A comparative study of the vibrational spectra of the anticancer drug melphalan and its fundamental molecules 3-phenylpropionic acid and l-phenylalanine

The structural stability and the vibrational spectra of the anticancer drug melphalan and its parent compounds 3-phenylpropionic acid and l-phenylalanine were investigated by the DFT B3LYP/6-311G∗∗calculations. Melphalan and its fundamental compounds were predicted to exist predominantly in non-planar structures. The vibrational frequencies of the low energy structures of melphalan, 3-phenylpropionic acid, and phenylalanine were computed at the DFT B3LYP level of theory. Complete vibrational assignments of the normal modes of melphalan, 3-phenylpropionic acid, and phenylalanine were provided by combined theoretical and experimental data of the molecules. The experimental infrared spectra of phenylalanine and melphalan show a significantly different pattern of the C=O stretching mode as compared to those of normal carboxylic acids. A comparison of the 3700-2000 cm^-1 infrared spectral region of the three molecules suggests the presence of similar intermolecular H-bonding in their condensed phases. The observed infrared and Raman spectra are consistent with the presence of one predominant melphalan conformation at room temperature.