The neutral-zwitterion transition of amino acid was studied for Gly-(H2O)(n) by geometry optimization. Quite distinctly from the smaller clusters, some initially neutral Gly-(H2O)(9) were found to be transformed to a zwitterionic form spontaneously in the optimization process through intramolecular proton transfer. Highly organized hydrogen bonds of the water network polarize the oxygen and nitrogen atoms of Gly so efficiently that the barrier for the proton transfer is essentially eliminated. (C) 2010 Elsevier B.V. All rights reserved.