Molecular Mechanism of Specific Ion Interactions between Alkali Cations and Acetate Anion in Aqueous Solution: A Molecular Dynamics Study

Specific ion interactions between alkali cations (i.e., Li+, Na+, and K+) and an acetate anion in aqueous solution were studied using molecular dynamics simulation techniques and polarizable potential models. The ions acetate systems were used as a model for understanding the interactions between ions and protein surfaces. We computed free energy profiles for different ion pairs using constrained mean force methods. Upon analyzing the computed free energy profiles for the Na+/K+-acetate ion-pairs, we observed a deeper contact ion minimum and also a larger association constant for the Ne+-acetate pair as compared to the corresponding K+-acetate pair. These observations help to demonstrate the preferential binding of Na+ over K+ to protein surfaces. We also applied various rate theories to study the kinetics of ion pair interconversion.