To investigate how many water molecules may induce the zwitterionic glycine isoenergetic with respect to the neutral form, mp2/6-31＋＋g**//b3lyp/6-31＋＋g** method has been applied to study the micromechanism of the glycine-H₂O interaction. It is found that the answer is five water molecules which confirms the experimental result. The most stable conformation of pentahydrated zwitterionic glycine com- plexes is a pair-chains structure, whose energy is near to the energy of the most stable pentahydrated neutral glycine conformation (the error is only 2.26 kJ/mol), so they can coexist. Via proton transfering, a pentahydrated neutral glycine complex can convert into the corresponding pentahydrated zwitterionic glycine com- plex. There are two kinds of proton transfer route in glycine-H₂O system, one is “direct proton transfer” and the other is “water-assisted proton transfer”. In the proton transfer process of the corresponding pair-chains structures, there is a lower energy barrier or no barrier, and the reaction can proceed spontaneously.

Key words: hydrated glycine, proton transfer, zwitterion, pair-chains, theoretical study