The functions of microbial and metazoa rhodopsins are usually triggered by the isomerization of their chromophore C13=C14 and C11=C12 bond respectively. To disclose a possible molecular-level basis for such a selection, multiconfigurational quantum chemistry is used to model two forms (ASR13C and ASRAT) of the sensory rhodopsin from the cyanobacterium Anabaena PCC 7120 and compare their simulated isomerizations with that of a vertebrate (bovine) rhodopsin (Rh). It is found that, in spite of their evolutionary distance, the selected eubacterial and vertebrate rhodopsins isomerize via distinct implementations of the bicycle-pedal mechanism originally proposed by Warshel. However, in spite of such similarity, ASR13C and ASRAT, hosting a 13-cis and all-trans chromophore respectively, have somewhat slower excited state dynamics with respect to Rh. By mapping the electronic structure progression of the ASR forms during their excited state lifetime, we provide evidence for the involvement of an extra excited state which is neglected in Rh. We show that such an involvement is modulated by the pre-organized structure of the chromophore and, most importantly, by the position of the reactive double bond. The results suggest that the 11-cis chromophore of vertebrate and invertebrate visual pigments as well as of photoentraining pigments has been selected to maximize photoisomerizaztion speed and light sensitivity.

Electronic progression during the photoisomerization of microbial and vertebrate light-sensing rhodopsins

249th National Meeting (2015)

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