Structure of a model salt bridge in solution investigated with 2D-IR spectroscopy

TL;DR

This study uses 2D-IR spectroscopy to elucidate the spatial structure and geometry of a model salt bridge between guanidinium and acetate in solution, revealing detailed molecular interactions.

Contribution

The paper demonstrates a novel application of 2D-IR spectroscopy to determine the geometry of salt bridges in solution, providing insights into their molecular structure.

Findings

Salt bridging causes significant changes in IR response.

Cross peaks in 2D-IR spectra indicate salt bridge formation.

Reconstructed geometry of the solvated salt bridge.

Abstract

Salt bridges are known to be important for the stability of protein conformation, but up to now it has been difficult to study their geometry in solution. Here we characterize the spatial structure of a model salt bridge between guanidinium (Gdm+) and acetate (Ac-) using two-dimensional vibrational (2D-IR) spectroscopy. We find that as a result of salt bridging the infrared response of Gdm+ and Ac- change significantly, and in the 2D-IR spectrum, salt bridging of the molecules appears as cross peaks. From the 2D-IR spectrum we determine the relative orientation of the transition-dipole moments of the vibrational modes involved in the salt bridge, as well as the coupling between them. In this manner we reconstruct the geometry of the solvated salt bridge.