POLYMERIC ELECTROPHORESIS MEDIA

Abstract: A water-soluble copolymer characterized wherein it has the structure (I) wherein L is alkylene, arylene, a divalent heteroatom, (1), or a combination of two or more thereof, wherein Z is a divalent heterocyclic group having 5 to 7 nuclear carbon and heteroatoms, -A- represents recurring units derived from one or more polymerized acrylamide monomers, R<1> is hydrogen or methyl, R<2'> is a haloacetamido group, -B- represents recurring units derived from any other polymerized vinyl monomers, and x, y and z represent weight percents of the recurring units, x being about 50-90 weight percent, y being about 1-50 weight percent, and z being about 0-45 weight percent is disclosed. This copolymer is useful in an element, a method and a kit for electrophoresis.

Description: POLYMERIC ELECTROPHORESIS MEDIA This invention relates to a medium or element for electrophoresis and to acrylamide based copolymers prepared from certain acrylic monomers such as those disclosed in EP-A-89107729.9 filed on April 28, 1989 entitled "Kit for Electrophoresis Gel Medium". More particularly, it relates to improved polymeric gel media suitable for electrophoretic separation of biopolymers such as proteins and polynucleic acids (DNA, RNA and their derivatives or fragments) prepared from acrylamide based copolymers. Acrylamide based copolymers having vinylsulfonyl or chloroethylsulfonyl groups as media for electrophoresis are described in a number of patent publications, for example, US-A-4,582,868. Such materials are also known to be gelatin hardeners and crosslinkable polymers useful as binders in film formation or for other uses. US—A-3,625,694 describes polymer mordants having active aldehyde, chloroalkanoyl (such as chloroacetyl and chloropropionyl), chloroalkyl (such as chloromethyl), vinylsulfonyl, and other active groups for anchoring (crosslinking) the mordant to a gelatin matrix. US-A-4,193,795 and US-A-4,201,840 describe polymer mordants having vinylsulfonyl and chloroethylsulfonyl groups to covalently bind dyes . In practice, various supporting media are used to minimize convection and diffusion, and to effect separation both on the basis of molecular size and of net charge. The most popular support media are sheets of paper or cellulose acetate, agarose, starch, and polyacrylamide. Paper, cellulose acetate, and similar porous materials are relatively inert and serve mainly for support and to minimize convection. Separation of proteins using these materials is based largely upon the charge density of the proteins at the pH selected. However, the use of starch, agarose and polyacrylamide gels not only minimize convection and diffusion but also actively participate in the separation process by materials providing a restrictive medium in which the average size of the polymeric network opening (or average pore size) can be controlled to achieve a molecular fractionation in a desired molecular size range. In this way, molecular sieving occurs and provides separation on the basis of both charge density and molecular size instead of upon the charge density of the proteins at the pH selected only. Agarose is rarely used at concentrations higher than 5% because such solutions are ver' viscous and not easily poured. Agarose is therefore widely used at concentrations < 5% (w/v) for the electrophoresis of large molecules, for example, high molecular weight proteins, and polynucleotides . The ability to produce gels having a wide range of polymer concentrations (and, therefore, since the gel network opening decreases with increasing polymer concentration, a wide range of controlled average pore sizes) as well as to form pore size gradients within the gels by virtue of polymer concentration gradients, are additional advantages of polyacrylamide as an electrophoresis gel medium. Control over pore size enables mixtures to be sieved on the basis of molecular size and enables molecular weight determinations to be performed. These determinations are especially accurate if the proteins are treated with a detergent, such as sodium dodecyl sulfate (SDS), which neutralizes the effects of inherent molecular charge so that all SDS treated molecules, regardless of size, have approximately the same charge density values. This technique is referred to as SDS-PAGE (Sodium Dodecyl Sulfate-PolyAcrylamide Gel Electrophoresis) . The popularity of polyacrylamide-based electrophoresis gels stems not only from the comparatively wide latitude in polymer content and buffer composition attainable with them, but also from the high degree of inertness in the gel with respect to both the voltages applied and the solutes being separated, the ease with which proteins are detected once separated and good reproducibility with carefully prepared gels. Further, it is desirable to have an acrylamide-based gel (that is, a gel comprising a crosslinked vinyl polymer comprising at least 50% by weight of monomeric units selected from acrylamide and N-substituted acrylamides or methacrylamides) that could be used at low concentrations for the separation of large molecules with the advantages of low viscosity, easy pourability and high purity. Additional advantages of using polyacrylamide as an electrophoresis gel medium are the wide range of polymer concentrations, wide range of controlled average pore sizes and the ability to form pore size gradients within the gels by virtue of polymer concentration gradients. Control over pore size enables mixtures to be sieved on the basis of molecular size and enables molecular weight determinations to be performed. These determinations are especially accurate if the proteins are treated with a detergent, such as sodium dodecyl sulfate (sodium dodecyl sulfate), which neutralizes the effects of inherent molecular charge so that all sodium dodecyl sulfate treated molecules, regardless of size, have approximately the same charge density values. This technique is referred to as (Sodium Dodecyl Sul te— olyAcrylamide Gel Electrophoresis, identified herein as PAGE). The popularity of polyacrylamide-based electrophoresis gels stems not only from the comparatively wide latitude in polymer content and buffer composition attainable with them, but also from the high degree of inertness in the gel with respect to both the voltages applied and the solutes being separated, the ease with which proteins are detected once separated and good reproducibility with carefully prepared gels. Conventionally, polyacrylamide gel media for use in sodium dodecyl sulfate—PAGE electrophoresis have been prepared in situ by free radical induced polymerization of a monomer such as acrylamide and a crosslinking agent, most commonly N.N'-methylenebisacrylamide, under oxygen—free conditions in the presence of water, a buffer, a polymerization initiator, and a polymeri¬ zation catalyst. The usual practice is to perform a free radical polymerization with acrylamide and a suitable bis monomer such as N,N'-methylenebisacrylamide (often simply referred to as "bis") in order to obtain a gel. Such gel formation is successfully done only as several precautions are taken, namely: a very high purity starting materials should be used; (b) the solution of monomers and buffer should be degassed to remove oxygen; (c) a free radical initiator and a catalyst must be quickly mixed into the degassed solution; (d) the solution should be quickly poured between two glass plates or down a glass tube, the lower end of which in either case is sealed to prevent leakage; and (e) the gelation should proceed with (i) oxygen largely excluded and (ii) adequate means for heat dissipation being present so that excess heat does not cause gel nonunifor ities. Thus, the preparation of a polyacrylamide gel medium for electrophoresis having the desired dimensions and consistency requires a great deal of skill and care, as well as keeping the solution free from oxygen. Precautions are also required in handling the monomers since both acrylamide and bis have been identified as known neurotoxins and suspected carcinogens. There are several alternatives to the above-described procedure whereby the user