Dynamic paths between neutral alanine–water and zwitterionic alanine–water clusters: single, double and triple proton transfer

Computations are presented for the alanine–(H2O)n (n=1–3) and its zwitterion–(H2O)n clusters. We find that at least two water molecules need to bind to alanine to give stable alanine zwitterion–water cluster. Structures of the conformers are predicted, and their relative energies are compared. Detailed analysis is presented on the dynamic (proton transfer) pathways between the neutral alanine–(H2O)n and the zwitterionic alanine–(H2O)n (n=2, 3) clusters, including the structures of the transition states. A variety of proton transfer pathways are predicted between alanine–(H2O)n and the zwitterion–(H2O)n clusters, depending on their structures: direct proton transfer, concerted double and triple proton transfer mechanism.