A MODEL FOR THE PREDICTION OF PRECIPITATION CURVES FOR GLOBULAR-PROTEINS WITH NONIONIC POLYMERS AS THE PRECIPITATING AGENT

A statistical thermodynamic model for the prediction of precipitation curves of globular proteins using nonionic polymers has been proposed. The model accounts for protein-polymer, polymer-solvent, electrostati c, and hydrophobic interactions as well as the entropy of mixing and e mploys simplifying assumptions such as spherical globular protein mole cule with uniform surface properties and linear, homogeneous polymer u niform with respect to molecular weight. The proposed model can only b e employed to predict precipitation curves of charged proteins at suff iciently high ionic strengths since it does not account for electrosta tic protein-protein interactions due to overlap of electrical double l ayers. The model predictions of precipitation curves of human serum al bumin (HSA) at the isoelectric point using polyethylene glycol (PEG) f or different initial protein concentrations and molecular weights of P EG agreed well with the experimental data. Higher polymer concentratio ns were found to be required to precipitate proteins for lower molecul ar weight polymers, lower initial protein concentrations, and more fav orable protein-polymer interactions. The HSA-PEG interaction parameter , obtained by fitting the model to experimental data for one molecular weight PEG, was found to be 0.122. Solubility of HSA in PEG solution was found to decrease with increasing salt concentrations, this effect being more pronounced at lower PEG concentrations. The net charge on HSA was found to result in a maximum in its solubility at intermediate salt concentrations as a result of competing salting-in and salting-o ut effects.