Path-integral molecular dynamics simulations of glycine·(H2O)n (n = 1-7) clusters on semi-empirical PM6 potential energy surfaces

Path-integral molecular dynamics simulations for the hydrogen-bonded glycine·(H2O)n (n = 1-7) clusters have been carried out using an on-the-fly direct dynamics technique at the semi-empirical PM6 level of theory. In the case of smaller clusters with n = 1-3, the simulations show that the cluster structure takes exclusively the hydrogen-bonded complex between a canonical neutral glycine and a water cluster moiety. In contrast, it was found that proton-exchange processes effectively occur between the COOH carboxylic group and water cluster moiety for n = 4-6 clusters although the overall structures are the complex between a neutral glycine and water clusters. In the case of the n = 7 cluster, glycine preferentially takes a zwitterionic form having NH3+ and COO− functional groups.