The photocycle of bacteriorhodopsin

As an example of a classic photo-induced biochemical process, we discuss the photocycle of bacteriorhodopsin, a model system for the biological basis of vision. The information presented was obtained using pump-probe optical spectroscopy and X-ray diffraction from flashfrozen samples. Pump-probe SAXS or Laue experiments at the SwissFEL offer the exciting possibility of directly determining the detailed molecular structure of even the shor test-lived intermediate states. In higher organisms, including invertebrates and humans, it is the membrane protein rhodopsin, found in the rod cells of the retina (see Fig. IV.6), which delivers the photon energy to a complex series of biochemical processes, leading ultimately to an electrical polarization of the membrane and hence to a nerve impulse. In 1967, Wald, Granit and Har tline received the Nobel prize for medicine for the discovery of the initial event in vision: a photon-induced trans → cis structural transformation in retinal, the photoreceptor located at the center of the rhodopsin molecule. This transformation is the fastest biological photoreaction known (see Fig. IV.7). Due to a highly complex photocycle and problems of photo-instability, rhodopsin is difficult to handle in the laboratory. Bacteriorhodopsin (bR), extracted from the “purple membrane” of Halobacterium salinarum, is easier to study and has a simpler photocycle (see Fig. IV.8) [17]. However, in spite of the fact that no membrane protein has been studied as extensively as bR, details of its photocycle are still controversial. Recent fs optical spectroscopy experiments have identified two additional shor t-lived intermediate states, not shown in Figure IV.8, which form just after the photoexcitation (see Fig. IV.7, above): The “I-state”, which forms within 200 fs, is believed to incorporate an incomplete (90°) bond rotation, and the “J-state”, with a rise time of 500 fs, may be a vibrationally-excited version of the K state. Finally, high-resolution flash-frozen X-ray diffraction investigations of bR indicate distor tions in the helices of the entire bR molecule [18] (see Fig. IV.9). For bacteriorhodopsin and other photo-sensitive biomolecules, a wealth of information is awaiting the greatly improved spatial and temporal resolutions of the Swiss- FEL.