Primary step in the bacteriorhodopsin photocycle: photochemistry or excitation transfer?

The absorption polarization of the first intermediate (K610) formed at room temperature in the proton-pumping photochemical cycle of bacteriorhodopsin (bR) shows a strong correlation with the polarization direction of the photolyzed parent molecule. The results suggest that, unlike other photosynthetic systems, excitation transfer does not take place prior to the primary photochemical change in bR. These observations together with the previously observed circular dichroism and the polarization temperature dependence are discussed in terms of the exciton structure and the nature of the absorption bandwidths (i.e., homogeneous vs. inhomogeneous) of the bR monomers within the trimer structure.

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

The photolysis-induced changes in the protein fluorescence intensity (at 320 nm) during the proton-pumping cycle of bacteriorhodopsin were examined by a delayed two-pulse technique in the time range 1 microsecond-20 msec 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 fluorescence intensity did, however, 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.