ABSTRACTSYNTHESES OF METAL-BINDING POLYMERS TO CREATE FUNCTIONAL FILMS THAT SELECTIVELY CAPTURE PROTEINSBySalinda Wijeratne Purification is often the most difficult step in producing proteins for both research and therapeutic applications. Among various protein-purification platforms, modified porous membranes are especially appealing because convective mass transport in small pores overcomes the diffusion limitations characteristic of bead-based columns. Polymer brushes are attractive for capturing proteins, but their high density may provide steric constraints on protein binding. I designed and synthesized several monomers with long, cleavable side chains. Removal of these side chains after polymerization should reduce brush chain density and provide the space necessary to capture large amounts of protein. Growth of the polymer brushes gave 100 nm-thick films, but unfortunately upon cleaving the side chains, the polymer brushes collapsed to prevent further functionalization. Additionally, synthesis of brush-modified surfaces is cumbersome, frequently requiring deposition of initiator molecules and polymerization under inert conditions. Thus, I developed much simpler methods for creating highly-swollen or porous films for protein binding. In an initial study, I synthesized the metal-binding polymers poly(N,N-dicarboxymethylallyl amine) (PDCMAA) and carboxymethylated polyethyleneimine (CMPEI). These polymers contains iminodiacetic acid groups, which readily form metal-ion complexes that may selectively capture proteins with hexahistindine clusters (His-tags) at their termini. LBL adsorption of multilayer protonated poly(allylamine) (PAH)/PDCMAA films is a simple, economical method to introduce metal-ion-binding groups onto a surface. Remarkably, 10-bilayer PAH/PDCMAA films are 1 μm thick, and these coatings have a very high Cu2+ binding capacity (~2.5 mmol/cm3 of film, or 2.5 M). However, PAH/PDCMAA films do not swell sufficiently for extensive protein capture. In contrast, sequential adsorption of PAH and CMPEI leads to membranes that bind Ni2+ and capture ∼60 mg of His-tagged ubiquitin per mL of membrane, which is higher than capacities of commercial beads. Compared to PDCMAA, the more hydrophilic polyethyleneimine in CMPEI might enhance swelling. In some cases minimizing the metal-ion leaching from membranes is important to avoid contaminating proteins. Therefore, I synthesized another series of polymers containing the stronger metal-ion-binding ligand nitrilotriacetate (NTA). Due to the high cost of commercial NTA derivatization reagents, I established a novel route to synthesize NTA-containing polymers, poly(NTA), at minimal cost. Sequential adsorption of PAH and poly(NTA) yields membranes that bind Ni2+ and capture ∼40 mg of His-tagged ubiquitin per mL of membrane. Introduction of porosity may enhance the kinetics of protein binding in polyelectrolyte films. Development of porous films through adsorption of star-poly(dimethylaminoethyl methacrylate) [PDMAEMA] and star-poly(acrylic acid) [PAA], creates highly swollen films that bind as much as 10-20 multilayers of lysozyme. Sequential adsorption of star-PDMAEMA and star-PAA leads to membranes that capture ∼120 mg of lysozyme per mL of membrane, which is about 3 times the capacity of commercial ion-exchange membranes.
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