1 Ion specific interactions between halides and basic amino acids in water

Ion specific behavior of halides at surfaces of aqueous basic amino acids is unravelled by means of molecular dynamics simulations employing both non-polarizable and polarizable force fields. Analysis in terms of density plots, cumulative sums, and residence times provides a clear, robust, and quantitative picture of specific ion effects. Small anions like fluoride, but not heavier halides, exhibits strong affinity for positively charged groups in the order guanidinium> imidazolium> ammonium. In contrast, large soft anions, such as iodide are weakly attracted to non-polar regions of the amino acids. Since interactions of halides with positively charged groups exhibit a local character and are not overwhelmingly strong, similar behavior will be observed (in an additive sense) also at surfaces of hydrated proteins. 2 1