Conformational and vibrational investigations of L-Tryptophan zwitterion

Theoretical computations of the molecular structures and vibrational parameters for all the possible conformations of L- Tryptophan in zwitterionic form have been carried out using the Gaussian 09 [DFT (B3LYP)] and GAR2PED softwares. The experimental IR and Raman data characterization and analysis have been made on the basis of the computed geometrical and vibrational parameters. The vibrational assignments of the experimentally observed vibrational bands have been proposed using the outcomes of the computed parameters. The geometrical parameters available in the literature and absence of C[dbnd]O stretching frequency in the IR spectrum suggest the molecule to exist in the zwitterionic form. Intramolecular H bonding is found between one O atom of the CO₂ group and one H atom of the NH₃ group. Intermolecular H bonding is expected between the H atom at the site N of the indole ring and O atom of another molecule and similarly, between one H atom of the NH₃ group and O atom of another molecule. The charge transfer resulting in the molecule has been analysed using the HOMO–LUMO analysis. The electron density mappings of the iso-surfaces with the molecular electrostatic potentials (MEPs) have been carried out to obtain a number of data concerned with the charge density distribution, size, shape and site of chemical reactivity of the molecule. The region in the neighbourhood of the two O atoms of the CO₂ group is the region of the most negative ESP in zwitterionic L-TRYP and is an active site for the neucleophilic attack. Similarly, the region in the neighbourhood of the three H atoms of the NH₃ group is the region of the most positive ESP and is an active site for the electrophilic attack.