Resonance Raman Methods for Proton Translocation in Bacteriorhodopsin

Resonance Raman spectroscopy is a useful technique for studying changes in the molecular structure of a biological chromophore. This chapter focuses on the resonance Raman methods used for proton translocation in bacteriorhodopsin. Bacteriorhodopsin is an ideal proton pump to study with resonance Raman spectroscopy. Its retinylidene chromophore has a visible absorption band that shifts significantly during the proton-pumping photocycle, which enhances the Raman scattering from each photointermediate selectively by using a suitable visible laser excitation wavelength. Resonance Raman studies yield a number changes in the state of protonation and double-bond configuration of the chromophore during the proton-pumping photocycle. It provides useful constraints on models for the mechanism of bacteriorhodopsin. Two recent applications of resonance Raman spectroscopy to the study of protein-induced perturbations of the K chromophore are described in the chapter. First, the vibrations of the Schiff's base proton are analyzed to investigate possible changes in hydrogen bonding of the C=NH group arising from the primary photoisomerization. Second, two-color, time-resolved resonance Raman spectroscopy is applied to detect a partial relaxation of a Schiff's base structural perturbation within 60 nsec after K formation.