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Description
Rhodopsins constitute a broad class of sensory photoreceptors with retinal chromophores bound to the protein via a retinal Schiff base (RSB). Microbial rhodopsins are mostly activated through an all-trans to 13-cis photoisomerization reaction, whereas animal rhodopsins are invariably activated through an 11-cis to all-trans isomerization reaction. The recently discovered bestrhodopsins constitute a subfamily of very special bistable microbial rhodopsins. The P. antarctica bestrhodopsin photochemistry involves a very peculiar all-trans to 11-cis isomerization and vice versa, rather than the all-trans to 13-cis photoreaction of canonical microbial rhodopsins, and hence resemble animal rhodopsins in that regard. Here, we present the 11-cis to all-trans photoreaction as determined by femtosecond to sub-millisecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). The primary photoreaction involves ultrafast isomerizations in 240 fs from the 11-cis RSB reactant to a mixture of highly distorted all-trans and 13-cis RSB isomeric photoproducts. The 13-cis RSB isomer fraction of the primary photoproduct then thermally isomerizes to a distorted all-trans RSB in 120 ps. To rationalize this highly unusual phenomenology, we propose bicycle pedal models for the branched photoisomerizations from the 11-cis reactant to all-trans and 13-cis RSB isomer products, with co-rotation of the C11=C12 and C13=C14 double bonds. The former fraction undergoes bicycle pedal motion aborted at the C13=C14 double bond, resulting in an all-trans RSB isomer. The latter fraction undergoes a full bicycle pedal motion of both C11=C12 and C13=C14 double bonds, resulting in a 13-cis RSB isomer. The primary products are trapped high up the ground state potential energy surfact (PES) owing to steric interactions with the protein binding pocket. Due to the resulting low energetic barrier on the ground state PES, thermal isomerization from 13-cis to all-trans RSB occurs in 120 ps. We suggest that the mechanism for simultaneous production of two different isomers may generally apply for rhodopsins, where the production of only one isomer as in most animal and microbial rhodopsins may be regarded as limiting cases.
