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 × 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 τ1 of ~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 ~30 nm. (c) The second
photoproduct K, which appears with a time constant of τ2 = 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
τ1 and τ2 remain unchanged upon deuteration within the experimental
accuracy of 20%.