| Abstract
| - The accurate calculation of the energetics of electrostatically driven binding of amino acid residues to otheramino acids (salt bridges) or to synthetic molecules is critical in numerous physiological and technologicalprocesses. Commonly used continuum methods are unable to capture differences in interaction energies resultingfrom specific chemical changes, such as the stronger retention of basic proteins on sulfated (strong) than oncarboxylated (weak) cation exchangers. The inadequacies of continuum models in describing hydration effects,specifically local solvent structure and associated polarization effects, are especially apparent in such systems.These shortcomings have been addressed by modeling the protein−cation exchanger interaction as that ofmethylammonium, a model for a protonated lysine residue, with the methylated analogues of the ion-exchangefunctionalities, namely methyl sulfate and acetate ion, respectively. A hybrid quantum-continuum treatmentof the solvent is able to capture the qualitative differences in binding that are not obtained by continuummethods. More remarkably, it is seen that a heuristic approach, based solely on the bulk solvation free energiesof individual species, is able to describe qualitatively the differences in binding.
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