Structures, vibrational absorption and vibrational circular dichroism spectra of L-alanine in aqueous solution: a density functional theory and RHF study

Abstract

A detailed comparative study of structures, vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra has been carried out for the zwitterionic structure of the amino acid L-alanine. Theoretically determined structures necessary for deriving VA and VCD spectra were calculated with three different solvation approaches: the zwitterion surrounded by explicit water molecules only, the zwitterion embedded in a self-consistent reaction field (Onsager model) and the zwitterion plus the explicit water molecules embedded in a self-consistent reaction field. The structures were optimized at the density functional theory level using the B3LYP functional with the 6-31G* basis set. The Hessians and atomic polar tensors and atomic axial tensors were all calculated at the B3LYP/6-31G* level of theory. An important result is the method of treating solvent effects by both adding explicit water molecules and a dielectric media through the Onsager model and only this combination gave excellent comparison to experimental VA and VCD spectra. Two dominant conformers with different backbone configurations are identified, and the inclusion of solvent effects clearly favours one of the conformers. The calculated VA and VCD spectra of this conformer are in better agreement with experimentally measured VA and VCD spectra previously reported.