Salt-induced protein separation in an aqueous electrolyte solution

Liquid-liquid phase separations of aqueous ovalbumin and bovine serum albumin (BSA) solutions are reported experimentally for a wide range of solution conditions. The temperature-induced clouding of protein solutions, which signals the onset of liquid-liquid phase separation, provides a simple means of assessing the effect of solution conditions on the strength of protein interaction. Our results show that the effect of salts on protein interactions depends sensitively on the ionic composition of solution and the identities of both the cation and the anion of the added salts. The results are used to test and refine theoretical models for the interaction energy between macromolecules. A modified perturbed hard-sphere chain (MPHSC) model is employed to determine the interaction energy for solvation forces playing an important role in protein interactions and to predict the osmotic pressures of protein solutions.