The photoisomerization of retinal in bacteriorhodopsin: Experimental evidence for a three-state model

Hasson, K. C.; Gai, Feng; Anfinrud, Philip

doi:10.1073/pnas.93.26.15124

Cited by 185 publications

(272 citation statements)

References 43 publications

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“…The above scenario agrees with the theoretical (33) and experimental evidence (9,11,17,23,34) for the so-called two-state͞two-mode model. Basically, after ultrafast (Ͻ50 fs) departure from the Franck-Condon region along high-frequency skeletal modes, the molecule undergoes low-frequency torsional motion, coherently excited by the former modes, eventually leading to the isomerization of retinal.…”

Section: Discussionsupporting

confidence: 89%

“…The instantaneous rise (within the cross correlation) of the F 3 component clearly speaks for a retinal signal, in sharp contrast with the slower rise of the Trp response (F 1 component). In addition, the decay times are very close to those reported in experiments probing retinal itself, either by stimulated emission (10,31) or excited-state absorption at 460 nm (23).…”

Section: Discussionsupporting

confidence: 79%

“…Here, we report on transient absorption measurements, with specific spectroscopic fingerprints of the all-trans retinal, the Trp, and the photoproduct, and we identify the formation of a transition state by an impulsive mechanism, caused by arrival of a wave packet of torsional modes along the isomerization coordinate. We believe that this particular behavior was hidden in the traditionally investigated red part of the spectrum (600-800 nm) (3,5,9,23), because of overlapping absorption and stimulated emission contributions of the all-trans and cis photoproduct. We also present mechanisms by which the translocation of charge along retinal triggers the isomerization, once the system is in the transition state.…”

mentioning

confidence: 97%

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Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

Schenkl

Mourik

Friedman

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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A visible-pump͞UV-probe transient absorption is used to characterize the ultrafast dynamics of bacteriorhodopsin with 80-fs time resolution. We identify three spectral components in the 265-to 310-nm region, related to the all-trans retinal, tryptophan (Trp)-86 and the isomerized photoproduct, allowing us to map the dynamics from reactants to products, along with the response of Trp amino acids. The signal of the photoproduct appears with a time delay of Ϸ250 fs and is characterized by a steep rise (Ϸ150 fs), followed by additional rise and decay components, with time scales characteristic of the J intermediate. The delayed onset and the steep rise point to an impulsive formation of a transition state on the way to isomerization. We argue that this impulsive formation results from a splitting of a wave packet of torsional modes on the potential surface at the branching between the all-trans and the cis forms. Parallel to these dynamics, the signal caused by Trp response rises in Ϸ200 fs, because of the translocation of charge along the conjugate chain, and possible mechanisms are presented, which trigger isomerization.ultrafast spectroscopy ͉ retinal proteins ͉ translocation of charge ͉ structural dynamics T he different biological functions of retinal proteins (vision, ion pumping, light signaling, etc.) rely on the ultrafast isomerization of the retinal chromophore (1). This process has been intensively investigated by fs pump-probe and f luorescence up-conversion spectroscopy in the visible (VIS) and͞or near-IR (2-13), revealing the retinal excited-state dynamics before isomerization and the build-up of the isomerized form. Time-resolved VIS-pump͞mid-IR probe experiments on bacteriorhodopsin (bR) established that the isomerization time is Ն400 fs (14). Indeed, the excited-state lifetime of 300 -500 fs (12, 15) goes hand-in-hand with the rise times observed for the 13-cis photoproduct (3,4,14), whereas a longer time scale of 3-4 ps has been attributed to vibrational relaxation of the photoproduct (14).The mechanism leading to the isomer is still subject to debate. Using Ͻ5-fs pulses, Kobayashi et al. (11) were able to monitor changes in the high-frequency vibrational spectrum of the excited state, suggesting that retinal is in a twisted, nonplanar configuration during the initial 200 fs. They also showed that the high-frequency modes were modulated with a period of Ϸ200 fs, corresponding to the low-frequency torsional modes observed in transient absorption experiments at 800-nm probe wavelength (16). That the system is all-trans-like during the first 200 fs is in line with Zhong et al. (17), who carried out transient absorption studies on wild-type bR and bR reconstituted with retinal analogs that cannot undergo isomerization and found identical dynamics during the initial 200 fs. In addition, McCamant et al. (18) recently showed by fs-stimulated Raman spectroscopy that the high-frequency modes (1,000-1,500 cm Ϫ1 ) decay on a time scale of Ϸ250 fs. They argued that this observation points to the excited-sta...

