Raman study and DFT calculations of amino acids

Intense Raman signals were observed of nano gram quantities of three amino acids (histidine, valine, glycine) using visible laser excitation (488nm) at relatively low laser power (5mW) and short acquisition time (2min). Considerable variations in the Raman signal were observed when changing the pH of the amino acid solution. Scanning electron microscopy reveals considerable differences in the crystallite structure of histidine, valine and glycine and for different pH. Narrow vibrational bands are correlated with formation of crystallites. H/D substitution and pH dependence experiments were used to identify vibrational bands associated with the functional groups able to exchange protons. The observed spectral bands are directly compared with density functional calculations to assign the vibrational bands in histidine, valine and glycine. The effect of hydration is studied at neutral pH in the theoretical