makes electrophoresis gels by free radical polymerization and crosslinking in situ. These include (a) the use of preformed gels in cassettes and (b) the use of preformed gels on flexible supports. With either of these alternatives, however, some operating freedom or flexibility with regard to gel size, polymer content in the gel and buffer content is lost. Also — especially with precast gels in cassettes made by free radical polymerization and crosslinking — there generally remain, after completion of the gel formation reaction, some unreacted monomers, initiator by—products and catalyst. The presence of such species poses some toxicological hazards to the user and may interfere with the electrophoretic separation to be performed. Also, such precast gels have been found to have limited shelf lives . US-A-4,582,868 describes the crosslinking of acrylamide—rich copolymers to form electrophoresis gel media by a non-free radical induced mechanism that does not require exclusion of oxygen. Typically, copolymers of acrylamide and a monomer that affords sites for subsequent non—free radical initiated crosslinking by treatment with a crosslinking agent, for example, an acrylamide derivative such as N-[3-(2-chloroethyl- sulfonyl)propionamidomethyl]acrylamide, an acrylate derivative such as 2-[3-(2-chloroethyl- sulfonyl)propionyloxy]ethyl acrylate, H CH 2 =C I-C-0(CH 2 ) 2 0C(CH 2 ) 2 S0 2 CH 2 CH 2 C1, or 0 0 an active ester such as N-[2-(ethoxycarbonylmethoxy- carbonyl)ethyl]acrylamide: CH 2 =CHC0NHCH 2 CH 2 C00CH 2 C00C 2 H 5 are prepared, in accordance with US-A-4,582,868, by a free radical initiated polymerization in the absence of oxygen. Thereafter, in a separate procedure, which can safely be performed in the presence of oxygen, the chloroethylsulfonyl or other pendent reactive group—containing polymers are crosslinked by reaction at a suitable pH with a bis-nucleophile crosslinking agent such as a diamine or a dithiol. In this regard, it is noted that electrophoresis often is performed at pH values that facilitate dehydrohalogenation of the chloroethyl¬ sulfonyl groups. If the vinylsulfonyl groups so formed are not all reacted with the intended crosslinking agent, they could react with amino groups on dissolved proteins during electrophoresis. Such reaction would artifactually retard the electrophoretic migration of proteins and consequently give misleading electrophoresis results in relation to the results obtained with electrophoresis gels formed by the free radical polymerization of acrylamide and bis alone. Therefore, enough crosslinking agent should be used to assure complete reaction of these groups. Despite the availability of the above- described alternatives, electrophoresis media are still generally prepared by the polymerization of vinyl monomers at the time of use. This necessarily involves exposure of the operator to monomers prior to use and to residual monomers during use. Such monomers are suspected carcinogens, and at least some are known to be neurotoxins. Although bis is the most widely used crosslinker for acrylamide-based electrophoresis gels, bis-crosslinked gels generally cannot be resolubilized. The ability to solubilize the gel after performing electrophoresis is advantageous in that it enables one to recover a resolved species from the gel. The use of an alternative, cleavable crosslinker such as diallyltartardiamide (DATD) 0 OH OH 0 CH 2 =CH-CH 2 NHC-CH-CH-CNHCH 2 CH=CH 2 permits the researcher to achieve such recovery. After electrophoresis, each portion of the gel to be solubilized is excised and treated in a dilute solution of periodic acid. The -CH0H-CH0H- linkage in the middle of the crosslink is cleaved in the periodic acid solution and a solution of the thus solubilized polymer is produced. From this solution one can easily recover the resolved species for further experiments. Obviously, it would be advantageous to provide researchers with the option of gel solubilization after electrophoresis. Also, if electrophoresis is to be performed soon after gelation, the chemical reaction (that is, crosslinking) responsible for gelation must be compatible with the buffers present for electrophoresis. Gel electrophoresis is often performed at pH values ranging from 5 to 9. A particularly popular system for the determination of molecular weights of proteins by electrophoresis was described by Laemmli, Nature. 227:680 (1970). In this system, two gels are used, one directly above the other, with a multi-phasic buffer system. The upper (or stacking) gel is at pH 6.8, which is achieved with tris(hydroxymethyl)amino- methane to which HCl has been added to lower the pH. The stacking gel has a low polymer concentration (generally from 4 to 6% w/v). Its purposes are (a) to provide a medium onto which samples can be loaded in discrete lanes and (b) to concentrate all species in a particular sample at the interface between the stacking gel and the lower (or resolving) gel. In meeting objective (b), the solutes, which are generally sodium dodecyl sulfate—denatured proteins, are "stacked" together (or very nearly so), that is, are concentrated at the interface of the two gels, just before entering the resolving gel. For the stacking to occur effectively, the proteins, rather than the buffer, should carry most of the current and there should be no molecular size separation. Molecular size separation occurs in the resolving gel, where the buffer carries most of the current and the solutes migrate at a velocity determined by the voltage gradient and the retardation due to the pore size distribution of the crosslinked polymer gel. The pH in the resolving gel is typically 8.8 and the polymer concentration is usually at least 10% (w/v) . In summary, the popular Laemmli procedure requires two gels, the lower of which (resolving gel) is larger (thereby providing a longer path for solutes to traverse) and contains a higher concentration of the gelled polymer, and therefore a smaller average pore size than that of the upper, or stacking gel. The conditions recommended by Laemmli are: 0 Stacking Gel Resolving Gel pH 6.8 8.8 buffer 0.125 M Tris(hydroxy 0.375 M Tris(hydrox methyl) aminomethane methyl)aminomethane hydrochloride hydrochloride gel cone. , % (w/v) 4-6 > 10 Despite the teachings of Laemmli over the prior art in regard to use of a stacking gel, it is still desirable to be able to coat the solutions to be gelled in the presence of oxygen so that they can be easily and safely poured to form gels quickly and solubilize gels or portions thereof for sample recovery after electrophoresis. However, for the electrophoretic separation of large molecules, such as high molecular weight proteins and polynucleotides, only a single (resolving) gel is typically used. This gel, (typically agarose in the prior art) is generally formed from a lower concentration of a high molecular weight polymeric material than the resolving gel used in the Laemmli procedure, in order to have appropriate larger pore size characteristics for the electrophoretic separation of the larger molecules. The problems noted above are overcome with a water—soluble copolymer characterized wherein it has the structure „1 wherein L is alkylene, arylene, a divalent heteroatom, -C-Z- , or a combination of two or more thereof, wherein Z is a divalent heterocyclic group having 5 to 7 nuclear carbon and heteroatoms, —A- represents recurring units derived from one or more polymerized acrylamide monomers, R is hydrogen or methyl, R 2' is a haloacetamido group, —B— represents recurring units derived from any other polymerized vinyl monomers, and x, y, and z represent weight percents of the recurring units, x being about 50-90 weight percent, y being about 1-50 weight percent, and z being about 0-45 weight percent. In another aspect of the invention, a method for preparing an electrophoresis gel in situ comprises mixing an aqueous solution of the copolymer as described above, with a selected crosslinking agent that will react with crosslinkable sites on the copolymer by a reaction with the selected crosslinking agent that does not involve a free-radical vinyl addition mechanism. In