Vibrational circular dichroism and IR absorption spectra of amino acids: a density functional study.

With density functional theory, vibrational circular dichroism (VCD) and infrared absorption (IR) spectra are obtained at the B3LYP/CC-pVTZ level of theory for 20 alpha-amino acids. The contribution of different vibration modes to the IR and VCD spectra is analyzed. Overall agreement between calculated results for amino acids in gas phase with the available experimental VCD data for matrix-assisted amino acid films is found. The analysis of the calculated IR and VCD spectra indicates that the functional groups in the backbones and side chains of amino acids contribute differently to the spectra line shape. It is obtained that molecular torsions are the characteristic vibrations of the amino acids at the low-frequency regime, whereas the bending of bond angles, the out-of-plane wagging of individual atoms, and some stretching modes dominate the intermediate frequency range. Specific modes like NH(2) scissoring, CO bond stretching, and the (symmetric and asymmetric) stretching of the hydrogen atoms in the NH(2) and OH groups characterize the high-frequency regime. A general trend emerging from these calculations indicates that the rho(OH) rocking and nu(C=O) stretching modes have the highest intensity in the VCD spectra of most amino acids.