Emission Spectroscopy of Rhodopsin and Bacteriorhodopsin

The detection of emission from the excited state of retinylidene chromophores complexed to membrane proteins is an area that has only recently begun to yield reliable and important information. This chapter reviews studies that aimed at critically assigning the emitting species, calculating the quantum yield of emission, elucidating the nature of the excited state responsible for the emission, studying the temperature dependence of the emission, and interpreting all of these data to help determine the nature of the primary photochemistry in bacteriorhodopsin. It has been generally assumed that bacteriorhodopsin emission emanates from a weakly allowed excited-to-ground-state transition from a state that lies lower in energy than the excited state responsible for the strongly allowed visible absorption. It is demonstrated that these previous assumptions on the nature of the transition could be incorrect and that the emission could emanate from a strongly allowed transition. It is reported that little activity in fluorescence spectroscopy of rhodopsin and bacteriorhodopsin is mainly the result of the extreme difficulties encountered in detecting the weak emissions from retinylidene membrane proteins.