still another aspect of the invention, an element for electrophoresis is characterized as comprising a crosslinked polymer which, before crosslinking with a crosslinking agent, is the copolymer described above. In a further aspect of the invention, an electrophoresis element is characterized as comprising a copolymer as described above, which copolymer has been prepared as a crosslinked gel by the method described above. Moreover, the copolymer described above, after crosslinking with a crosslinking agent, can be used as either a resolving gel or as a stacking gel. This invention also provides an electrophoresis element which comprises both a resolving gel and a stacking gel, the element characterized wherein the resolving gel comprises a first copolymer as described above and having an M w , weight molecular weight, of less than about 100,000 and an M , number molecular weight, of at least about 7,000, the stacking gel comprises a second copolymer as described above and having an M , weight molecular weight, of greater than 100,000 and an H , number molecular weight, of greater than 50,000. with each copolymer having been prepared as a crosslinked gel by the method as described above. In a final aspect of the invention, a kit for electrophoresis is characterized as comprising the copolymer as described above, or a combination of the variations of the copolymer as described above, and in a container separate from the copolymer(s), a crosslinking agent for crosslinking the copolymer by a reaction that does not involve a free-radical vinyl addition mechanism. Advantageous features of this invention include: the ability to coat the solutions to be gelled in the presence of oxygen so that they can be easily and safely poured, the formation of the gels quickly and solubilization of the gels or portions thereof for sample recovery after electrophoresis. The copolymers of the present invention relate to electrophoresis media comprising crosslinked polymers which, before crosslinking, conform to the following structure 10 wherein L is substituted or unsubstituted alkylene generally having 1 to 20 carbon and heteroatoms in the backbone. This definition of alkylene is meant to include alkylene groups interrupted or terminated •-c with oxy, thio, -NR 3- ,wherei.n R3 is hydrogen, substituted or unsubstituted alkyl of 1 to 6 carbon atoms (such as methyl, chloromethyl or 2—hydroxy- ethyl) or substituted or unsubstituted aryl of 6 to 10 carbon atoms (such as phenyl, naphthyl, or xylyl), 20 -Z- (as defined below), ester (-C00-), amide (-C0NH-), urylene (-NHC0NH-), urethane (-NHC00-), sulfonyl (—S0 2 —), carbonate, sulfonamide, azo, phosphone or other similar groups. Representative alkylene groups include methylene, ethylene, 25 isobutylene, hexamethylene, carbonyloxyethoxycar- bonyl, methylenebis(iminocarbonyl), carbonyloxy- dodecylenecarbonyloxyethylene, carbonyliminomethylene- iminocarbonyliminoethylene, carbonyliminomethylene- iminocarbonylethylene, carbonyliminotrimethylene- 30 iminocarbonyl, carbonyloxyethyleneiminocarbonyl, and other groups described or suggested by US-A-4,161,407 and US-A-4,548,870. L can also be a divalent heteroatom (or heteroatom—containing group) such as oxy, thio, 35 -NR 3- where R3 i•s as defined above, ester (-C00-), amide (-C0NH-), urylene (-NHC0NH-), urethane (-NHC00-), sulfonyl (-S0 2 -), carbonate, and sulfonamide. Also included in this definition of L are divalent groups which are combinations of two or more of any of the divalent 0 heteroatoms, -C II-Z- wherein Z is a divalent heterocyclic group having alkylene, and arylene groups defined above, for example, phenylene, arylenealkylene, alkylene-arylenealkylene, phenyleneiminocarbonyl, and others readily determined by one of ordinal skill in the art. L can further be substituted or unsubstituted arylene generally having 6 to 12 nuclear carbon atoms. Representative arylene groups include phenylene, tolylene, naphthylene, and others noted in the patents mentioned above. Preferably, L is substituted or unsubstituted alkylene interrupted and/or terminated with heteroatom-containing groups as defined or a combination of phenylene and alkylene groups. alkylene, arylene, a divalent heteroatom, ° —CII-Z- , or a combination of two or more thereof, wherein Z is as defined above. or arylene-R - wherein R is alkylene of to 6 carbon atoms, X is -0- or -NH-, and k, m, and n are each 0 or 1, provided that k is 0 when m is 1, and m is 0 when k is 1, where phenylene or phenylene— methylene is most preferred. In the copolymer structure above, —A- represents recurring units derived from one or more polymerized acrylamide monomers such as acrylamide, N—isopropylacrylamide, 2-acrylamido—2-hydroxymethyl—1,3—propanediol, N—isopropylacrylamide, N-hydroxymethylacrylamide, N-(l,l-dimethyl-3-oxo-butyl)acrylamide, N—methylmethacrylamide, 2—acryl¬ amide-—2-hydroxymethy1-1,3-ρropanediol, methacryla- mide, 3-(3—dimethylaminopropyl)acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N—isopropylmethacrylamide, and 3—(2-dimethylamino- ethyl)acrylamide. Particularly preferred is (unsubstituted) acrylamide, and preferably acrylamide. R 1 i.s hydrogen or methyl, R2• is a haloacetamido group, -B- represents recurring units derived from any other polymerized vinyl monomers including styrene or styrene derivatives, divinyl, diacrylic and diacrylamide monomers such as divinylbenzene, ethylene diacrylate, ethylene dimethacrylate, N.N 1 —methylene-bisacrylamide, and x, y, and z represent weight percents of the recurring units, x being about 50—90 weight percent, y being about 1-50 weight percent, and z being about 0-45 weight percent. The foregoing polymers can be crosslinked with agents having two or more amino, mercapto, sulfinic acid, or phenolic hydroxy groups such as ethylened amine, 1,3—propanediamine, 1,3-propane- dithiol, dithiothreitol, dithioerythreitol, 1,5—pen— tanediamine, hexamethylenediamine, diethylenetriamine, triethylenetetra ine, propylenediamine, di(amino— methyl)ether, l,8-diamino-4-(aminomethyl)octane, xylylenediamine, hydroquinone, bisphenol A, bisphenol sulfone, 1,4—butanedisulfinic acid, benzenedisulfinic acid, thioethanolamine, p-aminothiophenol, and butylenediamine. More particularly, preferred crosslinking agents are bis mercaptans having in its backbone chain the -CH-CH group. OH OH Examples of these more preferred crosslinking agents are dithiothreitol and dithioerythreitol, with dithiothreitol being most preferred. In a preferred embodiment, the reactive group R 2* is the chloroacetyl, especially in the form of a chloroacetamido group. This group is electrophilic and would be expected to react with a nucleophile such as an amine or a thiol. We have found that the reaction with a thiol occurs much more rapidly than with an amine at the pH values (7-10) of most interest for electrophoresis. Thus we have found in one preferred embodiment that dithiothreitol HSCH 2 (CH0H) 2 CH 2 SH is a very effective crosslinking agent for this class of polymers. The advantages of dithiothreitol, besides the rapid crosslinking reaction even in the presence of a primary amine (hydroxymethyl aminomethane buffer), include (a) water solubility, (b) low toxicity, and (c) susceptibility to post-electrophoresis reaction with periodic acid to solubilize the gel or selected portions thereof. The chloroacetamido-thiol reaction is pH sensitive and produces HCl as a by-product of the reaction: 0 0 II (base) || R 1 NHCCH 2 C1 + R 2 SH — 1 HCCH 2 SR 2 + HCl Consequently, it is advantageous to adjust to 7.8 and 9.4, respectively, the pH's of the hydroxymethyl aminomethane»HCl solutions used in the stacking gel and resolving gel buffers. These adjustments increase the rate of reaction and compensate for the generation of HCl. The electrophoretic separation of sodium dodecyl sulfate—complexed proteins on dithriothreitol crosslinked poly[acrylamide-co-N-(3-chloro- acetamidoproρyl)methacrylamide] copolymers compares very favorably to the separation obtained on acrylamide/bis gels. Selection and incorporation of a suitable buffer is well within the knowledge of skilled workers in the electrophoresis art and depends upon the materials to be separated by the electrophoresis process in which the medium is to be employed. Such buffers and bases for selecting them are described, for example, in Andreas Chrambach, "The Practice of Quantitative Gel Electrophoresis," VCH Publishers, Deerfield Beach, Florida, U.S.A. (1985), and U.K. Laemmli, Nature. 