Effect of solvent on the phase diagram of a simple anisotropic model of globular proteins

Anisotropic protein interactions play a role in globular protein phase transitions, including the shape of fluid-fluid coexistence curves and the formation of hemoglobin polymer fibers in sickle cell disease. Also, the solvent has been shown to play an important role in the phase behavior of some aqueous protein solutions, through the release or trapping of water molecules upon crystallization. Both anisotropy and solvent effects have been treated separately in earlier theoretical studies. Here we propose and analyze a simple, composite model that treats both anisotropy and solvent effects. We find that this model qualitatively explains some phase behavior that has been observed in protein solutions, including normal and retrograde solubility curves and lower critical points.