Theory of precipitation of protein mixtures by nonionic polymer

A theory is developed to predict the solubility of protein mixtures in solutions containing nonionic polymer. Effective protein‐protein interactions due to polymer are taken to be volume‐exclusion potentials derived using statistical mechanics. Statistical‐mechanical perturbation theory is used to calculate chemical potentials. The effects of protein size, mole fraction and polymer concentration on solubility are explored. The theory is extended to include electrostatic interactions. The excess chemical potential of the proteins due to the charges on all species is calculated using the mean spherical approximation for a mixture of charged hard spheres. The theory predicts: the larger protein is preferentially precipitated over the smaller one; the more concentrated protein is more likely to precipitate; and increasing the charge of a particular protein reduces its ability to precipitate.