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

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 soln. 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 IR response of Gdm+ and Ac-​ change significantly, and in the 2D-​IR spectrum, salt bridging of the mols. appears as cross peaks. From the 2D-​IR spectrum we det. 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.