Enthalpy-Entropy of Cation Association with the Acetate Anion in Water.

Negatively charged carboxylate and phosphate groups on biomolecules have different affinity for Na(+) and K(+) ions. We performed molecular simulations and studied the pair potential of mean force between monovalent cations and the carboxylate group of the acetate anion in aqueous solution at 298 K. The simulations indicate that a larger affinity of Na(+) over K(+) in the contact ion pair (CIP) state is of entropic origin with the CIP state becoming increasingly populated at higher temperature. Differences between the osmotic activities of these two ions are however governed by interactions with acetate in the solvent-shared ion pair (SIP) state as was previously shown (Hess, B.; van der Vegt, N. F. A. Proc. Natl. Acad. Sci. U.S.A.2009, 106, 13296). SIP states with Na(+) are slightly more stable than SIP states with K(+), resulting in a smaller osmotic activity of sodium. We discuss the different affinities of Na(+) and K(+) in the SIP state in terms of an enthalpy-entropy reinforcement mechanism which involves a water-mediated hydrogen-bonding interaction between the oppositely charged ions. SIP states are enthalpically favorable and become decreasingly populated at higher temperature.