METHOD OF COVALENTLY BONDING BIOPOLYMERS TO HYDROPHILIC SURFACES

Abstract: The invention describes a novel method of covalently bonding biopolymers, such as protein, to an organic polymer surface coated with a hydrophilic uncharged layer, by conducting the immobilisation reaction in a nonpolar or substantially nonpolar reaction medium. The especially preferred reaction medium is a microemulsion suitably containing 0.5-25 % by weight of water.

Description: M ET H OD O F CO V A LENTLYBONDINGBIOPOLYMERSTOHYDROPHILICSURFACES The immobilisation of proteins to solid surfaces is an established technique within a number of applications, such as solid phase diagnostics, analysis with biosensors, affinity chromatography, extracorporeal therapy, and bio- organic synthesis. In all of these cases, the protein which may be a pure protein or a conjugate, such as a glycoprotein or a lipoprotein, is bonded to a solid sur¬ face, whereupon the biological activity of the protein is utilised for a specific purpose. In solid phase diagnostics, an antibody is frequently immobilised to a plastic surface, usually consisting of polystyrene. When in contact with a body fluid, the immo¬ bilised antibody bonds any antigen that may be present. The antibody-antigen complex is then detected by means of a labelled antibody. The labelling may be in the form of a radioactive isotope, a fluorescent group, or an enzyme conjugate. In extracorporeal therapy, a biologically active sub¬ stance is bonded to a chamber through which the patient's blood is conducted. A current example of extracorporeal therapy is hemoperfusion across an immobilised immunosti- mulating substance. Interferons and interleukins are exam¬ ples of such substances. Examples of diseases that can be treated by this technique are cancer and AIDS. In bio-organic synthesis, use is made of enzymes for producing organic compounds. An appropriate use for bio- organic synthesis are lipid transformations, i.e. trans¬ forming a lipid, usually a triglyceride, into another. Most enzymes are expensive, and frequent reuse is neces¬ sary to ensure good process economy. In view hereof, the use of immobilised enzymes is of interest to most large- scale enzymatic processes. As mentioned above, the immobilisation of proteins to both organic and inorganic surfaces is today a well-estab¬ lished technique (see chapter 4 in "Principles of Immobi¬ lization of Enzymes", Handbook of Enzyme Biotechnology, Second Edition, Ellis Horwood Limited, 1985), and it is possible to bond a large amount of protein to the surface while retaining adequate biological activity. However, it has been found that most solid surfaces are so designed that they adsorb spontaneously proteins and other biomolecules. Adsorption in aqueous solution is promoted primarily by two types of physical forces, elec¬ trostatic attraction and hydrophobic interaction. Most surfaces are, at normal pH, negatively charged, but usual¬ ly contain also hydrophobic domains. A protein usually has both positive, negative and hydrophobic seats, which means that a protein is attracted to most surfaces, on the one hand by electrostatic attraction between positive seats and negatively charged groups in the surface and, on the other hand, by hydrophobic interaction between hydrophobic domains of the protein and the surface. This is described in, for example, "Surface and Interfacial Aspects of Bio- medical Polymers", Ed. J.D. Andrade, Plenum Press 1985, Vol. 2, p 81. This nonspecific adsorption is an undesired pheno- menon for the above-mentioned applications. In solid phase diagnostics, it results in an impaired sensitivity and a shorter life of the diagnostic kit. In both extracorporeal therapy and in bio-organic synthesis, spontaneous adsorp¬ tion causes impaired activity and a shorter kit life. One way of drastically reducing the adsorption of proteins and other biomolecules on organic polymer sur¬ faces is to provide the surfaces with a layer of an uncharged hydrophilic polymer. One example of a polymer that has been used for this purpose is polyethylene glycol (see C.