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 Gibbs 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 features of the protein-water phase diagram, such as the presence of a metastable liquid-liquid immiscibility region, An equation is derived which expresses the coefficient for the contribution of concentration gradients to the Gibbs free energy of the solution in terms of the interaction potential between the protein molecules, Calculations are presented which show that the presence of a metastable liquid-liquid immiscibility region in the phase diagram has a profound effect on the properties of the interface between a crystal and the solution, Near and below the critical temperature for metastable liquid-liquid phase separation the protein crystal is covered by a thin liquid film with a high protein concentration. This surface film causes a substantial lowering of the surface energy of the crystal. The effects of this film on the kinetic processes associated with the nucleation and the further growth of protein crystals are discussed.