Multistep 11- to All- Retinal Photoisomerization in Bestrhodopsin, an Unusual Microbial Rhodopsin.

Rhodopsins constitute a broad class of retinal-binding photoreceptors. Microbial rhodopsins are canonically activated through an all- to 13- photoisomerization, whereas animal rhodopsins are mostly activated through an 11- to all- 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 to isomerization for the D661 to P540 photoreaction and vice versa. Here, we present the bestrhodopsin 11- to all- 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- reactant to a mixture of highly distorted all- and 13- photoproducts. The 13- fraction then thermally isomerizes to a distorted all- 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- retinal. The other fraction undergoes a full bicycle pedal motion of both C11═C12 and C13═C14, resulting in 13- 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- to all- retinal in 120 ps. All- 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.