-G. GOlander, "Preparation and Properties of Func- tionalised Polymer Surfaces", Dissertation, Royal Insti¬ tute of Technology, Stockholm 1986), but other substances, such as polysaccharides, for example dextran, cellulose ethers and starch; polyvinyl alcohol; and neutral silica sol have also been used for this purpose. By coating the organic polymer surface with a layer of the uncharged hydrophilic polymer, both electrostatic attraction and hydrophobic interaction can be avoided. In this manner, the spontaneous adsorption of proteins to such surfaces can be reduced and occasionally almost entirely eliminated. Hydrophilised surfaces of this type are of great interest to, inter alia, the above-mentioned applications of immobilised proteins. To covalently bond protein to such a surface, it is necessary to introduce into the hydrophilic layer reactive functional groups serving as anchoring points for the protein. However, it has proved extremely difficult to covalently bond biopolymers, such as proteins, to thoroughly hydrophilised organic polymer surfaces, even if the surfaces contain a high concentra¬ tion of reactive groups. The uncharged hydrophilic surface does not attract protein. On the contrary, it acts as a repellent because it is energetically unfavourable for a protein in aqueous solution to approach such a surface. As a result, the amount of immobilised protein usually will be low, regardless of whether it is an antibody for solid phase diagnostics, an immunostimulating substance for extracorporeal therapy, or an enzyme for bio-organic syn¬ thesis. Thus, it is difficult to both adsorb and covalent¬ ly bond proteins from an aqueous solution to hydrophobic polymer surfaces coated with a thoroughly covering layer of a hydrophilic uncharged polymer, for example polyethy¬ lene glycol or a polysaccharide. Where mineral surfaces are concerned, the situation is somewhat different. A recently published work on the immobilisation of lipase on silica shows that consider- able amounts of protein can be immobilised also on silica treated with polyethylene glycol. Coupling occurs to reactive groups on the free ends of the polyethylene glycol (M.-B. Stark and K. Holmberg: "Biotech. Bioeng." 34 (1989) 942). This work describes the immobilisation of the protein to the polyethylene glycol-modified surface both from an aqueous solution and from a hexane solution, as well as from a microemulsion containing 94% hexane, 5% tenside and 1% water. A microemulsion is an isotropic and thermodynamically stable solution of a hydrophobic component, water and a surface-active component. The authors show that practically the same amount of lipase is bonded to the polyethylene glycol-coated silica sur¬ face in the three experiments conducted with different solvents. Bonding seems to occur independently of the polarity of the solvent. Surprisingly, it has now been found that biopolymers can be immobilised with good yield to hydrophobic polymer surfaces coated with a layer of a hydrophilic uncharged polymer, such as polyethylene glycol or a polysaccharide, if the immobilisation reaction is carried out in a non¬ polar or substantially nonpolar reaction medium. By "bio- polymer" primarily protein is meant, but the term also comprises lipids, carbohydrates and lipopolysaccharides. The observation made according to the invention is highly interesting since it implies that the surface, which by hydrophilisation has been made repellent to proteins from aqueous solution, in this manner can be functionalised with proteins, for example antibodies and antigens. When the reaction is over, the surface is suitably leached in water, causing the uncharged hydrophilic polymer layer to be rehydrated, as a result of which the protein is bonded to a highly protein-repellent base. The present invention thus comprises a novel method of covalently bonding biopolymers to hydrophilised organic polymer surfaces. The method is highly generic and may be expected to find wide application. The basic principle of the invention is that the immobilisation reaction is not carried out in water which is the conventional medium for reactions with biopolymers, but in a nonpolar or substan- tially nonpolar reaction medium. By "substantially non¬ polar reaction medium" is here meant that its dielectric constant is less than 10%, preferably less than 5%, of the dielectric constant of pure water. It has been found that the method according to the invention, under the same con¬ ditions in other respects, frequently increases the amount of bonded biopolymer by more than 10 times. An especially preferred form of reaction medium is a microemulsion. The amount of water in the microemulsion usually is 0.5-25% by weight, preferably 1-15% by weight. The nonpolar reaction medium and the hydrophobic com¬ ponent in the