Photophysics and Molecular Electronic Applications of the Rhodopsins

Abstract

This review examines the photophysical properties of an interesting and biologically important class of proteins known as the rhodopsins. The visual rhodopsins are responsible for the conversion of light into nerve impulses in the image resolving eyes of mullusks, arthropods, and vertebrates. Despite independent evolutionary development, these systems have converged on proteinand chromophore-binding site designs that are remarkably similar. The bacterial rhodopsins represent a much broader class of proteins that serve both photosynthetic and phototactic functions. The best known of this latter group is bacteriorhodopsin, which converts light into energy via photon-activated transmembrane proton pumping. Regardless of function, all rhodopsins appear to share common features, which include a retinyl chromophore in a protein-binding site formed within the inner segments of seven transmembrane helices. Upon the absorption of light, the chromophore isomerizes to generate a bathochromically shifted photoproduct that stores a significant fraction of the photon energy in both electrostatic and conformational forms. Subsequent dark reactions carry out the relevant biological function. The nature of the primary photochemical events in the rhodopsins has