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.