Spectral Fingerprints of Hydrogen-Bonding in Water Solvation of Amino Acids

Many amino acids are neutral in the gas phase but are found in the zwitterionic form in water due to their interaction with the hydrogen bond network of the solvent. For instance, glycine is known to switch from neutral to its zwitterionic form in about 6-10 solvating molecules. In this article, we employ terahertz (THz) vibrational spectroscopy at 0.37 K in helium nanodroplets in synergy with DFT molecular dynamics to study the H-bonding network changes from neutral to zwitterionic glycine. Zwitterionization is found to result in an increased absorption for the fully deuterated zwitterionic species solvated by 10 water molecules between 150 and 300 cm^-1 experimentally and confirmed by calculations based on DFT quasi-classical trajectory dynamics. The experimental observation of a spectral signal at about 320 cm^-1 for the zwitterionic complex is rationalized based on a suitable cross-correlation function between the H and O atoms which are involved in the solvating process. This study deciphers how zwitterionization implies a rearrangement of the H-bonding network.