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. 2022 Jun 22;10:879609. doi: 10.3389/fchem.2022.879609

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Copyright © 2022 de Grip and Ganapathy.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Chemical structures of the most common chromophore configurations in the rhodopsin families. The type-2 pigments contain an 11-cis, 15-anti retinylidene Schiff base of retinal A1 (A) in the “dark state” (or “ground state” in photophysical terminology), which is photo-excited into the all-trans configuration. The Type-1 pigments contain the all-trans configuration (B) in the “dark state.” This is photo-excited into the 13-cis, 15-anti configuration (C), which thermally relaxes and re-isomerizes, returning to the ground state. The ring-polyene chain orientation is different for type-2 (6-s-cis) and type-1 (6-s-trans) rhodopsins.