PH-DEPENDENCE OF THE RATE AND QUANTUM YIELD OF THE RETINAL PHOTOISOMERIZATION IN BACTERIORHODOPSIN

The quantum yield of the retinal photoisomerization, the primary step in the bacteriorhodopsin (bR) photosynthesis, is determined from the p icosecond bleach recovery of the absorption at 560 nm in the pH range 6.8-11.3. From this and the dependence of the observed retinal excited state decay on pH, the effects of pH on both the photoisomerization a nd internal conversion rates are also determined. The results show tha t while the quantum yield of photoisomerization is independent of pH, each of the rate constants decreased by more than a factor of 2 at ele vated pH. This suggests that the perturbation resulting from the pH ch anges affects both rate constants similarly. This observation is discu ssed in terms of the theoretically proposed potential energy surfaces of the ground and excited states of retinal in bR.