[FLuorescence of photoactive bacteriorhodopsin]

Fluorescence characteristics of all-trans-bacteriorhodopsin (BR570), 13-cis-BR (BR560), and the intermediate of the BR cycle (P585) are obtained by measuring variable fluorescence in the course of their phototransformations. The fluorescence spectra (lambda greater than 580 nm) have a maximum at 720, 730 and 740 nm, their respective excitation spectra with a maximum of 578, 565 and 585 nm coincide with the absorption spectra of the three states. The emission quantum yield for P585 is approximately 10(-4) at -62 degrees C and 4 +/- 2 10(-4) for BR560 at -196 degrees C. The effect of sharp increase of BR570 fluorescence quantum yield (from less than or equal to 10(-4) to approximately 10(-3)) under action of light at -196 degrees C (photoactivation or induction of emission) is found which points to the existence of photoprocess parallel to the photoconversion of BR570 into P600 (formula: see text). The emission of BR560 is not photoactivated. The photoactivated state BR570 is not identical with pseudo-BR of Gillbro et al., (1977) which has a maximum in the excitation spectrum at 597 nm. The greater part of the red fluorescence of the membrane belongs to the minor nonphotoactive forms of BR (B578, B630) with relatively high fluorescence yield (greater than or equal to 10(-2) at -196 degrees C). The fluorescence characteristics of BR are discussed in the framework of a scheme in which the existence of an intermediate excited level common for the emission and photoconversion and a ground level common for the initial state and photoproduct is suggested.