Glycine Zwitterion Stabilized by Four Water Molecules

TL;DR

This study uses density functional theory to explore how water molecules stabilize glycine's zwitterionic form, revealing that up to four waters significantly influence its energy landscape and tautomerization behavior.

Contribution

It provides detailed computational insights into glycine's hydration effects and the role of water molecules in stabilizing its zwitterionic form, which was previously not well understood.

Findings

Water molecules reduce energy barriers for tautomerization.

Four water molecules stabilize the zwitterion more than fewer waters.

The potential energy surface is smoother with fewer local minima.

Abstract

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.