Ab initio Molecular Dynamics Study of Glycine Intramolecular Proton Transfer in Water

TL;DR

This study employs ab initio molecular dynamics to investigate the structural and thermodynamic aspects of glycine's intramolecular proton transfer in water, revealing hydration changes and energy barriers.

Contribution

It provides detailed free energy profiles and hydration insights for glycine's proton transfer, including corrections for methodological approximations.

Findings

Hydration number increases from 5 to 8 during transfer

Quantified free energy difference between neutral and zwitterionic forms

Estimated energy barrier for proton transfer

Abstract

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.