SALTING-OUT OF AQUEOUS PROTEINS - PHASE-EQUILIBRIA AND INTERMOLECULAR POTENTIALS

Salting-out phase equilibria ale reported for lysozyme and alpha-chymotrypsin from concentrated ammonium-sulfate solutions. Supernatant and dense-phase protein concentrations and the resulting protein partition coefficients are given as a function of solution pH and ionic strength. Phase equilibria with a trivalent salt (sodium citrate) confirm that ionic strength, rather than salt concentration is the appropriate variable describing phase equilibria. The salting-out behavior of a mixture of an aqueous lysozyme and alpha-chymotrypsin is independent of the presence of the other protein. Parameters for a molecular-thermodynamic description of salting-out behavior are obtained from low-angle laser-light scattering (LALLS). Osmotic second viral coefficients from LALLS are reported over a range of pH for dilute chymotrypsin concentrations in aqueous electrolyte solutions at 0.01 and 1.0 M ionic strengths. Effective Hamaker constants, regressed from experimental osmotic second virial coefficients, are determined for models of the protein-protein potential of mean force. In addition to excluded volume, dispersion and shielded charge-charge potentials, the description of protein-protein interactions includes attractive charge-dipole and dipole-dipole potentials as well as an osmotic-attraction potential that becomes important at high salt concentrations. Protein dipole-dipole potentials are required to account for the observed pH dependence of osmotic second virial coefficients, especially at low ionic strength.