Thr Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:11: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 11: Arg Trp Cys Ser Lys Ser Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:12: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 12: Arg Leu Cys Pro Lys Thr Asp Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:13: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13: Arg Trp Cys Pro Lys Ser Ser Met Gly Cys Lys Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:14: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14: Arg Trp Cys Pro Arg Thr Val Gin Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:15: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15: Arg Trp Cys Pro Thr Ala Pro Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:16: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16: Arg Leu Cys Pro Lys Thr Asp Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:17: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 17: Arg Trp Cys Pro Lys Ser Ala Leu Asp Cys Lys Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 18: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18: Arg Trp Cys Thr Lys Thr Ser Arg Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:19: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:19: Arg Trp Cys He Arg Thr Asp Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:20: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:20: Arg Trp Cys Pro Lys Thr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:21: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO.-21: Arg Trp Cys Pro Arg Thr Val Arg Arg Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:22: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:22: Arg Trp Cys Pro Lys Thr His Lys Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:23: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:23: Arg Trp Cys Pro Lys Thr Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:24: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:24: Arg Trp Cys Pro Lys Ser Thr Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:25: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:25: Arg Trp Cys Pro Lys Ser Thr Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:26: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:26: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:27: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:27: Arg Trp Cys Pro Arg Ser Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:28: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:28: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:29: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:29: Arg Trp Cys Pro Arg Ser Asn Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 30: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:30: Arg Trp Cys Pro Arg Ser Asn Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:31: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:31: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 32: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 32: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 33: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:33: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO: 34: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:34 Arg Trp Cys Pro Arg Ser Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:35: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:35: Arg Trp Cys Pro Lys Thr Ser Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:36: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO.-36: Arg Trp Cys Pro Lys Tyr Thr Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:37: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:37: Arg Trp Cys Pro Arg Ser Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:38: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:38: Arg Trp Cys Pro Lys Ser Thr Leu Gly Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:39: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:39: Arg Trp Cys Pro Arg Ser Ser Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:40: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:40: Arg Trp Cys Pro Arg Ser Asn Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 (2) INFORMATION FOR SEQ ID NO:41: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:29 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:41: Arg Trp Cys Pro Arg Ser Asn Leu Glu Cys Met Lys Asp Ser Asp Cys 1 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 2) INFORMATION FOR SEQ ID NO:42: (i) SEQUENCE CHARACTERISTICS (A) LENGTH: 9 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:42: Arg Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Cys Xaa Lys Asp Ser Asp Cys 5 10 15 Leu Ala Glu Cys Val Cys Leu Glu His Gly Tyr Cys Gly 20 25 2) INFORMATION FOR SEQ ID NO:43: (i) SEQUENCE CHARACTERISTICS (A) LENGTH: 11 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:43: Arg Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Cys Xaa 5 10 (2) INFORMATION FOR SEQ ID NO.