Raman and Terahertz Spectroscopy of Low-Frequency Chiral Phonons in Amino Acids

TL;DR

This study uses Raman, ROA, and THz spectroscopy combined with DFT calculations to identify and analyze low-frequency chiral phonons in amino acids, revealing their vibrational properties and optical interactions.

Contribution

It provides the first spectral signatures of chiral phonons in amino acids using combined vibrational spectroscopy and theoretical modeling, advancing understanding of biomolecular chirality.

Findings

Chiral phonons produce bisignate peaks between 1 and 4.5 THz in amino acids.

ROA and TCD data strongly agree, confirming chiral phonon signatures.

DFT calculations link observed modes to twisting and shearing motions.

Abstract

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.