Early Picosecond Events in the Photocycle of Bacteriorhodopsin

Abstract

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%.

Primary reactions of sensory rhodopsins

“…1c). The resulting fluorescence quantum yield of pSRII is calculated to 1.2 ϫ 10 -4 , which is very close to the value of BR (1.3 ϫ 10 -4 ), which in turn is in good agreement with earlier publications (13). Estimation of excited-state lifetimes using ref.…”

“…In BR the kinetics at 490 nm (excited-state absorption) and 900 nm (stimulated emission) were used to assign the 500-fs transient to the transition to the first ground-state product J (13,31). Because sSRI has similar absorption properties as BR, the 490-nm absorption transient can be taken as an indication that both processes, the 5-ps and the 33-ps process, are related to the S 1 decay.…”

“…In recent years ultrafast spectroscopic experiments on the primary reaction dynamics in a number of photoreceptors such as rhodopsin, phytochrome, or photoactive yellow protein contributed to a better understanding of the construction principles and the molecular mechanisms of these systems (10)(11)(12). Also the primary events in the photocycles of the ion pumps BR and HR have been described in great detail (13)(14)(15)(16)(17)(18), but to our knowledge no data for the SRs in the ultrafast time domain were available until now.…”

A Photochromic Histidine Kinase Rhodopsin (HKR1) That Is Bimodally Switched by Ultraviolet and Blue Light

“…The I1 intermediate shows an overall blue shift of the difference spectrum compared with the excited state, along with a significant loss in signal. This difference spectrum represents a red shift of the absorbance of Rh-Bl, similar to that observed for J and K intermediates in bacteriorhodopsin (BR) (23). The final P560 intermediate is formed from I1 within 400 ps along with a blue shift, an increase in negative contributions, and an increase but no spectral shift in the induced absorption.…”

“…3, which show a spectrally silent deprotonated species before protonation occurs in 2.3 ms. The excited state lifetimes of 5 and 60 ps are unusually long for a rhodopsin (22,23). A study on unprotonated Schiff base model compounds showed that decreasing the polarity of the environment strongly increases the excited state lifetime (24).…”