The interface between a protein crystal and an aqueous solution and its effects on nucleation and crystal growth

In this paper we report calculations of the concentration profile and the G ibbs free energy of the interface between a protein crystal and an aqueous solution, The calculations are based on a simple, realistic expression for the Gibbs free energy of the solution, which reproduces characteristic feat ures of the protein-water phase diagram, such as the presence of a metastab le liquid-liquid immiscibility region, An equation is derived which express es the coefficient for the contribution of concentration gradients to the G ibbs free energy of the solution in terms of the interaction potential betw een the protein molecules, Calculations are presented which show that the p resence of a metastable liquid-liquid immiscibility region in the phase dia gram has a profound effect on the properties of the interface between a cry stal and the solution, Near and below the critical temperature for metastab le liquid-liquid phase separation the protein crystal is covered by a thin liquid film with a high protein concentration. This surface film causes a s ubstantial lowering of the surface energy of the crystal. The effects of th is film on the kinetic processes associated with the nucleation and the fur ther growth of protein crystals are discussed.