Molecular thermodynamics for fluid-phase equilibria in aqueous two-protein systems

With saline water as the continuous medium, a two‐component McMillan‐Mayer equation of state is used to describe liquid–liquid phase equilibria in a two‐protein system. The equation of state is based on a hard‐sphere reference with perturbations introduced through a potential of mean force to account for electrostatic forces and for attraction between protein particles. To illustrate the thermodynamic framework, one parameter each is fitted to experimental precipitation data for aqueous saline one‐protein systems containing either lysozyme or ovalbumin. A lysozyme–ovalbumin interaction parameter is then introduced to calculate phase behavior in the aqueous two‐protein system. These calculations are remarkably similar to classic vapor–liquid equilibrium calculations using an equation of state. For the aqueous two‐protein system, calculations give the light‐phase composition as well as the lysozyme and ovalbumin partition coefficients for a given dense‐phase composition. Agreement with sparse experimental data is reasonable over a range of pH and high ionic strength provided by the common precipitant ammonium sulfate.