227:680, (1970). The polymerized units bearing the chloroacetamido group should be present in the copolymer only to the extent needed to provide the desired degree of crosslinking density in the polymeric gel. Less than the necessary amount would lead to gels in which the crosslink density is too low, whereas greater amounts than needed would detract from the acrylamide-like character that we seek. The crosslink density and molecular weight requirements are satisfied if the copolymer contains on a weight basis 10% recurring units derived from N—(3-chloro— acetamidopropyl)methacrylamide monomer (on a mole basis this is 3.5% of the monomer. The weight average molecular weight T_ and the number average molecular weight Η are important parameters that should be properly selected in order for a preformed polymer that is subsequently crosslinked with a chemical crosslinking agent that does not involve a free-radical vinyl addition mechanism, first to produce a gellable polymer solution for forming a resolving electrophoresis gel that (1) is easily poured into a slot (mold, tube, or between glass plates) no greater than 0.15 cm thick, (2) has a short gel time (that is, about 4 to 10 minutes; by "gel time" is meant the time from addition of the crosslinking agent until the solution cannot be poured easily), but (3) remains pourable into said slot for a sufficient time to fill an electrophoresis mold of 14 x 14 x 0.15 cm (which takes about 4 to 6 minutes), and second, within about 10 minutes to 2 hours after addition of the crosslinking agent, to produce a firm gel having (1) sufficient crosslink density to afford good molecular sieving during electrophoresis, (2) good resolution during electrophoresis, and (3) sufficient physical integrity to permit manual removal of the hardened gel from the mold shortly after electrophoresis as well as its handling without causing detrimental flow, compression, fracture, stretch, tear or disintegration of the gel. With 10% (by weight) N-(3-chloroacetamido- ρroply)methacrylamide in the resolving gel (G R ) copolymer, the number of equivalents of crosslinking site per gram (y) can be computed as: y = °- 10 = 4.59(10 ~4 ) 218 Knowing y and the molecular weight between crosslinks (H ) desired permits the estimation of the minimum value of Η for the starting copolymer consistent with achieving the crosslink density s ought , _ y _ i = = ? 2.. ^3n0π(1o0 -4 ") - E n 2 E c 5 . 8 ( 10 J ) M n = 1.7 (10~ 4 ) The preferred weight average molecular weight M is not estimated so directly. We have found, nevertheless, that the elapsed time necessary will form a gel after addition of crosslinking of crosslinking agent to an aqueous solution of polymer in accordance with the invention decreases with increasing H (as expected) and that for a 12% (w/v)vpolymer concentration in the resolving gel, one can achieve a gelation time of six minutes (time between crosslinker addition and gelation) if T_f is approximately 4.6(10 —4). The requirements of and " E in said one copolymer component of electrophoresis media used as a resolving gel are: 1) M is small enough that the addition of 1.25 to 1.5 times the stoichiometric amount of the selected crosslinking agent will not raise the viscosity of the mixture so much that the media cannot be poured into a 0.15 x 14 x 14 cm mold within about 4 to 10, preferably about 6 to 8 minutes after addition of the crosslinking agent. 2) M is large enough to provide a crosslink density sufficient to form a gel after about 10 minutes after addition of 1.25 to 1.5 times the stoichiometric amount of the selected crosslinking agent, said gel ultimately having sufficient integrity within about 2 hours after pouring to be removed from the mold and handled gently without tearing or falling apart. 3) The number of equivalents of crosslinking sites per gram of polymer is in the range of 0.45(10 ) to 14(10 ), preferably at __ _4 least 2/M , more preferably from about 2.25(10 ) to 10(10 ), and most preferably about 4(10 -4 ) to 7(10 "4 ). More particularly, for a copolymer of the present invention used for preparing a resolving gel for electrophoresis, the water soluble copolymer should have a number average molecular weight, M_, of at least about 7,000, and a weight average molecular weight, Mw. of less than about 100,000. The number average molecular weights and the weight average molecular weight can be varied by methods known to those skilled in the synthetic polymer chemistry art. For example, the molecular weights can be decreased by increasing the amount of initiator used, increasing the amount of chain transfer agent used, decreasing the monomer concentration, and increasing the reaction temperature. They can also be varied by selection of the particular chain transfer agent and/or initiator. Also preferred is an electrophoresis element comprising both a resolving gel and a stacking gel. The resolving gel comprises a first copolymer as described above wherein the molecular weight, Mw,,, is less than about 100,000 and the M is at least about 7,000, and the stacking gel comprises a second copolymer as described above wherein the molecular weight, " w is greater than 100,000 and the " is greater than 50,000. For compatibility, the monomers from which the stacking gel copolymer is made should be the same as, or similar to, those used for the resolving gel. Preferably, acrylamide comprises at least 90 mole percent of the monomer mix. Consequently, the molecular weights, M n and Η w , of the stacking gel copolymers must be significantly greater than those of the corresponding resolving gel copolymers as described above. The rate of gelation can be expected to increase as (a) the concentration of copolymer in the gel, (b) the molecular weight of the starting copolymer (particularly M ), (c) the number of crosslinking sites present per gram of copolymer, and (d) the pH of the solution, increase. A polymer of the present invention useful as a stacking gel has an K of at least 50,000 and an w of at least 100,000, preferably of an " M w of at least 150,000, and more preferably at least 200,000 or 300,000. A preferred polymer, poly[acrylamide—co-N- (3—chloroacetamidopropyl) methacrylamide] (weight ratio of acrylamide to comonomer: 90/10; mole ratio: 96.5/3.5), preferably has the following molecular weights when used for both purposes : Mn ^w Resolving Gel (G R ) > 0.7(10 4 ) < 10.0(10 4 ) Stacking Gel (G s ) > 5(10 4 ) > 10.0(10 4 ) As indicated earlier, these preferred molecular weights should be similar for any other copolymers of the structure noted above that are crosslinked similarly. This is because: 1) the bulk of the polymer composition is derived from acrylamide or modestly substituted acrylamides, 2) the fraction of crosslinking sites per molecule should be similar, and 3) the molecular weight of polymer backbone between crosslinks should also be similar. It is expected, therefore, that the preferred polymers of the invention for forming resolving gels should have an Η I of about 7,000 to 30,000 and an M_W. of about 25,000 to 100,000; and those preferred for forming stacking gels should have an Ε of about 50,000 to 300,000 and an Ϊw of about 100,000 to 1,000,000. Regarding the number of crosslinking sites present on the polymer, the crosslinking reactions should ideally be done using equal equivalent (that is, stoichiometric) amounts of reactive crosslinking agent and sites on the polymer. However, we have found it advantageous to use 25-50% more dithio¬ threitol (or other suitable crosslinking agent) on a chemical equivalency basis than there are crosslink¬ ing sites present. The benefits of using 1.25 to 1.5 times the just-required stoichiometric amount of crosslinking agent are (a) better overall crosslinking as manifested by lower degrees of gel swell in high