Density functional theory investigation on the conformational analysis, molecular structure and FT-IR spectra of Tryptophan methyl-ester molecule

Tryptophan methyl-ester (TrpME) is derived from a natural amino acid, which contains an indole heterocycle. This gives tryptophan its absorption and fluorescence spectroscopic properties. TrpME has become a commonly tool used in many fields namely medicine which is considered as the most important one. In front of his major importance, we found interesting to make a theoretical study on the conformational analysis by DFT/B3LYP method with 6-31G(d,p) basis set to find the most stable form of TrpME. Seven staggered stable conformers were observed on the torsional potential energy surfaces, it is shown that TrpME_1 conformer (D1= 178.62°, D2= -69.77°, D3= -67.33° and D4= -140°). is the most stable form. The optimized geometrical parameters and vibrational frequencies of the most stable conformation of this molecule were calculated by HF and DFT methods with 6-31G(d,p) standard basis. The electronic and orbital properties like HOMO and LUMO energies of this molecule were also calculated by DFT/B3LYP with 6-31G(d,p), 6-31+G(d,p) and 6-311+G(d,p ) basis sets. This study is expanded to determine the gap energy, polarizability, Mulliken charges and thermodynamic properties of the TrpME. The correlation of certain thermodynamic functions with temperature has been determined in the form of quadratic equations. The UV spectrum is also recorded by TDDFT. It shows good conformity with the experimentally found spectrum and confirms that TDDFT is a powerful tool to study the electronic properties of molecules.