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Section: Discussionsupporting

confidence: 89%

Section: Discussionsupporting

confidence: 79%

mentioning

confidence: 97%

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Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

Schenkl

Mourik

Friedman

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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“…The observed constancy of the excited-state stimulated emission spectrum 13 dictates that the first step in RINE (i.e., the two initial electric field couplings shown in Figure 2 at t 1 and t 2 ) must occur with unchanged efficiency throughout the lifetime of the excited state. Thus, the faster decay of the RINE signal compared to the stimulated emission must be due to a loss in efficiency of the final field couplings, at t 1 ′ and t. The most likely scenario that would cause this is a rapid loss of vibrational coherence on the ground-state surface between times t 2 and t 1 ′ .…”

Section: -15mentioning

confidence: 97%

Femtosecond Stimulated Raman Study of Excited-State Evolution in Bacteriorhodopsin

2005

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Femtosecond time-resolved stimulated Raman spectroscopy (FSRS) is used to examine the photoisomerization dynamics in the excited state of bacteriorhodopsin. Near-IR stimulated emission is observed in the FSRS probe window that decays with a 400-600-fs time constant. Additionally, dispersive vibrational lines appear at the locations of the ground-state vibrational frequencies and decay with a 260-fs time constant. The dispersive line shapes are caused by a nonlinear effect we term Raman initiated by nonlinear emission (RINE) that generates vibrational coherence on the ground-state surface. Theoretical expressions for the RINE line shapes are developed and used to fit the spectral and temporal evolution of the spectra. The rapid 260-fs decay of the RINE peak intensity, compared to the slower evolution of the stimulated emission, indicates that the excited-state population moves in ∼260 fs to a region on the potential energy surface where the RINE signal is attenuated. This loss of RINE signal is best explained by structural evolution of the excited-state population along multiple low-frequency modes that carry the molecule out of the harmonic photochemically inactive Franck-Condon region and into the photochemically active geometry.

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“…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.…”

mentioning

confidence: 99%

Primary reactions of sensory rhodopsins

Lutz¹

2001

Proceedings of the National Academy of Sciences

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The first steps in the photocycles of the archaeal photoreceptor proteins sensory rhodopsin (SR) I and II from Halobacterium salinarum and SRII from Natronobacterium pharaonis have been studied by ultrafast pump͞probe spectroscopy and steady-state fluorescence spectroscopy. The data for both species of the bluelight receptor SRII suggests that their primary reactions are nearly analogous with a fast decay of the excited electronic state in 300 -400 fs and a transition between two red-shifted product states in 4 -5 ps. Thus SRII behaves similarly to bacteriorhodopsin. In contrast for SRI at pH 6.0, which absorbs in the orange part of the spectrum, a strongly increased fluorescence quantum yield and a drastically slower and biexponential decay of the excited electronic state occurring on the picosecond time scale (5 ps and 33 ps) is observed. The results suggest that the primary reactions are controlled by the charge distribution in the vicinity of the Schiff base and demonstrate that there is no direct connection between absorption properties and reaction dynamics for the retinal protein family.S ensory rhodopsin (SR) I and II are members of the family of retinal-containing membrane proteins of archaea, which also includes the well-known ion pumps bacteriorhodopsin (BR) and halorhodopsin (HR). SRI and SRII act as receptors in phototaxis. In this process light absorption triggers movement of the flagellated cells toward favorable or away from unfavorable environments, depending on the wavelength of the absorbed light. Thus, the SRs provide a simple color-sensing mechanism (1) for haloarchaea like Halobacterium salinarum and other species. Attractant and repellent reactions of the cell are triggered by light absorption of specific photochromic states of these photoreceptors. In SRI, the dark-adapted state SRI 587 is converted into a blue-shifted intermediate S 373 by absorbing orange light around 590 nm (2), leading to positive signaling via the closely coupled transducer protein HtrI (3). Absorption of a UV photon in the S 373 state leads to a repellent stimulus, resulting in a photophobic response of the cell. SRII is a blue-light receptor with maximum ground-state absorption around 490 nm. The repellent signaling is initiated by the intermediate SRII 360 (M) in the course of the SRII photocycle (4).SRI and SRII are structurally very similar to BR and HR; sequence homology is especially striking in the chromophore environment (5, 6). Although 13 of 21 residues in a 5-Å shell around the retinal atoms are strictly conserved in BR, HR, and SRI of all species sequenced to date, SRII has three residues less conserved (7). This might be one reason for the blue-shifted absorption maximum of SRII compared with the other archaeal receptor proteins (8). Photoexcitation triggers a series of conformational changes initiated by trans-cis isomerization of the retinal chromophore in all retinal-containing membrane proteins (9). The first events lead to the formation of a red-shifted intermediate observed on the subnanosecond ...

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The photoisomerization of retinal in bacteriorhodopsin: Experimental evidence for a three-state model

Cited by 185 publications

References 43 publications

Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

Femtosecond Stimulated Raman Study of Excited-State Evolution in Bacteriorhodopsin

Primary reactions of sensory rhodopsins

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