Mode-Specific Photoresponse of Retinal Protonated Schiff Base Isomers in the Reversible Photochromic Reactions of Microbial and Animal Rhodopsins

The primary photoisomerization reactions of the all-<i>trans</i> to 13-<i>cis</i> and 11-<i>cis</i> to all-<i>trans</i> retinal protonated Schiff base (RPSB) in microbial and animal rhodopsins, respectively, occur on a subpicosecond time scale with high quantum yields. At the same time, the isolated RPSB exhibits slower excited-state decay, in particular, in its all-<i>trans</i> form, and hence the interaction with the protein environment is capable of changing the time scale as well as the specificity of the reaction. Here, by using the high-level QM/MM calculations, we provide a comparative study of the primary photoresponse of <i>cis</i> and <i>trans</i> RPSB isomers in both the initial forms and first photoproducts of microbial <i>Krokinobacter eikastus</i> rhodopsin 2 (KR2) and <i>Halobacterium salinarum</i> bacteriorhodopsin (BR), and animal <i>Bos taurus</i> visual rhodopsin (Rho). By simulating photoabsorption band shapes of RPSB inside the proteins, we show that its photoresponse is highly mode-specific for the forward reactions, resulting in excitation of those vibrational modes that facilitate particular double-bond isomerization. The reverse reaction shows specificity only for 13-<i>cis</i> isomers in microbial rhodopsins, whereas the specificity is lost for all-<i>trans</i> RPSB in visual rhodopsin. This indicates evolutionary highly tuned 11-<i>cis</i> chromophore-protein interactions in visual rhodopsin. We also highlight the differences in the photoresponse of RPSB in two microbial rhodopsins and discuss the implications to their excited-state dynamics.