microemulsion usually are an aliphatic hydrocarbon, such as hexane or nonane, or a broader distillation fraction, such as petroleum ether 60-80. The hydrophobic component of the microemulsion usually con¬ stitutes 63-98.5% by weight. The surface-active component usually is a combination of a surface-active compound and a so-called auxiliary tenside. The surface-active substance may be anionic, cat- ionic, amphoteric or nonionic, while the auxiliary tenside usually is an alcohol or a low-molecular alkylene oxide adduct. Examples of conventional substances of this type are butanol, pentanol, hexanol, ethylene glycol monobutyl ether and diethylene glycol monobutyl ether. The amount of surface-active component usually constitutes 0.5-20% by weight of the weight of the microemulsion. It has been found especially advantageous to use a surface-active compound capable of forming microemulsions in the absence of an auxiliary tenside. Surface-active compounds having this ability are certain nonionic com¬ pounds which as hydrophilic group have a polyalkylene glycol chain produced by polymerisation of ethylene oxide or by combinations of ethylene oxide and propylene and/or butylene oxide, as well as certain ionic compounds having the ionic hydrophilic group in a non-terminal position on the hydrocarbon chain. As regards nonionic tensides, the preferred hydro¬ philic part is a polyethylene glycol chain which, in the case most preferred, has a length of between 3 and 8 ethy¬ lene oxide units on an average. The hydrophobic part may derive from hydroxyl compounds or carboxyl compounds con¬ taining an alkyl chain which consists of 8-20 carbon atoms, or of an alkyl aryl group which consists of 9-24 carbon atoms in all. Examples of such compounds are ethy¬ lene oxide adducts of nonyl phenol, octyl phenol and fatty alcohols. The preferred ionic tensides are anionic groups, such as sulphonate, sulphate, carboxylate, phosphate and phos- phonate, sulphonate being especially preferred. If desir¬ ed, these tensides may also contain alkylene oxide groups, such as ethylene oxide, as coupling agents between the anionic group and the hydrophobic group. The hydrophobic part may consist of an alkyl chain consisting of 10-22 carbon atoms, or of an alkyl aryl group consisting of 9-24 carbon atoms in all. A few ether, ester or amide bonds may occur in the hydrophobic part. Examples of suitable ionic compounds are di(2-ethylhexyl)sulphosuccinate and carboxy- methylated nonyl phenol ethoxilates containing 1-4 ethy¬ lene oxide groups. To covalently bond the hydrophilic polymer to the solid polymer surface, and to covalently bond the bio- polymer to the hydrophilic polymer, reactive functional groups are introduced in conventional manner to serve as anchoring points. Examples of reactive groups that can be attached to or occur on the solid polymer surface are amino, carboxyl or hydroxyl groups with which the hydro¬ philic nonionic polymer or an activated form thereof can react. The hydrophilic polymer preferably comprises reac¬ tive groups, such as epoxy, tresylate, carbonyl imidazole and acyl azide groups capable of reacting with reactive groups on the solid polymer surface and with protein which is normally bonded via one or more of its amino, thiol and/or phenolic hydroxyl groups. This immobilisation tech- nique has been described in detail in, inter alia, the above-mentioned references "Surface and Interfacial Aspects of Biomedical Polymers", Ed. J.D. Andrade, Plenum Press 1985, Vol. 2, p 81, and C.G. Golander: "Preparation and Properties of Functionalized Polymer Surfaces", Dis¬ sertation, Royal Institute of Technology, Stockholm 1986, which are hereby included as part of the description of this invention. In the event that the protein is a glyco¬ protein, glycolipip or carbohydrate, bonding may occur to aldehyde groups generated in the carbohydrate moiety by oxidation with, for example, sodium periodate. It is also possible to anchor the hydrophilic polymer to the solid polymer surface in conventional manner by physical adsorption. The nonionic hydrophilic polymer suitably is poly¬ ethylene glycol or randomly distributed or block-distri¬ buted polyalkylene glycols between ethylene oxide and alkylene oxides having 3-4 carbon atoms, or tetrahydro- furan. Other groups of suitable polymers are adducts of ethylene oxides, optionally in combination with higher alkylene oxides or tetrahydrofuran, with a dihydroxy or polyhydroxy