Specific and non-specific effects of sodium and potassium ions on the interactions between model charged groups of proteins

Potassium and sodium ions are crucial for many physiological processes in living systems and play different roles when interacting with proteins and enzymes. Intracellular concentration of potassium is always maintained higher than that of sodium, which provides a suitable environment for biochemical machinery. These cations also possess different properties in physico-chemical Hofmeister phenomena. It is now accepted that the main physical reason for these ion-specific effects is due to formation of ion pairs. The greater ability of sodium over potassium to destabilize protein solutions was previously rationalized by sodium stronger pairing with carboxylates, which are the main anionic moiety of proteins. While ion pairing of cations with carboxylates was studied in detail previously, understanding of the molecular mechanisms of cation-specific mediation of protein-protein interactions is still lacking. In this work with the use of molecular dynamics we studied the effect of sodium and potassium on interactions between model compounds that bear typical positively and negatively charged groups of proteins, namely, methylammonium and acetate molecules. We found only a weak difference in the contacts of charged groups depending on the cation type present in the solution. Our results suggest that a strength of cation-carboxylate binding is not critical, but structural features may be more important.