Infrared and Raman spectroscopic study of pH-induced structural changes of l-histidine in aqueous environment

Aqueous solutions of l-histidine have been analysed in parallel by infrared (IR) and Raman spectroscopy over the pH range 0-14 with increments of one pH unit. The vibrational spectra in the region 2000-500 cm−1 have been interpreted and band positions have been assigned tentatively, taking into account assignments from literature after critical evaluation. As a result, a complete and complementary set of vibrational data has been obtained that can be used to determine all possible states of protonation of histidine, i.e. H4His2+, H3His+, H2His0, HHis− and His2−. In addition, IR and Raman bands have been proposed as markers for the presence of the imidazole Nπ- or Nτ-protonated tautomeric forms of H2His0 and HHis−.