Time-resolved protein fluorescence studies of intermediates in the photochemical cycle of bacteriorhodopsin.

Abstract

The photolysis-induced changes in the protein fluorescence intensity (at 320 nm) during the proton-pumping cycle of bacteriorhodopsin were examined by a delayed 2-pulse technique in the time range 1 .mu.s-20 ms at room temperature. No detectable change in the protein fluorescence intensity was observed on the earliest time scale within the lifetime of the intermediate K590, when retinal apparently undergoes the largest structural changes. The time dependence of the relative changes in fluoresence intensity did display a close correlation with the population of the L550 and M412 intermediates. From a computer numerical fit of the data, with available published kinetic parameters, the protein fluorescence quantum yields of the K590, L550 and M412 intermediates are found to be 1.0, 0.92 and 0.80 of that for native bR570, respectively. The probable mechanisms of the observed fluorescence quenching during the photochemical cycle are qualitatively discussed.