purity water, and (b) assurance that most (if not all) of the electrophilic groups have been reacted. Unreacted electrophilic groups at potential crosslinking sites could react with nucleophilic groups on proteins and thereby confound the electrophoretic separation process. The number of equivalents of crosslinking sites per gram of polymer can be varied by adjusting the concentration of monomer(s) containing the reactive R 2 • groups shown in the copolymer structure above. In still another embodiment of the invention, the copolymer of the present invention is a component of a kit for electrophoresis comprising the copolymer described in detail above or a combination of more than one copolymer as described above, and in a container separate from the copolymer(s) , a suitable crosslinking agent, more particularly described above, for crosslinking the copolymer by a reaction that does not involve a free—radical vinyl addition mechanism. Optionally, the kit may also contain a selected buffer and other suitable ingredients for incorporation into the electrophoresis medium. In still another embodiment of the invention, there is provided a method of preparing an electrophoresis gel for the separation of large molecules in situ comprising mixing an aqueous solution of a copolymer as described above with a crosslinking agent as described above that will react with crosslinkable sites on the copolymer by a reaction with a selected crosslinking agent that does not involve a free—radical vinyl addition mechanism. The rate of gelation can be expected to increase as (a) the concentration of copolymer in the gel, (b) the molecular weight of the starting copolymer (particularly M ), (c) the number of crosslinking sites present per gram of copolymer, and (d) the pH of the solution, increase. Typically, concentration of the preformed polymer will be from about 8 to about 14%, preferably 10 to 12%, (w/v) for the resolving gel and from about 1.5 to about 5%, preferably 2 to 5%, more preferably 2.5 to 4% (w/v) for the stacking gel. For example, the preferred polymer, poly[acrylamide-co—N-(3-chloro- acetamidopropyl)methacrylamide] (weight ratio of acrylamide to comonomer: 90/10; mole ratio; 96.5/3.5), is employed at a concentration of about 12% to make a resolving gel, and about 4% to make a stacking gel. However, to ensure gel formation, the molecular weights M_ and " are both higher in the stacking gel polymer than in the resolving gel polymer. Preparation of N—(3—Chloroacetamidopropyl)- methacrylamide a preferred monomer useful in the practice of this invention: In a 3-liter 4-neck flask fitted with condenser, stirrer, and 2—dropping funnels were placed N-(3-aminopropyl)methacrylamide hydrochloride (157 g 0.88 moles) in methanol (1.2 ft) and 2,6-di- tert-butyl-p-cresol (1.0 g). In one funnel was placed chloroacetyl chloride (100 g, 0.89 mole), and in funnel two was placed triethylamine (178 g, 1.76 mole). The solution was cooled to 0-5 β C (ice-methanol) , and triethylamine was added in a slow stream over 30 minutes, and the chloroacetyl chloride was added over 1 hour. After the addition, the temperature was maintained at 0°C for 2 hours, the ice bath was removed, and stirring was continued at room temperature overnight. The solvent was removed, and to the residue was added hot ethyl acetate (500 mft). The mixture was filtered to remove triethylamine hydrochloride, then the solid was washed with hot ethyl acetate (500 mft), filtered again, the filtrate combined, and the solvent was removed on a rotory evaporator. The residue was crystallized from ethyl acetate (400 mft) by heating to dissolve, filtering to remove any solid present, and cooling to 0°C to crystallize. The crude monomer was purified by chromatography on a silica gel packed column. The product was eluted from the column using a 1:1 mixture of ethyl acetate and dichloromethane (4 ft). The collected solvent was evaporated, and the residue crystallized from ethyl acetate (300 mft) with 2,6-di-tert-butyl-p-cresol (500 mg) to give a white crystalline compound, mp 85-90 β C, 83 g (43% yield). Anal. Calc'd for C 9 H 15 C1N 2 0 2 : C, 49.4; H, 6.9; N, 12.8; cl, 16.2. Found: C, 49.0; H, 7.6; N, 13.2; Cl, 17.3. The following examples illustrate the practice of this invention: Exa ple 1 : Preparation of Poly[acrylamide—co-N—(3— chloroacetamidopropyl)methacrylamide] (weight ratio 95/5)- To a mixture of acrylamide (electrophoresis grade) (34.2 g, 0.48 mole) and N-(3-chloroacetamido- propyl)methacrylamide (1.8 g, 0.008 mole) in tert-butanol (400 mft) and isopropanol (40 mft), maintained under a nitrogen atmosphere, was added 2,2'-azobis(2-methylpropionitrile) (1.0 g). The solution was heated to 70 β C for 6 hours (the polymer precipitates from solution after 10 minutes). The polymer was then filtered, washed with methanol (2ft), acetone (2 ft), sucked dry and placed in a vacuum oven at 35°C overnight to give a white powder; 36 g 100% yield. The polymer had an inherent viscosity of 0.42 dft/g in a 1.0 molar sodium chloride solution measured at a concentration of 0.25 g/dL. Anal. Calc'd for 3i02 H 5170 C1 17 N 1017 0 1017 : C, 50.35, H, 7.04; N, 19.53; Cl, 0.76. Found: C, 48.3; H, 7.12; N, 18.18; Cl, 0.85. Other polymers of the same family were prepared by varying the weight proportions of acrylamide and 3-chloroacetamidopropyl methacrylamide. Example 2: Alternate Preparation of PolyCacrylamide- co—N—(3—chloroacetamidopropyDmethacrylamide] (weight ratio 95/5). This preparation employs a water/isopropyl alcohol solvent system instead of a t-butanol/isopropyl alcohol system. A mixture of 68.4 g of acrylamide, 3.6 g of N-(3-chloroacetamidopropyl)methacrylamide, 600 mft of Milli Q water, 200 mft of isopropyl alcohol, and 4 g of 2,2 * -azobis(2-methylpropionitrile) was placed in a 1000 ft flask, purged with nitrogen gas for 10 minutes, and placed in a 63°C constant temperature bath while stirring mechanically. After about 7 hrs . and 25 minutes, the flask was removed from the bath and allowed to cool overnight. The polymer product was precipitated by pouring into 4 ft of acetone with stirring. The solvent was removed by filtration, the solid washed with 4 ft of methanol, collected, washed again with 4 ft of acetone, collected again on a filter, and dried overnight under vacuum. Example 3: Description of Use of Copolymers for SDS Electrophoresis . 1. Stock Solutions a. A 21.8% stock solution of the 95:5 poly[acrylamide-co-N-(3-chloro-acetamidopropyl)methacry lamide] was made by dissolving 21.8 g in high purity, deionized water (Milli Q) to a final volume of 100 mft. b. a 22.5% stock solution of the 80:20 poly[acrylamide-co—N—(3—chloro-acetamidopropyl) methacrylamide] was made by dissolving 20 g in high purity, deionized water (Milli Q) to a final volume of 100 mft. 2. Preparation of 15% Resolving Gel To cast a 15% T gel, 6.88 mft of stock solution A representing [(1.5)( .05)/218]0.000344 chemical equivalents of N-(3-chloroacetamido-ρropyl) methacrylamide was mixed with 0.05 mft of 20% sodium dodecyl sulfate (SDS), IX equivalent of crosslinker (dithiothreitol), and the necessary volume of water to bring the total volume to 7.50 mft. After this solution is completely mixed, the solution is taken to a final volume of 10.0 mft with 2.5 mft of Tris-HCl, 1.5 molar, pH 8.8. The buffer is added last since the crosslinking is pH dependent, and, if the buffer is added before the crosslinking agent, the gelation occurs too quickly. After adding the buffer, and mixing thoroughly, the mixture is poured between two glass plates, separated by a 0.075 cm slot, on a gel casting stand that had been previously set up. The mixture is poured within 1.5 minutes after adding the buffer, and the gel is completely poured within 5 minutes. After approximately 20 minutes, gelation begins and continues until the gel is firm. If a stacking gel is to be used, step 3 is followed. If a stacking gel is not used, the Teflon comb to form wells is inserted between the glass plates after the gel solution has been poured and kept there until the gel is firm. 3. Preparation of 5% Stacking Gel After the a 15% T resolving gel was firm, a 5% stacking gel was layered over it. The 5% stacking gel was made by adding 2.2 mft of stock solution B representing [( .5)(.2)/218]0.000459 chemical equivalents of N-(3—chloroacetamido— proρyl)methacrylamide. To this was added 0.05 mft of 20% sodium dodecyl sulfate, 0.83 mft Tris-HCl (pH 6.8, 1.5 M), IX equivalents of dithiothreitol, and high purity, deionized water (Milli Q) to a final volume of 10 mft. After thorough mixing, this was overlaid on the 15% resolving gel, and a Teflon comb was inserted to form the stacking gel and loading wells for samples. The stacking gel became firm after approximately 6 hours. 