Salt-induced protein precipitation: Phase equilibria from an equation of state

Abstract

A molecular-thermodynamic model is developed for salt-induced protein precipitation. The model considers an aqueous solution of a globular protein as a system of interacting hard spheres in a continuum pseudo-solvent (water and salt ions). The protein molecules are considered to interact in a manner described by a set of spherically-symmetric two-body potentials of mean force. These include screened Coulombic repulsion, dispersion (van der Waals) attraction, osmotic attraction, and an attractive square-well potential intended to model specific protein-protein chemical interactions (including the hydrophobic effect and protein self-associations). Following Chiew et al. (1995), an analytical equation of state is derived using the Random Phase Approximation with the hard-sphere fluid as the reference system and a perturbation based on the protein-protein overall potential of mean force. This equation of state provides an expression for the chemical potential of the protein and determines liquid-liquid equilibria. The model is generalized for co-precipitation of several proteins. Experimental single-protein precipitation data are correlated for hen egg-white lysozyme and for α-chymotrypsin in concentrated aqueous solutions of ammonium sulfate.