Early picosecond events in the photo cycle of Bacteriorhodopsin
The primary processes of the photochemical cycle of light-adapted bacteriorhodopsin (BR) were studied by various experimental techniques with a time resolution of 5 \xd7 10-13 s. The following results were obtained. (a) After optical excitation the first excited singlet state S1 of bacteriorhodopsin is observed via its fluorescence and absorption properties. The population of the excited singlet state decays with a lifetime \u03c41 of ~0.7 ps (430 \xb1 50 fs) (52). (b) With the same time constant the first ground-state intermediate J builds up. Its absorption spectrum is red-shifted relative to the spectrum of BR by ~30 nm. (c) The second photoproduct K, which appears with a time constant of \u03c42 = 5 ps shows a red-shift of 20 nm, relative to the peak of BR. Its absorption remains constant for the observation time of 300 ps. (d) Upon suspending bacteriorhodopsin in D2O and deuterating the retinal Schiff base at its nitrogen (lysine 216), the same photoproducts J and K are observed. The relaxation time constants \u03c41 and \u03c42 remain unchanged upon deuteration within the experimental accuracy of 20%.