Molecular dynamics simulations of small glycine-(H2O)n (n = 2-7) clusters on semiempirical PM6 potential energy surfaces

A recently developed semiempirical PM6 method was applied to study small glycine-water clusters. Various low-energy configurations as well as proton-transfer transition-states were optimized to understand the ability of the PM6 method in describing the electronic structures of these systems by comparing to previous density-functional theory and ab initio molecular orbital studies. Moderately good agreement was obtained for structures and energetics of the glycine-(H2O)n clusters although PM6 seems to somewhat overestimate the stability of the zwitterionic form. Molecular dynamics simulations directly using the PM6 potential surface were then carried out to determine the statistical structures of the clusters. Although we did not observe the transformation between neutral ↔ zwitterionic forms in our simulations, an interesting proton-dissociated behavior was seen as the cluster size increases. The difference in water solvation structures around the neutral or zwitterionic glycine form is also discussed.