Rhodopsins constitute a broad class of retinal-binding photoreceptors. Microbial rhodopsins are canonically activated through an all-trans to 13-cis photoisomerization, whereas animal rhodopsins are mostly activated through an 11-cis to all-trans isomerization. Bestrhodopsins constitute a special microbial rhodopsin subfamily, with bistable rhodopsin domains that can be photoswitched between a far red-absorbing state D661 and a green-absorbing state P540. Its photochemistry involves a peculiar all-trans to 11-cis isomerization for the D661 to P540 photoreaction and vice versa. Here, we present the P. antarctica bestrhodopsin 11-cis to all-trans photoreaction as determined by femtosecond-to-submillisecond transient absorption, femtosecond stimulated Raman and flash-photolysis spectroscopy. The primary photoreaction involves ultrafast isomerizations in 240 fs from the 11-cis reactant to a mixture of highly distorted all-trans and 13-cis photoproducts. The 13-cis fraction then thermally isomerizes to a distorted all-trans RSB in 120 ps. We propose bicycle pedal models for the branched photoisomerizations with corotation of the C11═C12 and C13═C14 double bonds. One reactant fraction undergoes bicycle pedal motion aborted at the C13═C14 double bond, resulting in all-trans retinal. The other fraction undergoes a full bicycle pedal motion of both C11═C12 and C13═C14, resulting in 13-cis retinal. The primary products are trapped high up the ground-state potential energy surface with a low energetic barrier that facilitates thermal isomerization from 13-cis to all-trans retinal in 120 ps. All-trans retinal then structurally and energetically relaxes with subsequent time constants of 0.7 and 62 μs and 4.4 ms, along with counterion protonation, completing the P540 to D661 photoreaction.

Department of Physics and Astronomy Vrije Universiteit Amsterdam De Boelelaan 1100 Amsterdam 1081 HV the Netherlands

Department of Physics University of Ioannina Ioannina Gr 45110 Greece

ELI Beamlines Facility The Extreme Light Infrastructure ERIC Za Radnicí 835 Dolní Břežany 252 41 Czech Republic

Institut für Biologie Experimentelle Biophysik Humboldt Universität zu Berlin Invalidenstrasse 42 Berlin D 10115 Germany

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