Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids
Chiral phonons are mirror-symmetric vibrations that correspond to twisting and rotational motions of atoms. In chiral biomolecules, they correspond to low-energy terahertz (THz)-range vibrations of the molecular segments involving dozens of atoms whose energies are sensitive to the chirality of the molecules and local atomic geometries. Here we present spectral signatures of chiral phonons in circularly polarized low-frequency Raman and Raman optical activity (ROA) spectra from crystals of several amino acids in different enantiomeric forms. Along with complementary THz circular dichroism (TCD) measurements, our ROA data reveal two sets of bisignate peaks in valine, alanine, tyrosine and proline between 1 and 4.5 THz that are more intense than the ROA peaks in the fingerprint region. Density functional theory (DFT) calculations on L-alanine attribute these modes to twisting and shearing molecular motions. The strong agreement between the ROA and TCD data demonstrates the power of these complementary vibrational spectroscopy techniques to identify chiral phonons in biomolecules, and offers new insights into their vibrational properties and interactions with circularly polarized light.