Low-Molecular and Polymeric Organic Additives

It should be pointed out that the addition of substances, which could improve the biocompatibility of sol-gel processing and the functional characteristics of the silica matrix, is practiced rather widely. Polyethylene glycol) is one of such additives [110— 113]. Enzyme stabilization was favored by formation of polyelectrolyte complexes with polymers. For example, an increase in the lactate oxidase and glycolate oxidase activity and lifetime took place when they were combined with poly(N-vinylimida-zole) and poly(ethyleneimine), respectively, prior to their immobilization [87,114]. To improve the functional efficiency of entrapped horseradish peroxidase, a graft copolymer of polyvinylimidazole and polyvinylpyridine was added [115,116]. As shown in Refs. [117,118], the denaturation of calcium-binding proteins, cod III parvalbumin and oncomodulin, in the course of sol-gel processing could be decreased by complexation with calcium cations. 

These examples demonstrate that additives can have a beneficial effect on the entrapped biopolymers. Unfortunately, they are generally not universal. The additives need to be found for individual immobilized biopolymers and that is not so easy to do. For instance, lactate oxidase retained its activity in a silica matrix if the enzyme was taken as a complex with poly(N-vinylimidazole) prior to the immobilization, but the polymer did not stabilize glycolate oxidase [87,114], Its stabilization was observed after an exchange of poly(N-vinylimidazole) for poly(ethyleneimine). This is a decisive disadvantage of the approaches because they do not offer a general solution that might be extended to any immobilized biopolymer. 

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