Protein complexes with polyelectrolytes

Specific interactions between starch and proteins were observed as early as the beginning of the twentieth century. Berczeller996 noted that the surface tension of aqueous soap solutions did not decrease with the addition of protein (egg albumin) alone, but it did decrease when starch and protein were added. This effect was observed to increase with time. Sorption of albumin on starch is inhibited by bi- and trivalent ions and at the isoelectric point. Below the isoelectric point, bonding between starch and albumin is ionic in character, whereas nonionic interactions are expected above the isoelectric point.997 The Terayama hypothesis998 predicts the formation of protein complexes with starch, provided that starch exhibits the properties of a polyelectrolyte. Apart from chemically modified anionic starches (such as starch sulfate, starch phosphate, and various cross-linked starch derivatives bearing ionized functions), potato starch is the only variety that behaves as a polyelectrolyte. Its random phosphate ester moieties permit proteins to form complexes with it. Takeuchi et a/.999-1002 demonstrated such a possibility with various proteins and a 4% gel of potato starch. 

With respect to the ion-selective electrodes themselves, there are several sources of error to be considered. First, the ions of interest may be in a form to which the electrode is insensitive. For example, if the ions are in an insoluable or complexed form, such as bound to cellular constitutents or structures (e.g., the cell membrane), or complexed with polyelectrolytes (i.e., proteins). 

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. 

PPEs in water and prevent aggregation and self-quenching [53]. Heeger and co-workers paired anionic PPV with a nonconjugated cationic polymer to form a charge-neutral complex and saw both reduction of nonspecific effects and some loss in sensitivity in protein detection [54]. Polyelectrolytes may be best used in assay schemes, such as DNA hybridization assays or drug discovery screens, where the assay conditions are well controlled and nonspecific interactions reduced or avoided. 

From the physics point of view, the system that we deal with here—a semiflexible polyelectrolyte that is packaged by protein complexes regularly spaced along its contour—is of a complexity that still allows the application of analytical and numerical models. For quantitative prediction of chromatin properties from such models, certain physical parameters must be known such as the dimensions of the nucleosomes and DNA, their surface charge, interactions, and mechanical flexibility. Current structural research on chromatin, oligonucleosomes, and DNA has brought us into a position where many such elementary physical parameters are known. Thus, our understanding of the components of the chromatin fiber is now at a level where predictions of physical properties of the fiber are possible and can be experimentally tested. 

Polysaccharides, being polyelectrolytes, could form ternary complexes with the protein-tannin aggregate, enhancing its solubility in aqueous medium. 

One example of a practical application where polyelectrolytes are of crucial interest are immunoassays, where charged polymers are attached to surfaces and are then exposed to protein solution with the aim of loading of the polymer layer with a reproducible amount of these proteins. The protein-loaded particles or planar films thus obtained are in turn exposed to analyte solutions containing other proteins. If the proteins in the film match the proteins in solution, protein-protein complexes are formed, which are then visualized and/or quantified. In order to complete such a process successfully knowledge about the swelling of the polyelectrolyte layer in buffer solutions and the interaction of such a layer with proteins has to be established. Further questions, which are of great importance for such polyelectrolyte systems are the behavior of the monolayers in contact with common impurities present in contacting solutions especially traces of multivalent ions or tensides. 

The ability of polyelectrolytes to remove oppositely charged proteins from solutions has been exploited through the incorporation of protein-polymer precipitation steps into a variety of protein purification procedures, wherein precipitated proteins are recovered from the insoluble complex aggregate via redissolution by pH or ionic strength adjustment (14-16). Furthermore, preferential complexation of polyelectrolytes with specific proteins has been substantiated (13). Although there are reports of the optimization of bulk complexation yield through the adjustment of solution parameters such as pH and ionic strength (17), very little has been accomplished in the optimization of the selectivity of complex formation. 

FIG. 11 Schematic illustration of the different types of complexes (a) with long polymers (interchain aggregates are formed essentially in the case of Coulombic complexation of polyelectrolyte and protein) (b) with short amphiphilic polymers (hydrophobic association). 

Functionalization of nanorods with polyelectrolytes has been carried out by layer-by-layer deposition (92). First, CTAB-coated nanorods are prepared. Since these nanorods are positively charged, they can adsorb cationic and anionic poly electrolytes. Functionalization of nanorods with dyes is possible a fluorescent dye, 4-chloro-7-nitrobenzofurazan has been functionalized on the surface of Ti02 nanorods (93). Functionalization with a photoactive molecule such as ruthenium(II) tris(bipyridine) is also possible (94). A thiol derivative of the bipyridyl complex (Ru(bpy)3+-Cs-SH) in dodecane thiol is used for the functionalization of gold nanorods. Functionalization of block magnetic nanorods is very useful (95), for example, in the separation of proteins. Consider a triblock nanorod consisting of only two metals, Ni and Au. If the Au blocks are functionalized with a thiol (e.g. 11-amino-1 undecane thiol) followed by covalent attachment of nitrostreptavidin, then one can... 

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