Infrared and Raman spectra of Histidine : an ab initio DFT calculations of Histidine molecule and its different protonated forms

The infrared spectra of Histidine molecule have been recorded in the Nujoll mull as well as in aqueous solution in the range 400-4000 cm(-1). The Raman spectrum of the same molecule has also been measured. The different protonated/deprotonated forms of imidazole ring which contains different forms of Histidine1, Histidine2, Histidine3 and Histidine4 have been studied with OFT and RHF methods using several basis sets. A comparison of energies of the two neutral tautomers (Histidine1 and Histidine2) indicates that Histidine1 is more stable as compared to Histidine2 while Histidine3 (imidazolium cation) is the most stable in gas phase. The selected geometrical parameters and theoretically calculated frequencies for the above-mentioned form of Histidine were also proposed. The observed IR and Raman bands of Histidine molecule are assigned to different modes on the basis of calculated frequencies, their intensities and available literature values.