-44: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:13 amino acides (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:44: Ala Glu Gly Xaa Cys Xaa Xaa Xaa Xaa Cys Xaa Ser Tyr 1 5 10 (2) INFORMATION FOR SEQ ID NO: 5: (i) SEQUENCE CHARACTERISTICS (A) LENGTH: 18 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 5: Ala Glu Gly Ala Xaa Cys Xaa Xaa Xaa Xaa Xaa Xaa Xaa Xaa Cys Xaa 1 5 10 15 Gly Pro (2) INFORMATION FOR SEQ ID NO:46: (i) SEQUENCE CHARACTERISTICS (A) LENGTH: 18 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:46: Ala Glu Gly Ala Ser Cys Xaa Xaa Xaa Pro Xaa Xaa Xaa Xaa Cys Ala 1 5 10 15 Gly Pro 18 (2) INFORMATION FOR SEQ ID NO:47: (i) SEQUENCE CHARACTERISTICS (A) LENGTH: 58 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:47: Met His Ser Phe Cys Ala Phe Lys Ala Asp Asp Gly Pro Cys Lys Ala 1 5 10 15 He Met Lys Arg Phe Phe Phe Asn He Phe Thr Arg Gin Cys Glu Glu 20 25 30 Phe He Tyr Gly Gly Cys Glu Gly Asn Gin Asn Arg Phe Glu Ser Leu 35 40 45 Glu Glu Cys Lys Lys Met Cys Thr Arg Asp 50 55 (2) INFORMATION FOR SEQ ID NO:48: (i) SEQUENCE CHARACTERISTICS (A) LENGTH:58 amino acids (B) TYPE: amino acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:48: Met His Ser Phe Cys Ala Phe Lys Ala Asp Asp Gly Xaa Cys Lys Xaa 1 5 10 15 Xaa Xaa Xaa Arg Phe Phe Phe Asn He Phe Thr Arg Gin Cys Glu Glu 20 25 30 Phe He Tyr Gly Gly Cys Glu Gly Asn Gin Asn Arg Phe Glu Ser Leu 35 40 45 Glu Glu Cys Lys Lys Met Cys Thr Arg Asp 50 55

Claims: CLAIMS 1 A method for isolating an affinity ligand suitable for separating a targd molecule from a solution containmg it, the method compnsing (a) sdecting, with respect to the targd molecule, a first solution condition at which it is desired that an affinity ligand will bmd to said targd molecule, (b) selecting, with resped to the targd molecule, a second solution condition at which it is desired that the affimty ligand will not bmd to said targd molecule and wherem said second solution condition is different from said first solution condition, (c) providing a library of analogues of a candidate bmding domam, wherem each analogue differs from said candidate bmdmg domam by vanation ofthe ammo aαd sequence at one or more ammo acid positions within the domam, (d) contactmg said library of analogues with a solution containmg said targd molecule at the first solution condition, for sufficient time to permit analogue target bmding complexes to form, (e) removing analogues that do not bmd the targd under the first solution condition, (f) altenng the conditions ofthe contacting step (d) to the second solution condition, and (g) recovenng the analogues rdeased under the second condition, wherem the recovered analogues identify isolated affinity ligands 2 The method of claim 1, wherem, as a preliminary step of said method, a range of stability for the targd molecule the solution containmg it is ascertamed with respect to two or more parameters seleded from temperature, pH, ionic strength, dielectric constant, concentration of solutes, presence or absence of metal ions, and presence or absence of metal chelating agents, thereby defining a stabihty envdope for said targd molecule, wherem said first and second solution conditions are within said stability envelope 3 The method of claim 2, wherem said stability envelope is defined by a range of pH, a salt concentration range, and a concentration range for urea or EDTA 4 The method of claim 1 , wherem the library provided in step (c) is designed by making substitutions at ammo aαd positions within the candidate bmdmg domain that promote hydrogen bonding between a bmdmg domam analogue and said targd molecule under said first solution condition, the substitute ammo aαds being selected from the group consisting of D, E, H, K, N, Q, R, S, T, and Y 5 The method of claim 1, wherem the library provided m step (c) is designed by making substitutions at ammo aαd positions within the candidate bmdmg domam that promote sensitivity of the bmding between the targd and the analogues with respect to changes pH between the first solution condition and the second solution condition, wherem the substituted ammo aαds mclude H, E, D, Y and K at any ofthe vaned ammo aαd positions 6 The method of claim 1, wherem, m step (d), said library includes analogues which differ from said candidate bmding domam by including additional