Fig. 1. Photocycle models of CrChR2 and GtACR1.

a Model of the branched photocycle of CrChR2 (reproduced and modified from Kuhne et al.¹⁵). The “dark-adapted” cycle (left) exclusively comprises a C=N-anti-conformation of the retinal and two well-conducting open states that decay relatively fast. In the “light-adapted” ground state (right), the retinal has a 13-cis,C=N-syn conformation, and its photoproducts comprise a poorly conducting open state that decays at a slower rate. During continuous illumination, molecules are accumulated in the syn-cycle, which causes the inactivation of CrChR2. b Current photocycle model of GtACR1 (reproduced and modified from Sineshchekov et al.²⁴), showing the l-intermediate state as the open state followed by M and N/O state. The question of the existence of a branched photocycle as in CrChR2 is a topic of this investigation.