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.
Highlights
The retinal protonated Schiff-base (RPSB) in its all-trans form is found in bacterial rhodopsins, whereas visual rhodopsin proteins host 11-cis RPSB
The excited-state dynamics of RPSB-containing proteins as well as of RPSB in solutions shows a variety of time constants, typically in the sub-ps and few-ps regime
For the first time, the dynamics of the bare RPSB chromophore in vacuo and found timescales of the very same order
Summary
The retinal protonated Schiff-base (RPSB) in its all-trans form is found in bacterial rhodopsins, whereas visual rhodopsin proteins host 11-cis RPSB. The cis isomer is found to behave much faster than all-trans in MeOH at early times, but showed the same slow (4 ps) behavior on the longer timescale[24,25,26] To account for both fast and slow dynamics, as well as the low quantum yield of only 0.22 in solution, it was proposed that a primary role of the protein is to adjust the ratio between ground-state chromophore conformations that lead to reactive and nonreactive decay channels[27]. Interactions inside proteins, as well as in solutions, significantly change the energy gap between the ground and excited electronic states in the Franck–Condon region, and may change the excited-state potential-energy surface around conical intersections This may cause optimum conditions for fast isomerization and high quantum yields in proteins. The difference may be ascribed to significant perturbations taking place primarily in solutions, which slow down the isomerization and change the quantum yield significantly
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