compound, such as glycerol and pentaerythri- tol. Polysaccharides, such as dextran and starch; cellu¬ lose ethers, such as methyl cellulose, methyl hydroxy- propyl cellulose or ethyl hydroxyethyl cellulose; poly- vinyl alcohol; and neutral silica sol are other hydro¬ philic polymers suitable for use with the present inven¬ tion. The hydrophilic polymers are water-soluble and their polymer weight usually is from 1,000 to 200,000, prefer¬ ably from 2,000 to 100,000. The invention will be further illustrated by the following Examples. Example 1 Immobilisation of IgG antibody to hydrophilised poly¬ styrene surface A polystyrene plate having the dimensions 2 x 2 cm was made carboxyl-functional by plasma polymerisation of acrylic acid on the surface. The carboxyl-functional group was treated with a 10% solution of diamino propane in water in the presence of water-soluble carbodiimide (0.6%) at pH 4.5-5.0. The plate was rinsed with distilled water and treated with a 10% solution of diepoxidised polyethy¬ lene glycol of molecular weight 4,000 at pH 9.5 for 15 h at room temperature. To a microemulsion (about 3% of the dielectric con¬ stant of water) containing 86% isooctane, 4% sodium bis(2- ethyl hexyl)sulphosuccinate and 10% water (carbonate buf¬ fer, pH 9.5) was added 0.05 mg/ml IgG. After mixing, the hydrophilised plate was treated in this mixture for 4 h at 40°C. Parallelly, a hydrophilised plate (a) as above and a plate (b) hydrophilised with polyethylene glycol without epoxide groups were treated in aqueous carbonate buffer of pH 9.5. The amount of immobilised IgG was measured spectro- photometrically with enzyme-conjugated antibodies against IgG according to ELISA technique. The amount of adsorbed proteins is proportional to the absorbency at 490 mm. Sample OP 495 mm Hydrophilised plate (a) with epoxide groups 0.20 Hydrophilised according to the invention 2.21 Hydrophilised plate (b) without epoxide groups 0.17 Example 2 Immobilisation of gamma-Interferon to a hydrophilised PVC surface A PVC plate having the dimensions 12 x 8 cm and intended for an extracorporeal chamber was grafted with crotonic acid under irradiation with UV light of wave¬ length 320 nm and in the presence of benzophenone as ini¬ tiator. The resulting carboxyl-functional surface was treated with a 10% solution of polyethylene imine in water in the presence of water-soluble carbodiimide (0.6%) at pH 4.5-5.0. The plate was then treated with a 10% solution of polyethylene glycol 4000 dicarbonyl imidazole at pH 8.0 for 3 h at room temperature. To a microemulsion (about 3% of the dielectric constant of water) containing 82% isooc- tane, 8% pentaethylene glycol mono(n-dodecyl ether) and 10% water (borate buffer, pH 8.0) was added 50,000 gamma- Interferon. The plate was treated with this microemulsion for 2 h at 40°C. Peripheral mononuclear blood cells were isolated by gradient centrifugation on Lymphoprep. The cells were diluted to l«10 6 /ml in RPMI 1640, 10% FCS, 1% PEST and incubated on the gamma-Interferon-immobilised plate for 7 days at 37°C and 5% C0 2 - As reference, use was made of a plate which after hydrophilisation was treated with gamma-Interferon in aqueous borate buffer of pH 8. Neopterin was used as a marker for cell stimulation. It is well known that lymphocytes and macrophages secrete neopterin when stimulated by, inter alia, gamma-Inter- feron. Neopterin (nmol/1) Immobilisation according to the invention 67 Reference Example 3 Comparison between adsorption and immobilisation on a glass surface and on a polystyrene surface A glass surface having the dimensions 2 x 6 cm was washed first with ethanol, then with 1 M HC1 and finally with 30% hydrogen peroxide. The plate was then reflux boiled for 16 h in 150 ml of a 10% solution of 3-amino propyl trimethoxy silane in toluene. To the glass amino- functionalised in this manner was added a solution con- sisting of 10 g diepoxidised polyethylene glycol (mole¬ cular weight 1,500) in 40 g 0.1 M carbonate buffer of pH 9.0. After 24 h at 37°C, the glass plate was removed from the solution. ESCA analysis shows that a dense poly¬ ethylene glycol layer has been formed on the surface. Then a solution of 0.05 mg/ml IgG antibody in 0.1 M carbonate buffer, pH 9.0, was added to the plate. The immobilisation reaction was finished after 24 h at 37°C. A corresponding immobilisation of IgG to the modified glass surface was made also from a microemulsion, use being made of the same microemulsion