4. Gel Electrophoresis Conditions After the gel had been formed from part 2 (and part 3 if a stacking gel is used), the gel was used for a poly(acrylamide) gel electrophoresis experiment in the presence of sodium dodecyl sulfate and denatured proteins (SDS-PAGE). The experimental conditions include: a. Protein standards purchased from Bio-Rad Laboratories identified as high-molecular—weight and low-molecular—weight standards (c/n 161-0303 and 161-0304, respectively) that were dissolved in Tris-HCl, ph 6.5 (final concentration of 0.063 molar), sodium dodecyl sulfate, bromophenol blue tracking dye, and 2-mercaptoethanol (final concentration of 0.2%, 0.001%, and 5%, respectively) with glycerol added as a weighing agent to help concentrate the dissolved samples to the bottom of the sample well. Before the samples were loaded, the standards were heated at 95°C for 5 minutes to denature the proteins. b. The electrode running buffer was Tris/glycine/SDS as described by U. K. Laemmli (Nature. 227:680, 1970), specifically 0.025 molar Tris, pH 8.8, 0.19 molar glycine, and 0.1% SDS. c. The Teflon combs were removed from the gel (stacking gel if one is used) and an appropriate volume of standards from paragraph (4a) above were added so the final protein concentration of high-molecular—weight standards was 7.5 μg (or 1.5 μg/protein standard) and 9.0 μg of low-molecular—weight standard (or 1.5 μg/protein standard). The high-molecular-weight standard contained 5 proteins each having different molecular weight, and the low-molecular-weight standard had 6 proteins each having a different molecular weight. The actual equipment used was the Hoefer model SE 250 vertical slab gel cell (San Francisco, CA 94107), and the procedure was described in the manual accompanying the Mini Protein II Dual Slab Gel (Electrophoresis) Cell sold by Bio-Rad Laboratories, Richmond, CA 94804. d. The electrical parameters for the experiment were 100 volts, constant during the course of the experiment. The experiment was stopped when the bromophenol blue tracking dye was at the bottom of the gel, which was after approximately 2.3 hours under these conditions. After the electrophoresis was completed, the gel was removed from the plates, and stained with 0.05% Coomassie Blue R250 dye solution containing acetic acid (10%), methanol (40%), and water (50%) for a minimum of 30 minutes. The dye preferentially absorbs to the protein-rich areas, giving dark protein bands (after destaining with 10% acetic acid, 40% methanol, and 50% water) that leaves no doubt that the electrophoretic migration had taken place according to molecular size with good resolution. A graph of the migration distances (corrected for swelling due to destaining) plotted as abscissae and the logarithm molecular weights plotted as ordinates was nearly linear which is what one typically observes for SDS-PAGE. Thus, the gel medium provided results very similar to those obtained in SDS-PAGE gels made from acrylamide/bis polymerization. Example 4 Preparation of electrophoresis gel. An electrophoresis gel was made with poly[acrylamide-c_e—N-(3—chloroacetamidopropyl)meth- acrylamide] (weight ratio of acrylamide to comonomer: 90/10; mole ratio: 96.5/3.5) copolymers disclosed herein. This gel included a lower resolving gel made from poly[acrylamide—co—N—(3— chloroacetamidopropyl)methacrylamide] (weight ratio of acrylamide to comonomer-: 90/10; mole ratio, 96.5 ^ /3.5) of Ml = 17.8 (10 3 ) and w = 78.2 (10 ) and a stacking gel from a poly[acrylamide- co-N-(3—chloroacetamidopropyl)methacrylamide] (weight ratio of acrylamide to comonomer, 90/10, mole ratio 96.5/3.5) of Example 2 [M w • * 500 (10 3 )]. The molecular weight averages of the acrylamide/N-(3-chloroacetamidopropyl)methacrylamide copolymers were estimated using an aqueous gel permeation chromatography system in which (a) the fractionation was accomplished with four TSK-GEL (type PW) columns of 6000, 5000, 3000 and 2000 Angstrom permeability limits (Altex Scientific, 1780 Fourth St., Berkeley, CA 94710), (b) the eluent was 0.05 M Na 2 S0. in 5% ethylene glycol in water (v/v), (c) the calibrating standards were Shodex STANDARD P-82 polysaccharides of 853, 380, 186, 100, 48.0, 23.7, 12.2 and 5.8 (10 3 ) (Showa Denko K.K. , 280 Park Ave., 27th Floor West Building, New York, NY 10017) used at 0.1% (w/v) concentration, (d) the flow rate was 1.5 ml/min and (e) the detection of solute in the column effluent was done refractometrically. Each copolymer was crosslinked with dithiothreitol used at 125% of the stoichiometric amount based on the number of crosslinking sites present. The pH of, and buffers used in, the gels and the electrode chambers are summarized in Table I. Table I Location Buffer pH Compos ition Cathode 8.3 0.025 molar Tris, 0.192 molar glycine, 0.1% SDS Stacking gel 7.8 0.125 molar Tris-HCl (4% polymer) Resolving gel 9.4 0.375 molar Tris-HCl (12% polymer) Anode 8.3 same as cathode except SDS was omitted and 0.1 M sodium acetate beneficially added (These variations from cathode conditions are optional.) Tris = Tris(hydroxymethyl)aminomethane SDS = sodium dodecyl sulfate In electrophoresis experiments conducted according to the Laemmli procedure with the buffer compositions given in Table I, the dithiothreitol crosslinked poly[acrylamide-co-N-(3-chloroacetamido- propyl)methacrylamide] (weight ratio of acrylamide to comonomer: 90/10; mole ratio 96.5/3.5) copolymers of the present invention perform comparably to gels prepared from acrylamide and N,N'—methylenebisacryla- mide. Specifically, the gels prepared from copolymers of this invention permit good electrophoresis separations, with SDS—complexed proteins with molecular weights from about 14.4 3 (10 ) to about 200 (10 ) appearing at the anode and cathode ends of the gel, respectively, after electrophoresis under conditions of voltage and time ordinarily used by those skilled in the art of electrophoresis for SDS-PAGE electrophoresis on acrylamide/bis gel media. Not only is the degree of separation comparable to that achieved with acrylamide/bis-based gels but the sharpness of the separated bands is also very good. These results can be achieved with gels in the so-called "mini" format (0.15 cm x 7 cm x 8 cm) (thickness x height x width) and in the popular larger format (0.15 cm x 16 cm x 14 cm). Example 5 Preparation of a low molecular weight copolymer with an adequate level of crosslinking site to form a resolving gel in about four minutes . To a 60°C solution of 2,2*-azobis(methyl- propionitrile) (1.0 g) in Milli Q water (i.e., water purified with a Millipore C/N ZD40115-84 unit) (200 mft), isopropanol (20 mft) and cone, sulfuric acid (0.5 g), which had been purged with nitrogen, was added the following solution, which also had been purged with nitrogen: electrophoresis grade acrylamide (64.8 g; 0.91 moles), N-(3—chloroacetamidopropyl)- methacrylamide (7.2 g, 0.033 moles), Milli Q water (600 mft), and isopropanol (60 mft), the chain transfer agent. This was added to the first solution dropwise and under nitrogen over a 2—hour period. After the addition was complete, the resultant solution was held for 5 hours more at 60°C and then allowed to stand at ambient temperature overnight. The next day the solution was concentrated at 40"C on a rotary evaporator to a volume of about 500 mft. The concentrated solution was then added to 8 liters of reagent grade methanol, with stirring, to precipitate the copolymer. The precipitate was filtered and washed in an additional 4 liters of reagent grade methanol, then dried