  We performed plane wave density functional theory calculations to survey the
potential energy surface of neutral glycine (GlyNE) and its zwitterion (GlyZW)
solvated by up to four water molecules. Our previous conformation study of Gly
suggests inadequacy of the commonly used local basis function sets in dealing
with a high-energy isomer, such as Gly. We find the potential energy surface of
GlyNE and GlyZW smoother than usually was thought and without many local
minima. Two water molecules can create a local minimum around GlyZW. With three
water molecules, the energy difference between GlyNE and GlyZW is reduced to a
mere 27 meV with an energy barrier of 115 meV from GlyNE. GlyZW becomes
energetically more stable by 122 meV when solvated by four water molecules.
Water molecules become catalysts in the tautomerization and sometimes engage in
a switching transfer of a proton over the water bridge. Our results are
consistent with experimental findings that the effective hydration number of
Gly is 3 ~ 4.
