Polyelectrolyte-protein complexes solubility

Polyelectrolytes Binding to the protein may increase solubility high molecular weight protein complexes may also form which could decrease solubility Increased charge density or conformational change Potential pharmacological effect or toxicity of the excipients... 

Figure 5.19 Schematic depiction of a particle covered with a brush consisting of a polyelectrolyte block (dark line) and a neutral (water soluble) polymer block (gray line). Upon the addition of oppositely charged protein molecules (gray spheres), the polyelectrolyte block complexes with the protein, forming a net neutral phase. The neutral polymer block stabilizes this particle.

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

Earlier work (9) has shown that the size of the precipitate, but not the protein recovery, depends on the method of addition of the polymer to the protein solution. Mixing conditions in the precipitation vessel also affect the precipitate size (lOh The solubility of the protein-polyelectrolyte complex depends strongly on the solution conditions—pH, ionic strength, polymer dosage level, and the nature of the protein and polyelectrolyte. These factors are discussed below ... 

Xia J., Dubin P. L., Kim Y., Muhoberac B. B., Klimkowski V. J. Electrophoretic and quasi-elastic light scattering of soluble protein-polyelectrolyte complexes. J. Phys. Chem. 1993 97 4528-4534. 

Complex coacervation is a phase-separation process, in which two water-soluble polymers are brought to physicochemical conditions leading to their desolvation. The desolvated polymers separate from the equilibrium phase, in the form of a coacervate, that tend to deposit onto dispersed core materials. The polymers forming the coacervate can be two polyelectrolytes susceptible of bearing opposite net charges, and the dispersed cores can be oily droplets of an L/H emulsion. Pectin has been associated to soy proteins to encapsulate propolis, a polyphenol-rich compound produced by bees, for stabilization, water dispersibility, and controlled release in food (Figure 36.9). ... 

Cationic polymers are of particular interest carriers in drug and gene deliveiy because of their ability to promote cellular uptake.This holds also for the delivery of proteins, especially for those that possess an overall anionic charge at pH values above the isoelectric point (p/). These proteins can form soluble, nanosized, polyelectrolyte complexes with natural or synthetic cationic polymers by simply mixing the oppositely charged protein and polymer that self-assemble by electrostatic attraction, as represented in Scheme 14.1. 

Apart from specific interactions between a target protein and a ligand-polymer conjugate, nonspecific interactions of protein impurities with the polymer backbone could take place. The nonspecific interactions limit the efficiency of the affinity precipitation technique, and significant efforts were made to reduce these interactions. The advantage of polyelectrolyte complexes as carriers for affinity precipitation is low nonspecific coprecipitation of proteins when the polymer undergoes a soluble-insoluble transition (10). 

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