in a vacuum oven with a nitrogen bleed at 30-35°C. The yield was 95% based on the original recipe. The inherent viscosity of the copolymer, as determined with a 0.25% solution of the copolymer in aqueous 1.0 molar NaCl at 25°C, was 0.35 dl/g. Cl analysis: theory, 1.55%; found 1.46%. Molecular weight analysis by aqueous size raphy showed — = 1.51(104) A polymer solution cast from a 12% solution of this copolymer at pH 9.4 began to gel (to a resolving gel) four minutes after addition of the crosslinker (dithiothreitol — 44.2 mg per gram of copolymer) . in this and the following examples, the molecular weight averages of the acrylamide/N—(3—chloroacetamidopropyl)methacrylamide copolymers were estimated using an aqueous gel permeation chromatography system in which (a) the fractionation was accomplished with four TSK—GEL (type PW) columns of 6000, 5000, 3000 and 2000 Angstrom permeability limits (Altex Scientific, 1780 Fourth St., Berkeley, CA 94710), (b) the eluent was 0.05 molar Na 2 S0, in 5% ethylene glycol—in—water (v/v), (c) the calibrating standards were Shodex STANDARD P-82 polysaccharides of 853, 380, 186, 100, 48.0, 23.7, 12.2 and 5.8 kDa (Showa Denko K.K. , 280 Park Ave., 27th Floor West Building, New York, NY 10017) used at 0.1% (w/v) concentration, (d) the flow rate was 1.5 ml/min and (e) the detection of solute in the column effluent was done refractometrically. Example 6 Preparation of a high molecular weight copolymer with adequate crosslinking site level for forming gels in approximately 40 minutes . To a 50°C solution of ( H 4 ) 2 S 2 0 g (0.75 g) and NaHS0 3 (0.0375 g) in deionized water which had been purged of dissolved oxygen by bubbling with nitrogen, was added dropwise in a nitrogen atmosphere over 2 hours the following solution: electrophoresis grade acrylamide (64.8 g; 0.91 moles), N-(3-chloroacetamidopropyl)methacrylamide (7.2 g, 0.033 moles), and NaHSO- (0.225 g) in deionized water (300 ml). The resultant solution was held at 50 C C for an additional hour after the addition of the monomer-containing solution was finished. Then 250 mft more of deionized water were added with stirring. The copolymer was precipitated by adding the above solution to 8 liters of reagent grade methanol. The precipitate was filtered, washed and dried (oven at 30-35 β C). The yield based on the original recipe was 100% and the inherent viscosity of a 0.25% solution of the copolymer in aqueous 1.0 molar NaCl was 1.13 dl/g, which indicates a very high molecular weight copolymer. Cl analysis: theory, 1.55%, found, 1.40%. When a gel is cast from a 4% solution of this copolymer at a pH of 7.8 using dithiothreitol as the crosslinker (44.2 mg per gram of copolymer), gelation occurs 40 minutes after crosslinker addition. Example 7 a) Synthesis of high molecular weight copolymer for very low polymer content gels. Into a reaction vessel held at 50°C which initially contained a nitrogen-purged solution of 1.5 gram ammonium persulfate dissolved in 400 ml high purity water, were pumped (i) a (previously nitrogen-purged) solution of 259.2 grams of electrophoresis grade acrylamide, 28.8 grams of N-(3—chloroacetamidopropyl)methacrylamide, and 1200 ml of high purity water and (ii) another (nitrogen- purged) solution of 0.525 g sodium bisulfite and 10 ml of high purity water. Solutions (i) and (ii) were added to the reaction vessel over a period of 41 minutes and the combined solutions were held for four hours at 50°C, after which the reaction mixture was permitted to cool to room temperature. Before further use, an additional 500 ml of high purity water were added to this solution, with thorough mixing, to yield a solution that contained 12.5 to 13% (w/v) high molecular weight copolymer. b) Forming a gel for DNA electrophoresis. To form a gel of very low [2.5% (w/v)] polymer concentration, 1.88 ml of the final solution from Example 7a, 0.5 ml of concentrated buffer [0.2 moles tris(hydroxymethyl)aminomethane (hydroxymethyl aminomethane), 0.022 moles boric acid, 0.002 moles ethylenediamine tetraacetic acid and enough high purity water to make 100 ml of solution], 7.59 ml high purity water, 5 microliters of ethidium bromide in water solution (1 mg dye per ml solution) and 26.5 microliters of dithiothreitol in water solution (0.5 g/ml) were mixed well and poured into a shallow plastic tray, the ends of which had been taped in order to contain the copolymer solution. Just after pouring, a multi—toothed plastic well former was inserted into the gel perpendicular to the plane of the gel and perpendicular to the direction of electrophoresis. This assembly was permitted to stand in a covered container (so as to retard evaporation of water) for three hours, after which the gel was overlaid with a buffer solution made by a twenty—fold dilution of the concentrated buffer solution previously described. Then the plastic lane former was carefully removed and the tray (from which the tape at each end was also removed) was inserted into a horizontal electrophoresis cell, to which diluted buffer was added so that the face of the gel was one mm under the surface of the buffer. Then solutions of undenatured DNA fragments (double-stranded DNA) were loaded into the wells. Electrophoresis at 100V (voltage gradient 10 V/cm) for 73 minutes yielded good sample separation and resolution from 123 to at least 1434 base pairs. Example 8 a) Scaled-up synthesis of high molecular weight copolymer for very low polymer content gels. Into a 20 gallon capacity reaction vessel which initially contained a solution of 60.64 grams of ammonium persulfate dissolved in 25,000 grams of deionized (high purity) water, that had been and was continuously being purged with nitrogen, was thermostated at 50°C and was stirred at 112-120 rpm, were pumped two solutions: (i) starting at time 0 and ending 76 minutes later, a solution of 21.22 grams of sodium bisulfite dissolved in 404.3 grams of deionized water, (ii) starting at time one minute and ending 74 minutes later, a solution of 10,479 grams of acrylamide and 1,164 grams of N—(3—chloroacet- amidopropyDmethacrylamide dissolved in 34,953 grams of deionized water that had been purged with nitrogen and was kept under a nitrogen atmosphere. Solutions* (i) and (ii) were at ambient temperature (ca 25°C) before being added to the reaction but on addition to the well—thermostated reactor rapidly warmed to 50 β C. As these solutions were added, the viscosity of the reactor solution rose, indicating the formation of a high polymer. After 235 minutes, approximately 15,000 more grams of deionized water at 25°C were added, with stirring, to the reactor, and the reactor was allowed to cool. The final solution of copolymer in high purity water was 14.3% (w/w), and an aliquot of this solution was the stock solution for the gel made in Example 8b. An assay for the Cl content of the copolymer itself found 1.44%. (weight basis) as compared to 1.55% theoretical, indicating that the N-(3-chloroacet- amidopropyl)methacrylamide had been well incorporated into the acrylamide-rich chains. The average molecular weights of this particular acrylamide/N-(3-chloroacetamidopropyl)- methacrylamide copolymer were estimated at number average, M_ = 8.47(10 ), and weight average, IT = 20.2(10 ), using an aqueous gel permeation chromatography system in which (a) the fractionation was accomplished with BioGel TSK 60-XL, TSK 50-XL, TSK 30-XL and SEC 20-XL columns (Bio Rad Laboratories, 220 Maple Ave., Rockville Centre, NY 11571), (b) the eluent was 0.05 M Na £ S0 4 in 5% ethylene glycol-in-water (v/v), (c) the calibrating standards were poly(ethylene oxide) samples of known molecular weights ranging from 950 kDa down to 1.06 kDa (Scientific Polymer Products, 6265 Dean Parkway, Ontario, NY 14519), (d) the flow-rate was 1.0 ml/min and (e) the detection of solute in the effluent was done refractometrically. b) Forming a gel for DNA electrophoresis. Using the chemicals and procedures summarized in Example 7b above, a 3% (w/v) electrophoresis gel was made and used to separate undenatured DNA fragments. Excellent separation and resolution were achieved over the range of 72 to 2,674 base pairs.

