Specific interactions of ammonium functionalities in amino acids with aqueous fluoride and iodide.

Molecular dynamics simulations were performed to examine the relative strength of interactions of fluoride versus iodide with the ammonium (NH(4)(+)) ion, the alkyl ammonium side chain of lysine (R(Lys)NH(3)(+)), and the zwitterionic ammonium group of glycine (R((-))(Gly)NH(3)(+)) in aqueous solution. Clear trends were observed in the ammonium-anion association, with iodide showing essentially constant affinity for all three groups, whereas fluoride interacted most favorably with the ammonium ion, less with R(Lys)NH(3)(+), and comparably with iodide with R((-))(Gly)NH(3)(+). Neutron scattering experiments show little difference in the interaction of fluoride and iodide with glycine, confirming this last observation. The experimental neutron scattering data also suggests that the calculated coordination number of fluoride (about 6 in the constant pressure simulations) is about 20% too high, indicating certain inaccuracies in the classical force field description of this small anion.