No abstract

A statistical thermodynamic model based on the lattice model proposed by Baskir et al. (1987) was employed to predict the solubilities of globular proteins such as ovalbumin and lysozyme a t their isoelectric points in dextran solutions of different molecular weights. The model accounted for protein-polysaccharide and polysaccharide-solvent interactions as well as entropy of mixing, and it employed simplifying assumptions such as a linear homogeneous polysaccharide molecule and a spherical globular protein molecule of uniform surface properties. The protein-polysaccharide interaction parameter xs, obtained by fitting the model t o experimental data for one molecular weight of dextran, was found to be 0.132 and 0.115 for ovalbumin and lysozyme, respectively. Solubilities of ovalbumin and lysozyme in dextran solutions of different molecular weights exhibited a shallow maximum at intermediate dextran concentrationsand compared well with model predictions. Protein solubility was found to be very sensitive to protein-polysaccharide interactions and increase with more favorable proteinpolysaccharide interactions (larger XS), less favorable polysaccharide-solvent interactions (larger x), smaller size protein molecule, and lower molecular weight polysaccharide.

No abstract

Flexible random-coil structure of polymer molecules and protein−polymer and polymer−solvent interactions were accounted for in the evaluation of the interaction potential between two globular protein molecules which consisted of attraction because of depletion of polymer segments in the vicinity of protein and electrostatic repulsion because of the net charge of protein. The Gibbs free energies of protein in the solid and liquid phases, evaluated using a second-order perturbation theory around hard sphere as the reference, were equated to predict the phase diagram. Protein solubility was found to be lower and the concentration of protein in the solid phase was higher for higher polymer concentrations, larger protein sizes, higher molecular weight of polymer, weaker protein−polymer interactions, poorer solvent, higher ionic strengths, and pH values closer to the isoelectric point. Experimental measurements of solubility of bovine serum albumin, human serum albumin, and ovalbumin for different molecular weights of poly(ethylene glycol) agreed well with the model predictions.