Preferential interactions of proteins with salts in concentrated solutions.

The preferential interactions of proteins with solvent components were studied in concentrated salt by densimetric measurements. Proteins were found to be preferentially hydrated in NaCl, NaCH3COO, and Na2SO4. The resulting unfavorable free-energy change was related to the effects of these salts on solubility and stability of the proteins. This unfavorable free-energy change was correlated with the large, positive surface tension increment of these salts, i.e., their perturbation of surface free energy. On the other hand, KSCN, CaCl2, and MgCl2 showed considerable binding to bovine serum albumin, which could be related to their destabilizing and salting-in effects on macromolecules. Since the last two salts have high surface tension increments, it was concluded that this does not necessarily lead to protein preferential hydration and stabilization.