The Glycine Zwitterion is Stabilized by Four Water Molecules

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 suggested the 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 to be smoother than previously thought and to be 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.