Structural and spectroscopic study of L-tryptophan dimer state using DFT and MD: Computational and experimental analysis

The Fourier Transform Infrared (FTIR), and Raman techniques are experimentally and theoretically employed to study the structural and spectroscopic analysis of L-Tryptophan (LTp) and its dimer. Density functional theory, DFT, using the 6-311++G (d, p) basis set is used to compute the equilibrium geometrical parameters, harmonic vibrational wavenumbers, Frontier orbitals, Mulliken atomic charges, molecular electrostatic potential, non- covalent interaction, density of states, electron localization function, and some optimized energy parameters. Additionally, molecular dynamics simulations between water molecules and LTp residue are done. For the dimer state, the HOMO-LUMO energy gap is found to be 4.774 eV which is smaller than the band gap of the monomer state, 5.060 eV. Thus, the dimer state is likely more reactive and can be considered more bioactive compared to the monomer. Intermolecular hydrogen bond formation contributes to the elongation of the C--O bond length from 1.20 & Aring; in the monomer to 1.23 & Aring; in the cyclic dimer, which exerts additional electron-donating effects, resulting in alteration of bond lengths. Dimer states also have higher thermal total energies. It is realized that the dimer state induced redistribution of charges on the atoms causes the positive charge to be displaced and the decrease of the molecule's dipole moment to be 1.4752 D. Simulated charge transfer, NCI-RDG, and electron localization function confirm the presence of intermolecular hydrogen bonds in the dimer state. It is found that the computed wavenumbers of both LTp dimer and monomer agree well with the experimental IR and Raman values.