ammo aαds inserted at positions so as to enhance bmdmg affimty ofthe analogue for the targd molecule at the first solution condition or to enhance reduction of bmdmg affimty at the second solution condition 7 The method of claim 1, wherem analogues of said candidate bmding domam are prepared so as to alter their affimty for the target based on changes m pH, salt concentration, urea concentration or EDTA concentration, and said library of analogues is produced so as to mclude analogues differing from the candidate bmding domam by the substitution of ammo aαds selected from the group consisting of I, M, T, N, K, S, R, V, A, D, E, G, Y, H, Q at one or more ammo aαd positions within the domam 8 The method of claim 7, wherem the library of analogues is produced by employing a vanegation DNA codon that encodes a sd of ammo aαds, wherem said vanegation DNA codon is sdected from RNS, allowing for the sd of ammo aαds I, M, T, N, K, S, R, V, A, D, E, G, NAT, allowing for the set of ammo aαds Y, H, N, D, NAS, allowing for the sd of ammo aαds Y, H, Q, N, K, D, E, NRT, allowing for the sd of ammo aαds Y, C, H, R, N, S, D, G, and RRS, allowing for the sd of ammo aαds N, K, S, R, D, E, G 9 The method of claim 1 , wherem said targd molecule is stable within a stability envelope defined by a range of pH, a temperature range, and a range of salt concentration, and wherem said library of analogues is produced so as to mclude analogues differing from the candidate bmdmg domam by the substitution, at one or more ammo aαd positions within the domam, of a mo aαds seleded from one ofthe following groups Y, H, Q, N, K, D, E, H, Q, N, K, D, E, and H, Q, N, K, D, E, R, S, Y, T 10 The method of claim 1 , wherem said targd molecule is stable within a stability envdope defined by a range of solubility m an organic solvent, a temperature range, and a range of salt concentration, and wherem said library of analogues is produced so as to mclude analogues differing from the candidate bmding domam by the substitution, at one or more ammo aαd positions within the domain, of ammo aαds seleded to promote a hydrophobic mteraction between said bmdmg domam analogue and said targd molecule under said first solution condition 11 The method of claim 10, wherem said substitutions employ one or more ammo aαds sdected from the group consisting of H, C, F, G, I, L, M, P, V, W, and Y 12 The method of claim 10, wherem said library of analogues is produced by employing a vanegation DNA codon that encodes a sd of ammo aαds, wherem said vanegation DNA codon is selected from NYT, allowing for the sd of amino aαds S, P, T, A, F, L, I, V, NTT, allowing for the sd of ammo aαds F, L, I, V, and TNS, allowing for the sd of ammo aαds F, L, S, Y, C, W 13 The method of claim 1, wherem said targd molecule is stable within a stability envdope partly defined by a range of temperature and wherem said library of analogues is produced so as to mclude analogues differing from the candidate bmdmg domam by the substitution, at one or more ammo aαd positions within the domam, of ammo aαds seleded from the group consisting of Ala, Leu, Phe, Tyr, Tφ, lie, Ala, Gly, Pro, Md and Thr 14 The method of claim 1, wherem the targd molecule is stable under the conditions l) a range ofpH 2 - pH ll, 2) a range of 1 mM - 250 mM NaCI, and 3) a range of4°C - 40°C 15 The method of claim 14, wherem said first solution condition is at pH 7, 150mM NaCI, and 22°C, and said second solution condition is at pH5, 150mM NaCI and 22°C 16 The method of daim 1, wherem the targd molecule is stable under the conditions 1) a range of pH 6 2 - pH 7 8, 2) a range of 100 mM - 5 M NaCI, and 3) a range of 4°C - 40°C 17 The method of daim 16, wherem said first solution condition is at pH 7 2, 3M NaCI, and 22°C, and said second solution condition is at pH7 2, 2M NaCI and 22°C 18 The method of claim 1 , wherem the targd molecule is stable under the conditions 1) a range of pH 6 2 - pH 7 8, 2) a range of lnM - IM NaCI, 3) a range of lnM - IM KJIPCλ,, 2) a range of 0-60% by volume acetonitnle, and 3) a range of 0°C - 40°C 19 The method of claim 18, wherem said first solution condition is at pH 7 2, lOOmM NaCI, lOOmM KzHPO f c and 37°C, and said second solution condition is at pH7 2, 5nM NaCI, 37°C and 50% acdonitπle by volume 20 The method of claim 1 , wherem said targd molecule is stable in the presence of up to 50% by volume of an organic solvent 21 The method of daim 20, wherem said organic solvent is sdeded from the group consisting of ethanol, methanol, and isopropyl alcohol, acdone, methylethylkdone, ethylacetate, acdonitnle, and CH 2 C1 2 22 The method of claim 1, wherem said first and second solution