Resonance Raman Spectroscopy of Metalloproteins Using CW Laser Excitation

The study of metalloproteins by resonance Raman (RR) spectroscopy began two decades ago, with the publication by Long and coworkers of the first RR spectrum of the iron-sulfur protein rubredoxin (1 ,2 ). This simple spectrum, which contained only four bands attributed to the Fe-S stretching and bending vibrations of the protein FeS4 cluster, generated much interest because of the potential of RR spectroscopy for monitoring structures of metal centers in complex biological systems (3 ,4 ). The unique ability of this technique to study the coordination environment of transition metals in proteins derives from its dramatic increase in detection sensitivrty and selectivity for vibrations closely associated with atoms at the absorbing center(s) in the molecule. When the molecule is excited with a strong monochromatic light whose energy matches that of an electric-dipole allowed electronic transition, a vibronic coupling with the electronically excited state increases the probability of observing Raman scattering from vibrational transitions in the electronic ground state, and the modes that do show enhancement are localized on the chromophore (i.e., on the group of atoms that gives rise to the electronic transition). Since vibrational frequencies are sensitive to molecular bond strength, number of atoms, geometry, and coordination environment, the positions of the enhanced Raman bands can be used to monitor the chromophoric structure. Metalloproteins frequently exhibit allowed electronic transitions, owing to π-π* and/or ligand-metal charge-transfer (CT) transitions (5 ), and consequently, they give wide scope to the application of RR spectroscopy. A great many RR studies of heme proteins, cobalamin, chlorophylls, carotenoids, flavin nucleotides, the visual pigments, and bacteriorhodopsin, and a variety of iron and copper metalloprotein sites have been carried out in laboratories around the world (6 –11 ).