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Protein surface recognition by polymers

Sun, Wei

Recognition of protein surfaces using synthetic receptors is an effective strategy for providing access to enzyme inhibitors, protein antagonists, and diagnostic biosensors. In the artificial protein receptors, polymers represent promising fields due to their sizes (commensurate with proteins) and the ability to tailor a wide range of functionalities. Furthermore, the ability of polymers to adapt their conformations to protein surfaces renders them attractive candidates for protein surface binding. This work emphasized firstly on the synthesis of functional monomers which are specific for the amino acid residues on protein surfaces, and then on the development of polymers which can recognize protein surfaces in aqueous solution tightly and selectively. Eighteen monomers were prepared by multi-step synthesis. The interactions between their functional groups and amino acid residues covered several binding motifs in molecular recognition: hydrogen bond, electrostatic, hydrophobic, metal-ligand and π-cation interaction. For the later polymerization, these functional groups were incorporated with polymerizable structures. Thirteen of them were based on methylacylamide, and the other five were based on a 10, 12- tricosadiyne structure. Functional monomers based on methylacrylamide were polymerized by radical polymerization under heat or UV light in organic solvent or aqueous solution, with AIBN, V-50, benzophenone or ammonium persulfate as initiators. In some cases polymer lengths were controlled by RAFT method with water-soluble S, S´-bis(α,α’- dimethyl-α’’acetic acid)-trithiocarbonate as the chain transfer reagent (CTA) in aqueous buffer. Functional monomers based on a 10, 12- tricosadiyne structure composed of two parts: a polar functional headgroup and a hydrophobic tail containing the diacetylene moiety. The polymerization could proceed without any initiator, only when the diacetylenes were arranged in a lattice with appropriate geometry, ex., at water-air surface or in the form of liposome. In this work, polymers were prepared mainly in several forms: dendrimers, linear polymers, grafted polymers on PET membrane and crosslinked polymer materials. The binding events between these polymers and proteins were studied either in solution or at interface by fluorescence, UV, ITC titrations, BCA assay, HPLC and computer modeling. To prepare the dendrimers, the lysine and arginine specific monomer, bisphosphonate dilithium salt, was incorporated on the periphery of different polypropyleneimine (PPI) generations by reductive amination with NaBH4. They were purified with dialysis with a 1 KDa membrane. Their 1H and 31P NMR spectra in D2O displayed clear sets of slightly broadened signals, however MALDI TOF-MS showed that the dendrimers were not monodisperse. The binding events between proteins/peptides and these dendrimers were studied in buffered aqueous solution by three different spectroscopic methods (PFGLED, UV/Vis, and fluorescence). Results showed that the single weak binder (Ka in pure water <10 M -1) was turned into powerful dendrimeric receptors for basic proteins (KD <250 nM for the hexadecamer and histone H1), and the binding affinity and stoichimetry didn’t only depend on the dendrimer size, but also on the size and distribution of basic domains on protein surfaces. Series of linear copolymers were prepared with methylacrylamide based monomers by radical polymerization under 60°C with AIBN or V-50 as initiators. After preparation the polymers were purified by precipitation in ethylacetate and in some cases by dialysis. Several copolymers decorated with o-aminomethylphenylboronates for covalent ester formation and/or alkylammonium ions for noncovalent Coulomb attraction, showed exceptionally high affinities for a series of glycans: heparin, chondroitin-4-sulfate, hyaluronic acid and dextran. Heparin, a constant repeat of a 1, 4-glycosidic sugar dimer, carrying hydroxycarboxylates (iduronic acids) and hydroxysulfates (glucosamine), could be quantitatively detected with an unprecedented 30 nM sensitivity. One of these polymers was used to develop a fluorescence microplate assay for quantification of heparins and other sulfated carbohydrates by a cooperation group from Kiel, Germany. Based on these results, we proposed a new 7-member-ring binding scheme between the boronate and a sulfated ethylene glycol or aminoethanol unit, and this propose was studied by NMR titration and computer modeling with D-glucosamine 2-sulfate sodium salt (a fragment of heparin) and 1:1 complex of phenylboronic acid and piperidine. For the preparation of affinity membrane, track-etched PET400 membrane was grafted with a copolymer with 1:4 of bisphosphonate ester monomer and amino alcohol monomer via a synergist immobilization method from acetonitrile solution. In the synergist immobilization method, the synergist (tertiary amino groups) for photo-initiator benzophenone (BP) was introduced onto the membrane surface via an aminolysis reaction with diethylenediamine (DEEDA). The polymer formation was examined with solid state 31P NMR spectrum, contact angle and water permeability measurements. The resulting affinity membrane showed high binding capacity for selected proteins. Especially, it was found that markedly higher binding capacity and affinity have been achieved for lysozyme than for cytochrome C, both proteins with similar pI value and protein size. With this affinity membrane, the protein separation has been realized in the 1:1 mixture solution of lysozyme and cytochrome C with a very high selectivity. Crosslinked polymer materials for protein recognition and separation were developed in two ways, water-soluble microgels and MIPs (molecular imprinting polymers). All microgels were prepared by radical copolymerization in dilute aqueous solution. A typical monomer feed composition consisted of 80 mol% N-isopropylacrylamide, 10 mol% of crosslinker methyl bisacrylamide, and 10 mol% of an anionic comonomer such as sodium methacrylate, tetrazolate or bisphosphonate. Polymerizations were carried out at 70°C in the presence of a surfactant (sodium dodecylsulfate) and an initiator (ammonium persulfate). Microgels were subsequently purified by ultrafiltration against deionized water, using cross-flow membranes with a 100 kDa and 2 mm cut-off, to remove low-molecular-weight impurities and macroscopic gel particles, respectively. ITC titrations showed that they could recognize protein guests in buffered aqueous solution at neutral pH with Ka values of up to 1011 M-1 (averaged affinity towards each single protein). Switching between the functionalities allowed distinction between basic proteins of similar PI and size. Nearly all the other bindings was entropy driving. The research of MIPs with IgG as the final recognition target is still being under research. In this thesis only a little finished part is introduced. In the previous work, an exposed 9-mer fragment from IgG C-terminal was used as a template, and for comparing imprinting effects, a shorter peptide with only six amino acids but having more effective binding sites with respective to my monomers, as the other template. MIPs were prepared with ethylenebisacrylamide (crosslinker), methacrylamide (comonomer) and functional monomers in 10 mM hepes buffer on a microplate with the aiding of a liquid-handling robot. Until now only one functional monomer was used for each MIPs. For each functional monomer, six polymers were prepared. Three of them were MIPs (with template in polymerization) with varied crosslinker percentages (20%, 50% and 70%), and the other three were corresponding NIPs (without template in polymerization) as references. After the preparation, polymers were washed with buffer and 10 nM HCl solution until no residue signal could be observed in HPLC measurement. The following rebinding experiments were performed at three concentrations, 50 μM, 500 μM and 5000 μM of the template. The total template amount in 5000 μM solution was same as the original used in MIPs preparation. Between each rebinding, polymers were washed as described above. All rebinding experiments were monitored by HPLC and BCA assay. Results showed that several monomers had comparatively good imprinting effects. The larger template showed a better effect than the shorter one. No apparent difference was observed for the three crosslinker percentages. These results provided us much information for the further development.


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Sun, Wei: Protein surface recognition by polymers. 2009.


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