Hydrogen bond mediated stabilization of the salt bridge structure for the glycine dimer anion.

S Heiles,Jos Oomens,Evan R Williams,Giel Berden,Richard J Cooper

doi:10.1039/c5cp06120b

Abstract

The formation of a salt bridge in deprotonated glycine dimer anions in a solvent-free environment is investigated using both infrared multiple photon dissociation spectroscopy between 600 and 1800 cm(-1) and theory. The zwitterionic and nonzwitterionic forms of glycine in this complex are computed to be nearly iso-energetic, yet predominantly the zwitterionic form is observed experimentally. The zwitterion stability is attributed to both the Coulombic attraction and the high stabilization from intramolecular hydrogen bonding that drives the energetic cost of proton transfer in a solvent free environment. These results show that there is a fine balance between the stabilities of these two forms of the anion. Elucidating the role of intrinsic factors, such as hydrogen bonding, can lead to a better understanding of the stabilities of salt bridges in the interiors of large proteins or at protein interfaces.

Highlights

Proteins are the workhorse molecules of life as they carry out much of the function in cells and organisms
The zwitterion stability is attributed to both the Coulombic attraction and the high stabilization from intramolecular hydrogen bonding that drives the energetic cost of proton transfer in a solvent free environment
Glycine is not involved in salt bridges (SBs) in proteins, these results provide compelling evidence about the role of hydrogen bonding in stabilizing SB interactions that should lead to new insights into the stabilities of SBs in solvent-inaccessible environments, such as the interior of proteins and at protein– protein interfaces

Summary

Introduction

Proteins are the workhorse molecules of life as they carry out much of the function in cells and organisms. Understanding what roles various proteins perform and how they function on a molecular level requires a detailed understanding of both intramolecular interactions and the interactions of the proteins with their environments. Basic and acidic residues with ionized side chains play an important role in the functionality and structure of proteins owing to long-range Coulombic interactions.[1,2,3] Charged groups can reside on the surface of a protein or can be buried in the interior, and are important for the reactivities of active sites and protein solubility. Charged groups can be distant from each other, or can form salt bridges (SBs)[1,2,3,4] in which protonated and deprotonated residues interact directly. SBs are stabilized by a number of non-covalent interactions and solvation, but predicting SB stabilities remains challenging

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