Solvent effect on vibrational circular dichroism of chiral amino acids

This work presents a computational insight into the vibrational circular dichroism (VCD) spectra of methionine, serine and glutamine in gas and solvent (water and methanol) phases with the aid of density functional theory. VCD spectra are simulated using long-range-corrected functional CAM-B3LYP/6-31++G (d, p) level of theory. Molecular dynamics simulations provide the solute–solvent superstructures on which VCD calculation is performed. The combination of both implicit and explicit solvation models is employed to account for the solvent effect. Substantial changes in VCD spectrum are observed in solvent phase mainly due to the formation of zwitterions and non-covalent interaction with the solvent molecules. Vibrational modes of the –COO stretching, –NH3 wagging, scissoring and –CH2 scissoring vibration of methionine, –NH3 wagging, –COO stretching, O–H stretching vibration of serine and –NH2 wagging, –NH2 scissoring, –COO stretching vibration of glutamine produce characteristic VCD signature band in both gas and solvent phases. Molecular dynamics simulation depicts the probability of hydrogen bonding between the C=O groups of the amino acids and added solvent molecules. Solvent polarity shows the immense effect on VCD signals for the different vibrational modes at the interacting rigions of amino acids. In all cases, values of the rotational strength in the solvent phase are higher than that of in the gas phase. This study discloses the profound effect of non-covalent interactions on VCD signatures, and this technique may be applied for probing non-bonding interaction.