Raman Spectroscopy of Biological Molecules

Research efforts are underway to probe biological molecules with Raman spectroscopy in three states - crystal, semi-solid, and solution - to illuminate the structural transformations that occur across phases. The optical characterization of biological molecules using vibrational spectroscopy supplies detailed structural information. To observe Raman-active vibrational modes at physiological concentrations, enhancement of the Raman scattering cross-section is often necessary. Enhancement factors of orders of magnitude are achievable through resonance Raman (i.e., matched laser excitation with electronic transition) or surface enhanced Raman, where anisotropic, metallic nanoparticles of silver or gold are placed in close proximity to the molecule, either in solution or on a surface.

We are studying the low-frequency torsional modes (<200 cm-1) of proteins and polynucleotides. This region of the spectrum is rich with dynamical and structural information. A triple-grating monochromator provides the rejection capabilities necessary for observing these low-frequency vibrations. A companion molecular modeling effort assists in interpreting data in this complex spectroscopic region. The combination of theory and experiment, with the complementary CW Terahertz Spectroscopy effort in the group, greatly increases our ability to assign torsional vibrational modes to the flexibility of the biological molecule and provides the force field information needed to delineate the driving forces responsible for protein structure, folding, and function.