An experimental and theoretical study of l-tryptophan in an aqueous solution, combining two-layered ONIOM and SCRF calculations

ONIOM and SCRF calculations were performed to study the most stable theoretical structure of l-tryptophan in an aqueous solution phase and to observe the changes occurring in the structural and vibrational properties of l-tryptophan in the aqueous media. l-Tryptophan was characterized by infrared and Raman spectroscopies in the solid state and as an aqueous solution. Optimized geometries and relative stabilities for the l-tryptophan zwitterion were calculated while taking into account solvent effects using the self-consistent reaction field (SCRF) theory. The obtained results in the aqueous solution were compared with those calculated for the zwitterion in the gas phase by means of two-layered ONIOM calculations. For the complete assignments of the IR and Raman spectra of l-tryptophan in the aqueous solution phase, density functional theory (DFT) calculations were combined with Pulay's scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental values. Additionally, the roles of specific and bulk contributions from solvent effects on the properties of l-tryptophan were analyzed. Furthermore, bands corresponding to the normal modes of vibration were localized and assigned, and they served as the bases for the calculations of the corresponding force constants. Significant effects on the geometrical and vibrational frequencies were found for the studied zwitterion.