conditions require the presence of solvents seleded from the group consisting of acdomtnle, ethanol, acdone, hexane, methane, isopropyl alcohol, ethyl ether, methylethylkdone, butyl acetate, dichloromethane, chloroform, water and mixtures of such solvents, wherem the concentration of said solvent at said first condition differs from the concentration at said second condition by at least 10% by volume 23 The method of claim 1 , wherem said first and second solution conditions require the presence of solvents selected from the group consisting of acdomtnle, ethanol, acetone, hexane, methane, isopropyl alcohol, ethyl ether, methylethylkdone, butyl acetate, dichloromethane, chloroform, water and mixtures of such solvents, wherem the dielectπc constants ofthe solution at said first condition and the solution at said second condition differ by at least 10% 24 The method of daim 1 , wherem said first and second solution conditions require the presence of solvents selected from the group consisting of acetonitnle, ethanol, acetone, hexane, methane, isopropyl alcohol, ethyl ether, methylethylkdone, butyl acetate, dichloromethane, chloroform, water and mixtures of such solvents, wherem the concentration of said solvent at said first condition differs from the concentration at said second condition by an amount suffiαent to alter the surface tension by at least 10% 25 The method of claim 1 , wherem one or more ofthe affinity ligands identified at step (h) is used as a candidate bmdmg domam, and steps (d) through (h) are repeated 26 The method of claim 1 , wherem said library of analogues is prepared by inserting a synthetic DNA encoding each analogue m a replicable genetic package, resultmg on expression in display of an analogue bmdmg domam on the surface of said genetic package 27 The method of claim 26, wherein said replicable genetic package is a badenophage 28 The method of claim 27, wherem said badenophage is Ml 3 and said synthetic DNA is mserted m gene IU 29 The method of claim 1 , where the targd is immobilized pnor to said contacting step (e) 30 A method for obtaining DNA encodmg an engineered affinity ligand that bmds to a target molecule with a high degree of speαfiαty under a first bmdmg condition and releases from the target molecule under a second condition, compnsing (a) obtaining one or more affinity ligands expressed on the surface of a replicable genetic package accordmg to the method of claim 25, (b) allowing said genetic package to multiply, and (c) isolating the synthetic DNA from said multiplied genetic packages 31 A method for obtaining a substantially pure engineered affinity ligand, compnsing (a) expressing the isolated synthetic DNA obtained by the method of claim 30 m a gene expression system to produce an engineered affinity ligand, and (b) recovenng said engineered affinity ligand 32 A method of punfymg a targd molecule from a solution containmg it, compπsmg (a) immobilizing an engineered affinity ligand obtained accordmg to the method of claim 1 , (b) contactmg said immobilized affinity ligand with said solution under conditions which permit bmding of said target molecule to said affinity hgand, and (c) duting bound taiget molecule 33 The method accordmg to claim 32, wherein, m step (c), said targd molecule is eluted usmg said second solution condition 34 The method according to claim 32 wherem, m step (b), said taigd molecule is contacted usmg said first solution condition 35 A method for engineering desired bmdmg and release properties mto a polypeptide affimty ligand havmg a structure analagous to a canώdate bmding domam, to produce an engineered affinity ligand useful for separating a targd molecule from a solution containmg it, said method compπsmg (a) selecting, with resped to the targd molecule, a first solution condition at which it is desired u at the engineered affinity hgand will bmd to said targd molecule, (b) sdecting, with respect to the taigd molecule, a second solution condition at which it is desired that the engineered affinity ligand will not bmd to said targd molecule and wherem said second soluϋon condition is different from said first solution condition, (c) providing a multiplicity of polypeptide analogues denved from a canώdate bmding domam, wherein each analogue differs from said canώdate bmdmg domam by vanation ofthe ammo aαd sequence ofthe canώdate binding domam at one or more ammo aαd positions within the domam, (d) contactmg said multipliαty of analogues with a solution containmg said target molecule at the first solution condition, for suffiαent time to permit analogue target bmdmg complexes to form, (e) removing analogues that do not bmd the