Molecular thermodynamics for salt‐induce 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 take 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.