VIBRATIONAL ANALYSIS AND HYDROGEN-BONDING EFFECTS ON THE VIBRATIONAL MODES OF ZWITTERIONIC DL-TRYPTOPHAN: IR SPECTROSCOPY AND DFT CALCULATIONS

The vibrational behavior of zwitterionic DL-tryptophan has been investigated using infra-red spectroscopy and density functional theory calculations. In order to study the hydrogen bonding effects on the molecular structure and vibrational normal modes of the under-investigated amino acid, both the solid state and aqueous solution calculations have been presented. The three dimensional molecular structure optimized by the solid state calculations shows the best agreement with that previously published using X-Ray diffraction technique. In addition, the calculated and experimentally observed wavenumbers have been compared. However, only the calculations performed in the solid state allowed us to attribute the band observed at around 2532 cm(-1) to the N-H stretching vibration. Because some normal modes are strongly affected by the extensive hydrogen bonding network present in DL-tryptophan crystal, the solid state model is the most suitable for interpreting the experimental infra-red spectrum.