MOLECULAR THERMODYNAMICS FOR SALT-INDUCED PROTEIN PRECIPITATION

A molecular-thermodynamic model is developed for salt-induced protein precipitation, which considers an aqueous solution of globular protein molecules as a pseudo-one-component system containing macroions that interact through Coulombic repulsion, dispersion attraction and hydrophobic interactions and forces arising from ion-excluded volume. Forces from ion-excluded volume rake into account formation of ion pairs and ionic clusters at high salt concentrations; they are calculated in the context of the Percus-Yevick integral-equation theory. Hydrophobic interactions between exposed nonpolar amino-acid residues on the surfaces of the protein molecules are modeled as short-range, attractive interactions between ''spherical caps'' on the surfaces of the protein polyions. An equation of state is derived using perturbation theory. From this equation of state we calculate liquid-liquid equilibria: equilibrium between an aqueous phase dilute in protein and another aqueous phase rich in protein, which represents ''precipitated'' protein. In the equation of state, center-to-center, spherically symmetric macroion- macroion interactions are described by the random-phase approximation, while the orientation-dependent short-range hydrophobic interaction is incorporated through the perturbation theory of associating fluids. The results suggest that either ion-excluded-volume or hydrophobic-bonding effects can precipitate proteins in aqueous solutions with high salt concentrations.