targd under the first solution condition, (f) altenng the conditions ofthe contacting step (d) to the second solution condition, and (g) recovenng the analogues rdeased under the second condition, wherem the recovered analogues identify engmeered affinity ligands 36 The method accordmg to claim 35, wherem said multiphαty of analogues amounts to at least IO 6 analogues 37 The method accordmg to daim 35, wherem the multipliαty of analogues is prepared by inserting a synthetic DNA encodmg each analogue m a replicable genetic package, resultmg on expression m ώsplay of an analogue bmdmg domam on the surface of said genetic package 38 The method accordmg to daim 37, wherem said replicable genetic package is a bactenophage 39 The method accordmg to claim 38, wherem said badenophage is Ml 3 and said synthetic DNA is mserted m gene in 40 The method of daim 35, wherem the targd is immobilized pπor to said contactmg step (d) 41 A synthetic engmeered affinity ligand havmg an affimty for bmding with a targd molecule at a first solution condition and havmg a substantially decreased affinity for bmdmg with said targd molecule at a second solution condition, wherem said affinity hgand compπses an engmeered polypeptide bmdmg domam havmg an ammo aαd sequence that is analogous to a known canώdate bmdmg domam, wherem the ammo aαd sequence of said engmeered polypeptide bmding domam differs from that of the canώdate bmdmg domam in that ammo aαds at one or more ammo aαd positions ofthe canώdate bmdmg domam have been substituted with amino aαds that lend properties to the engmeered polypeptide bmdmg domain favoπng a decreased bmding affinity at said second solution condition 42 The engmeered affinity ligand accordmg to daim 41 , havmg a conformation constraint which is conferred by a chemical bond other than a peptide bond joining ammo aαds ofthe bmdmg domam together and which is suffiαently stable in structure to have a melting pomt of at least 40°C 43 The engmeered affinity hgand accordmg to claim 41 , wherem said substituted ammo aαds are sdected from a group of ammo aαds conferring a particular property, which groups of ammo aαds are seleded from I,M,T,N,K,S,R,V, A,D,E, and G, in order to make the affimty ligand responsive to changes in pH and the presence of organic solvent and salt, Y,H,N, and D, order to make the affinity ligand responsive to changes m pH, Y,H,Q,N,K,D, and E, m order to make the affinity ligand responsive to changes pH, Y,C,R,N,S,D, and G, m order to make the affinity ligand responsive to changes in pH and the presence of organic solvent and salt, N,K,S,R,D,E, and G, m order to make the affinity hgand responsive to changes in pH and the presence of organic solvent and salt, S,P,T,A,F,L,L and V, m order to make the affinity ligand responsive to changes m pH and the presence of organic solvent and salt, F,L,I, and V, m order to make the bmdmg protem responsive to changes m the concentration of organic solvent or salt, and F,L,S,Y,C, and W, m order to make the bmdmg protem responsive to changes in the concentration of organic solvent or salt 44 The engmeered affinity ligand accordmg to claim 41 which bmds to a targd molecule at a preselected pH and rdeases from said targd molecule at a different preselected pH 45 The engmeered affinity ligand according to claim 41 which bmds to a targd molecule at a preselected salt concentration and releases from said targd molecule at a different preselected salt concentration 46 The engmeered affinity ligand accordmg to claim 41 which bmds to a targd molecule at a presdected concentration of urea and rdeases from said targd molecule at a different preselected concentration of urea 47. The engineered affinity ligand according to claim 41 which binds to a targd molecule at a preseleded EDTA concentration and releases from said targd molecule at a ώfferent preseleded EDTA concentration. 48. The engineered affinity ligand according to claim 41 which binds to a targd molecule at a preselected volume % solution of an organic solvent and releases from said targd molecule at a ώfferent preselected volume % solution of said organic solvent. 49. The engineered affinity ligand according to claim 41 which binds to a targd molecule at a preselected temperature and releases from said targd molecule at a ώfferent preseleded temperature.

Inventors: LADNER ROBERT C, MACLENNAN JOHN M, LADNER, ROBERT, C., MACLENNAN, JOHN, M.
Applicants: DYAX CORP, LADNER ROBERT C, MACLENNAN JOHN M, DYAX CORP., LADNER, ROBERT, C., MACLENNAN, JOHN, M.
Application year: 1997
Main classification: G01N33
IPC codes: B01J20, C07K1, C07K7, C07K14, C07K17, C12N9, C12N15, C40B30, C40B40, G01N30, G01N33