composition and the same reaction conditions as in Example 1. A polystyrene plate having the same dimensions as the glass plate was activated with a solution of KMnO. in con¬ centrated H 2 S0 4 for 30 sec. at 20°C. The plate was then made amino-functional by adsorption of polyethylene imine, whereupon treatment with polyethylene glycol derivative and immobilisation of IgG antibodies, both from water and from microemulsion, was carried out in the same way as described above for the glass plate. The amount of immobilised IgG antibody was determined spectrophotometrically with enzyme-conjugated antibodies against IgG according to ELISA technique. Nonspecific adsorption of the enzyme-conjugated antibodies was also determined on polyethylene glycol-modified plates which had not been subjected to protein coupling, but instead had been made nonreactive by treatment with 1 M HC10. for 1 h at 20°C, the epoxide rings at the ends of the poly¬ ethylene glycol chains being opened. The amount of covalently bonded and adsorbed IgG on the modified glass and polystyrene plates is shown in the Table below. The amount of adsorbed proteins is propor¬ tional to the adsorbency at 490 nm. Sample OP 495 nm Hydrophilised glass plate, covalent bonding from water 2.30 Hydrophilised glass plate, covalent bonding from microemulsion 2.15 Hydrophilised glass plate, adsorption from water 2.05 Hydrophilised polystyrene plate, covalent bonding from water 0.25 Hydrophilised polystyrene plate, covalent bonding from microemulsion 1.90 Hydrophilised polystyrene plate, adsorption from water 0.15 The results indicate that good immobilisation of pro¬ teins is obtained when carried out in a reaction medium according to the invention. If the carrier is a hydro¬ philic inorganic material, the effect of the reaction medium on the immobilisation is marginal, and at the same time a high undesirable adsorption is obtained.

Claims: CLAIMS 1. A method of covalently bonding biopolymers to an organic polymer surface coated with a hydrophilic uncharged layer, the solid polymer serving as carrier being coated with a nonionic hydrophilic polymer having groups capable of reacting with the protein, c h a ¬ r a c t e r i s e d in that the biopolymer is caused to react with the reactive groups of the nonionic hydrophilic polymer in the presence of a reaction medium having a dielectric constant of less than 10% of the dielectric constant of pure water. 2. A method as claimed in claim 1, c h a r a c - t e r i s e d in that the reaction is carried out in a reaction medium having a dielectric constant of less than 5% of the dielectric constant of pure water. 3. A method as claimed in claim 2, c h a r a c ¬ t e r i s e d in that the reaction is carried out in a reaction medium consisting of aliphatic hydrocarbons. 4. A method as claimed in claims 1-2, c h a r a c ¬ t e r i s e d in that the reaction is carried out in a microemulsion having a water content of 0.5-25% by weight. 5. A method as claimed in claim 4, c h a r a c - t e r i s e d in that the reaction is carried out in a microemulsion having a water content of 1-15% by weight. 6. A method as claimed in claims 4-5, c h a r a c ¬ t e r i s e d in that the biopolymer is a protein which is bonded, via at least one of its amino, thiol and phe- nolic hydroxyl groups, to an epoxy, tresylate, carbonyl imidazole or acyl azide group in the hydrophilic polymer. 7. A method as claimed in claims 4-6, c h a r a c ¬ t e r i s e d in that the protein is bonded to a poly¬ ethylene glycol having reactive groups. 8. A method as claimed in claims 4-7, c h a r a c ¬ t e r i s e d in that the surface active component is a nonionic compound in which the hydrophilic group consists of 3-8 ethylene oxide units on an average. 9. A method as claimed in claims 4-7, c h a r a c ¬ t e r i s e d in that the surface-active component is a sulphonate. 10. A method as claimed in any one of claims 5-9, c h a r a c t e r i s e d in that the reaction is carried out in a microemulsion containing 0.5-20% by weight of a surface-active component containing a sulphonate and/or a nonionic compound in which the hydrophilic group consists of 3-8 ethylene oxide units on an average and 63-98.5% by weight of one or more aliphatic hydrocarbons.

Inventors: BERGSTROEM KARIN, HOLMBERG KRISTER, BERGSTROEM, KARIN, HOLMBERG, KRISTER
Applicants: BEROL NOBEL AB
Application year: 1991
Main classification: C07K17
IPC codes: C07K17, C12N11, G01N33