Raman Spectroscopy of Proteins and Their Assemblies

Abstract

Laser Raman spectroscopy, like infrared spectroscopy, is a method for determining molecular structure by measuring the energies (frequencies) of molecular vibrations. Although the two methods differ fundamentally in the mechanisms of interaction between radiation and matter, one obtains in both cases a vibrational spectrum consisting of a number of discrete bands, the frequencies and intensities of which are determined by the nuclear masses in motion, the equilibrium molecular geometry, and the molecular force field. An important advantage of Raman over infrared spectroscopy for biological applications is the virtual transparency of water (both H2O and D2O) in the Raman effect. This greatly simplifies the analysis of aqueous solutions and facilitates the investigation of hydrogen-isotope exchange phenomena. Changes in molecular geometry—particularly the conformational transitions characteristic of biological macromolecules—can produce large shifts in Raman band positions, often referred to as frequency shifts, empowering the technique in the diagnosis of protein secondary structure, determination of side-chain configurations, and detection of interacting side-chain groups. Since the molecular geometry and force field may be sensitive to interactions between molecules, the Raman method also has the potential for investigating intermolecular interactions, including the formation of biologically important protein complexes. Raman spectroscopy is gaining wide use as a method for probing protein structure, dynamics, assembly, and recognition.