Homocysteine is an intermediate non-essential amino acid produced during the metabolism of methionine to cysteine. At elevated levels in plasma, homocysteine is a risk factor for Alzheimer’s and cardiovascular diseases. The FTIR and FT Raman spectra of Methionine, Homocysteine and Cysteine molecules were recorded in the regions 4000-450 and 4000-100cm- 1 respectively. The optimized geometry, wave number and other properties were studied using MNDO, AM1 and PM3 semi-empirical methods. A complete vibrational assignment aided by the theoretical harmonic wave number analysis was proposed. The calculated harmonic vibrational frequencies were compared with experimental FTIR and FT Raman spectra. Based on the comparison between calculated and experimental results and the comparison with related molecules, assignments of fundamental vibrational modes were made. The X-ray geometry and experimental frequencies were compared with the results of theoretical calculations.
