  We use ab initio molecular dynamics simulations to quantify structural and
thermodynamic properties of a model proton transfer reaction that converts a
neutral glycine molecule, stable in the gas phase, to the zwitterion that
predominates in aqueous solution. We compute the potential of mean force
associated with the direct intramolecular proton transfer event in glycine.
Structural analyses show that the average hydration number Nw of glycine is not
constant along the reaction coordinate, but rather progresses from Nw=5 in the
neutral molecule to Nw=8 for the zwitterion. We report the free energy
difference between the neutral and charged glycine molecules, and the free
energy barrier to proton transfer. Finally, we identify approximations inherent
in our method and estimate corresponding corrections to our reported
thermodynamic predictions.
