Exploring the active site structure of a photoreceptor protein by Raman optical activity.

We have developed a near-infrared excited Raman optical activity (ROA) spectrometer and report the first measurement of near-infrared ROA spectra of a light-driven proton pump, bacteriorhodopsin. Our results demonstrate that a near-infrared excitation enables us to measure the ROA spectra of the chromophore within a protein environment. Furthermore, the ROA spectra of the all-trans, 15-anti and 13-cis, 15-syn isomers differ significantly, indicating a high structural sensitivity of the ROA spectra. We therefore expect that future applications of the near-infrared ROA will allow the experimental elucidation of the active site structures in other proteins as well as reaction intermediates.