Spectroscopic and computational study to the interaction mechanism of L-Tryptophan in acetone-DMSO liquid mixture, implication for browning reaction

The present work aims to understand the intermolecular interactions between L-Tryptophan (L-TRP) in acetone (ACN) and dimethyl Sulfoxide (DMSO) solvent. Density functional theory at the B3LYP and DFT-D3 levels, with a 6-311++G(d,p) basis set, is used to optimize the structures of the monomers and the mixtures. The Natural Bond Orbital (NBO) analysis validates the inter- and intramolecular charge transfer responsible for the system’s stability. Electronic properties such as the Molecular Electrostatic Potential (MEP), Frontier Molecular Orbital (FMO), and HOMO-LUMO energy gap have been investigated using DFT calculations. AIM and NCI analyses reveal van der Waals interactions and the weak and moderate nature of hydrogen bonding. NLO analysis shows that the complex is a promising candidate for a nonlinear optical material. Vibrational assignment has been performed using the VEDA software. Raman, FTIR and UV-Vis spectra are recorded for the system. The variation in peak wavenumber of C = O and S = O stretching has been investigated for the system consisting of L-TRP and the mixture of DMSO and ACN solvent (DMACN) at different concentrations (1 mg/ml, 3 mg/ml, 5 mg/ml, 7 mg/ml, 9 mg/ml). Both the C = O and S = O bond stretches are found to exhibit a red shift due to the formation of a hydrogen bond. This result is in good agreement with the theoretical calculation. Further, the Electron Localization Function (ELF) and Local Orbital Locator (LOL) are performed for topological analysis. The PDOS spectrum shows the contribution of a specific atom group to the total electronic density of state.