Terahertz Chiroptical Spectroscopy of an α-Helical Polypeptide: A Molecular Dynamics Simulation Study

Vibrational spectroscopy has provided incisive information on the structure of biological molecules. Here, using a molecular dynamics simulation method, infrared vibrational circular dichroism and vibrational optical rotatory dispersion spectra of a right-handed a-helix in the terahertz (THz) frequency range are calculated. Both the autocorrelation function of an electric dipole moment and the cross-correlation function of electric and magnetic dipole moments of the a-helix are calculated and Fourier-transformed to obtain THz absorption and optical activity spectra, which reveal characteristic features of the helical polypeptide structure. The anharmonicity and delocalized nature of the low-frequency modes in the THz frequency domain are taken into account to obtain statistically convergent results on the THz optical activity spectra. In addition, the magnitude of the THz vibrational optical activity signal of the a-helix is directly compared with those of typical, previously studied mid- and near-infrared chiral molecules. We anticipate that THz chiroptical spectroscopy that has not yet been demonstrated experimentally would provide highly important and complementary information on protein structure and dynamics.