Claims: CLAIMS : 1. A water-soluble copolymer characterized wherein it has the structure 10 wherein L is alkylene, arylene, a divalent heteroatom, -C-Z- , or a combination of two or more thereof, wherein Z is a divalent heterocyclic group having 5 to 15 7 nuclear carbon and heteroatoms. —A- represents recurring units derived from one or more polymerized acrylamide monomers, R is hydrogen or methyl, R 2' is a haloacetamido group, —B— represents recurring units derived from any other 20 polymerized vinyl monomers, and x, y, and z represent weight percents of the recurring units, x being about 50-90 weight percent, y-being about 1-50 weight percent, and z being about 0-45 weight percent. 2. The copolymer as claimed in claim 1 ,-, wherein L is an -(R 3 ) - (COXR 4 ) - (NHCO) - group where R is arylene or arylene-R — wherein 4 R is alkylene of 1 to 6 carbon atoms, X is —0- or —NH-, and k, m, and n are each 0 or 1, provided that k is 0 when m is 1, and m is 0 when k is 1. 2 Q 3. The copolymer as claimed in either of claims 1 and 2 wherein L is phenylene or phenylenemethylene. 4. The copolymer as claimed in any of claims 1 to 3 wherein -A- represents recurring units 3 5 derived from one or more of acrylamide, N-isopropylacrylamide and 2-acrylamido-2-hydroxymethyl-l,3-propanediol, N-isopropylacrylamide, N-hydroxymethylacrylamide, N-(l,l-dimethyl-3-oxo-butyl)acrylamide, N-methylmethacrylamide, 2-acryl- amido—2—hydroxymethyl—1,3—propanediol, methacryla¬ mide, 3-(3—dimethylaminopropyl)acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N—isopropylmethacrylamide, and 3—(2—dimethylamino- ethyl)acrylamide. . 5. The copolymer as claimed in any of claims 1 to 4 wherein —A- represents recurring units derived from acrylamide. 6. The copolymer as claimed in any of claims 1 to 5 wherein —B— represents recurring units derived from one or more styrene or styrene derivatives, divinyl, diacrylic and diacrylamide monomers. 7. The copolymer as claimed in any of claims 1 to 6 wherein —B- represents recurring units derived from divinylbenzene, ethylene diacrylate, ethylene dimethacrylate or N,N'—methylenebisacrylamide. 8. The copolymer as claimed in any of claims 1 to 7 comprising recurring units derived from N—(3—chloroacetamidopropyl)methacrylamide. 9. The copolymer as claimed in any of claims 1 through 8 characterized as being useful for making an electrophoresis medium for the electrophoretic separation of large molecules by crosslinking by a reaction with a selected crosslinking agent that does not involve a free—radical vinyl addition mechanism, the copolymer being derived from a minor proportion of a comonomer that contains a site for the crosslinking reaction with the selected crosslinking agent, the copolymer further having a " FTn greater than 50,000 and a " ITw greater than 100,000. 10. The copolymer as claimed in any of claims 1 through 8 characterized as being useful for making an electrophoresis medium by crosslinking by a reaction with a selected crosslinking agent that does not involve a free-radical vinyl addition mechanism, the copolymer being derived from a minor proportion of a comonomer that contains a site for the crosslinking reaction with the selected crosslinking agent, the preformed copolymer further having the following properties: 1) TT small enough that the addition of 1.25 to 1.5 times the stoichiometric amount of the selected crosslinking agent will not raise the viscosity of the mixture so much that the medium cannot be poured into a 0.15 x 14 x 14 cm mold within 4 to 10 minutes after addition of the crosslinking agent, and 2) M_ large enough to provide a crosslink density sufficient to form a gel after about 10 minutes after addition of 1.25 to 1.5 times the stoichiometric amount of the selected crosslinking agent, the gel ultimately having sufficient integrity within 2 hours after pouring to be removed from the mold and handled gently without tearing or falling apart. 11. The copolymer as claimed in claim 10 having a number average molecular weight, " M , of at least about 7,000 and a weight average molecular weight, w , of less than about 100,000. 12. The copolymer as claimed in either of claims 10 and 11 wherein the Η τ w, is such that the addition of the crosslinking agent will not raise the viscosity so much that the medium cannot be poured into the mold within 6 to 8 minutes after the addition. 13. The copolymer as claimed in any of claims 1 to 12 wherein the number of equivalents of crosslinking sites per gram of the copolymer is in the range of 0.45(10~ 4 ) to 14(10 *~4 ). 14. A method of preparing an electrophoresis gel n situ comprising mixing an aqueous solution of a copolymer as claimed in any of claims 1 through 13, with a selected crosslinking agent that will react with crosslinkable sites on the copolymer by a reaction with the selected crosslinking agent that does not involve a free-radical vinyl addition mechanism. 15. An element for electrophoresis which is characterized as comprising a crosslinked polymer which, before crosslinking with a crosslinking agent, is the copolymer as claimed in any of claims 1 to 13. 16. An electrophoresis element characterized as comprising a copolymer as claimed in any of claims 1 through 13, which copolymer has been prepared as a crosslinked gel by the method claimed in claim 14. 17. The use of the copolymer as claimed in either of claims 10 and 11, after crosslinking with a crosslinking agent, as a resolving gel. 18. The use of the copolymer as claimed in claim 9, after crosslinking with a crosslinking agent, as a stacking gel. 19. An electrophoresis element which comprises a resolving gel and a stacking gel, the element characterized wherein the resolving gel comprises a first copolymer as claimed in either of claims 10 and 11, and, the stacking gel comprises a second copolymer as claimed in claim 9, with each copolymer having been prepared as a crosslinked gel by the method claimed in claim 14. 20. The element as claimed in claim 19 wherein the resolving gel is prepared using an aqueous solution of the first copolymer which is present in a concentration of from 8 to 14% w/v, and the stacking gel is prepared using an aqueous solution of the second copolymer which is present in a concentration of from 1.5 to 6% w/v. 21. A kit for electrophoresis characterized as comprising the copolymer as claimed in any of claims 1 through 13, or a combination of the copolymer as claimed in any of claims 1 through 13, and in a container separate from the copolymer(s), a crosslinking agent for crosslinking the copolymer by a reaction that does not involve a free-radical vinyl 0 addition mechanism. 22. The invention as claimed in any of claims 14 and 16 to 21 wherein the copolymer is crosslinked with a crosslinking agent which is a compound having two or more amino, mercapto or 5 phenolic hydroxy reactive groups. 23. The invention as claimed in any of claims 14 and 16 to to 22 wherein the crosslinking agent is a bis mercaptan having in its backbone chain the -CH-CH group. 0 OH OH 24. The invention as claimed in any of claims 14 and 16 to 22 wherein the crosslinking agent is ethylenediamine, 1,3—propanediamine, 5 1,3-propanedithiol, dithioerythreitol, dithiothreitol, 1,5-pentanediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, propylenediamine, di(aminomethyl)ether, 1,8—diamino-4—(aminomethyl) Q octane, xylylenediamine, hydroquinone, bisphenol A, bisphenol sulfone, 1,4—butanedisulfinic acid, benzenedisulfinic acid, thioethanolamine, p—aminothiophenol or butylenediamine. 25. The invention as claimed in any of 5 claims 14 and 16 to 24 wherein the crosslinking agent is dithiothreitol or dithioerythreitol.

Inventors: CHRISTY KENNETH GEORGE JR, EIKENBERRY JON NATHAN, LA TART DAVID BREWER, OSTERHOUDT HANS WALTER, PONTICELLO IGNAZIO SALVATORE, CHRISTY, KENNETH, GEORGE, JR., EIKENBERRY, JON, NATHAN, LA TART, DAVID, BREWER, OSTERHOUDT, HANS, WALTER, PONTICELLO, IGNAZIO, SALVATORE
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1990
Main classification: G01N27
IPC codes: B01J13, C08F20, C08F220, G01N27