Research on the Terahertz Absorption Spectra of Histidine Enantiomer (L) and its Racemic Compound (DL)

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption spectra of polycrystalline L- and DL-histidine in the frequency range of 10–100 cm^-1. The spectra exhibit distinct differences in peak frequencies between the enantiomer (L-histidine) and racemic compound (DL-histidine). The observed spectral differences are attributed to the intermolecular interactions. With the density function theory (DFT) method, the frequencies of vibrational modes of L-histidine and DL-histidine in the THz range are calculated and well assigned according to the measured spectra. The origin of the observed vibrational modes is found to be non-localized and of a collective (phonon-like) nature, which points to the lattice and skeleton vibrations mediated by the hydrogen bond. Furthermore, we propose and demonstrate a method for determining the composition ratio of histidine mixtures based on the THz absorption spectra.