Stabilization of proteins by solvents. Effect of pH and anions on the positive cooperativity of 2-nonanone binding to bovine serum albumin.

The binding of 2-nonanone to bovine serum albumin exhibited positive cooperativity at low molal ratios of binding, indicating stabilization of the hydrophobic binding sites at low concentrations of 2-nonanone. The degree of cooperativity was affected by the type of anion present as evidenced from the changes in the Hill coefficient. The effectiveness of anions in increasing the Hill coefficient followed the Hofmeister series for anions, i.e. F- < SO42- < Cl- < Br- < SCN- < Cl3CCOO-. Also, the positive cooperativity decreased as the pH was increased from 3.0 to 7.0. The possible mechanism for the changes in the positive cooperativity of 2-nonanone binding to albumin in the presence of anions is discussed in terms of the stabilizing effect of ketones at low concentrations (Asakura, T., Adachi, K., and Schwartz, E. (1978) J. Biol. Chem. 253, 6423-6425) and the destabilizing effect of lyotropic anions on protein structure.