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 relativel y low laser power (5mW) and short acquisition time (2min). Considerable variations in the Raman signal were ob served when changing the pH of the amino acid solution. Sc anning electron microscopy reveals considerable difference s in the crystallite structure of histidine, valine and glyc ine and for different pH. Narrow vibrational bands are correlated with formation of crystallites. H/D substitution and pH dependence experiments were used to identify vibrat ional bands associated with the functional groups able to exchange protons. The observed spectral bands are d irectly compared with density functional calculations to as sign the vibrational bands in histidine, valine and glycine. The effect of hydration is studied at neutral pH in the theore tical calculations.