Early picosecond events in the photocycle 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 x 10(-13) s. The following results were obtained. (a) After optical excitation the first excited singlet state S(1) of bacteriorhodopsin is observed via its fluorescence and absorption properties. The population of the excited singlet state decays with a lifetime tau(1) of approximately 0.7 ps (430 +/- 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 approximately 30 nm. (c) The second photoproduct K, which appears with a time constant of tau(2) = 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 D(2)O and deuterating the retinal Schiff base at its nitrogen (lysine 216), the same photoproducts J and K are observed. The relaxation time constants tau(1) and tau(2) remain unchanged upon deuteration within the experimental accuracy of 20%.