Rigid-body Monte Carlo simulations were carried out to study the differential hydration of zwitterionic and neutral forms of glycine in water. To account for the solute polarization by the rather polar liquid environment, initial geometries were chosen as minima on the MP2/aug-cc-pVTZ potential energy surfaces of neutral and zwitterionic glycine continuously solvated by water, implementing the polarizable continuum model (PCM) within the integral equation formalism (IEFPCM). The dynamically changing hydrogen bonding network between the solute and solvent molecules was analyzed imposing distance, energy and angular distribution-based criteria. It was found that, on average, the zwitterionic form of glycine acts as an acceptor of 4.53 hydrogen bonds, while it plays the role of a proton donor in (on average) 2.73 hydrogen bonds with the solvent water molecules. In particular, we have found out that 2.73 solvent water molecules are involved in hydrogen bonding interaction with the ammonium group, acting as proton-acceptors. This is in excellent agreement with the recent experimental neutron diffraction studies, which have indicated that 3.0 water molecules reside in the vicinity of the NH3+ group of aqueous zwitterionic glycine. Neutral form of aqueous glycine, on the other hand, on average donates protons in 1.63 hydrogen bonds with the solvent water molecules, while at the same time it accepts 2.53 hydrogen bonds from the solvent molecules. The greater charge polarization in the zwitterionic form thus makes it much more exposed to hydrogen bonding interaction in polar medium such as water, which is certainly the main reason of the larger stability of this form of glycine in condensed media.
