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Intrinsic photoisomerization dynamics of protonated Schiff-base retinal.

ABSTRACT: The retinal protonated Schiff-base (RPSB) in its all-trans form is found in bacterial rhodopsins, whereas visual rhodopsin proteins host 11-cis RPSB. In both cases, photoexcitation initiates fast isomerization of the retinal chromophore, leading to proton transport, storage of chemical energy or signaling. It is an unsolved problem, to which degree this is due to protein interactions or intrinsic RPSB quantum properties. Here, we report on time-resolved action-spectroscopy studies, which show, that upon photoexcitation, cis isomers of RPSB have an almost barrierless fast 400?fs decay, whereas all-trans isomers exhibit a barrier-controlled slow 3?ps decay. Moreover, formation of the 11-cis isomer is greatly favored for all-trans RPSB when isolated. The very fast photoresponse of visual photoreceptors is thus directly related to intrinsic retinal properties, whereas bacterial rhodopsins tune the excited state potential-energy surface to lower the barrier for particular double-bond isomerization, thus changing both the timescale and specificity of the photoisomerization.

SUBMITTER: Kiefer HV

PROVIDER: S-EPMC6418104 | biostudies-literature | 2019 Mar

REPOSITORIES: biostudies-literature

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Intrinsic photoisomerization dynamics of protonated Schiff-base retinal.

Kiefer Hjalte V HV Gruber Elisabeth E Langeland Jeppe J Kusochek Pavel A PA Bochenkova Anastasia V AV Andersen Lars H LH

Nature communications 20190314 1

The retinal protonated Schiff-base (RPSB) in its all-trans form is found in bacterial rhodopsins, whereas visual rhodopsin proteins host 11-cis RPSB. In both cases, photoexcitation initiates fast isomerization of the retinal chromophore, leading to proton transport, storage of chemical energy or signaling. It is an unsolved problem, to which degree this is due to protein interactions or intrinsic RPSB quantum properties. Here, we report on time-resolved action-spectroscopy studies, which show, t ...[more]

PMID: 30872581

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Project description:Channelrhodopsins (ChRs), which form a distinct branch of the microbial rhodopsin family, control phototaxis in green algae. Because ChRs can be expressed and function in neuronal membranes as light-gated cation channels, they have rapidly become an important optogenetic tool in neurobiology. While channelrhodopsin-2 from the unicellular alga Chlamydomonas reinhardtii (CrChR2) is the most commonly used and extensively studied optogenetic ChR, little is known about the properties of the diverse group of other ChRs. In this study, near-infrared confocal resonance Raman spectroscopy along with hydrogen-deuterium exchange and site-directed mutagenesis were used to study the structure of red-shifted ChR1 from Chlamydomonas augustae (CaChR1). These measurements reveal that (i) CaChR1 has an all-trans-retinal structure similar to those of the light-driven proton pump bacteriorhodopsin (BR) and sensory rhodopsin II but different from that of the mixed retinal composition of CrChR2, (ii) lowering the pH from 7 to 2 or substituting neutral residues for Glu169 or Asp299 does not significantly shift the ethylenic stretch frequency more than 1-2 cm(-1) in contrast to BR in which a downshift of 7-9 cm(-1) occurs reflecting neutralization of the Asp85 counterion, and (iii) the CaChR1 protonated Schiff base (SB) has stronger hydrogen bonding than BR. A model is proposed to explain these results whereby at pH 7 the predominant counterion to the SB is Asp299 (the homologue to Asp212 in BR) while Glu169 (the homologue to Asp85 in BR) exists in a neutral state. We observe an unusual constancy of the resonance Raman spectra over the broad range from pH 9 to 2 and discuss its implications. These results are in accord with recent visible absorption and current measurements of CaChR1 [Sineshchekov, O. A., et al. (2013) Intramolecular proton transfer in channelrhodopsins. Biophys. J. 104, 807-817; Li, H., et al. (2014) Role of a helix B lysine residue in the photoactive site in channelrhodopsins. Biophys. J. 106, 1607-1617].

Dataset Information

Intrinsic photoisomerization dynamics of protonated Schiff-base retinal.

Publications

Intrinsic photoisomerization dynamics of protonated Schiff-base retinal.

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