Rapid Terahertz Dichroism Near Field Microscopy for Biomolecular Intramolecular Vibrational Spectroscopy

Intramolecular vibrations of the protein backbone provide access to functional conformational change, however the characterization of these motions has presented a major challenge. Previously we show that while the vibrational density of states and isotropic absorption are relatively insensitive to protein functional state and mutation, anisotropic optical absorbance reveals the reorientation of vibrational displacements. Those measurements used a novel technique where anisotropic terahertz transmission of protein crystals are measured in the near field. The rotation of the protein sample and the corresponding reimaging significantly extend data acquisition time. Here we present our unique technique-ideal polarization-varying anisotropic terahertz microscopy (Ideal PV-ATM) for fast and direct measurement of protein intramolecular vibrations. It avoids the rotation of the sample and shortens the measurement time by a factor of 16. And ideal PV-ATM spectrum of test sample sucrose, which has strong linear dichroism in the THz range, agrees well with the solid-state density functional theory and anisotropic birefringence calculations.