Fig. 4. Revised photocycle model of GtACR1.

a The extended photocycle model is based on our ambient temperature time-resolved FTIR-spectroscopic data (Fig. 2) along with our UV/VIS spectroscopic data (Supplementary Fig. 4) and previously published electrophysiological measurements³². The K to L transition precedes channel opening indicating a non-conducting L₁ state. Channel opening occurs in a two-step process with a fast (18 µs) and a slow (1.9 ms) opening process to the conducting state L₂ (red). Therefore, L₁ was subdivided into L₁ and L₁’. The possible kinetic correlations of L₁ and L₂ are detailed in Supporting Fig. 5. Mechanistically, channel closing occurs exclusively upon M formation. However, the photocurrent fully disappears only upon M decay, resulting in a fast and a slow observed channel closing due to the reversible reaction between M and L₂. Next, GtACR1 slowly relaxes into the ground state from the N/O intermediate state. b Different retinal states during the photocycle. All-trans,C=N-anti-structure (black box) is taken from chain A of the X-ray structure of PDB-ID 6EDQ²⁷ and protonated according to our measurement. Structures in the blue and orange boxes are obtained by energy minimization of the retinal conformations with a restraint on the 13-cis,C=N-anti-conformation. These structures indicate the 13-cis,C=N-anti-retinal conformation with protonated and deprotonated Schiff base observed by our measurements. No C=N-anti-species is present.