Fig. 1. Canthaxanthin single bond isomerization in the bicycle-pedal manner triggers OCP light activation.

a Structure of OCP in its inactive state (OCP^O) displaying the C-terminal domain (CTD, yellow colour), N-terminal domain (NTD, violent colour), N-terminal extension (NTE, pink colour), and loop (wheat colour). The canthaxanthin (CAN) carotenoid is embedded between the two domains. Cl⁻ (in green) is resolved in the CTD where it coordinates four water molecules (in red) through ion-dipole interactions at distances of 3.0–4.0 Å. b Absorbance changes of OCP in solution (solid lines) and crystalline state (dotted lines) upon illumination with violet light (at 410 nm with 3.5 mWcm⁻² power; FWHM 20 nm of the emission spectrum). Inactive state OCP^O and active state OCP^R are shown in black/grey and red/pink respectively. The OD intensities are unavoidably polluted by the LED emission in 380–430 nm and therefore were omitted from the graph. Insert: Absorbance changes (pink dots) in the crystalline state detected at 550 nm upon 10 min of illumination. Black line represents double exponential fit of the curve ($Y=A_1{\mathrm{e}}^{\frac{x}{t1}}+A_2{\mathrm{e}}^{\frac{x}{t2}}$, where A₁ = 0.02, t₁ = 38 s and A₂ = 0.44, t₂ = 291 s. Grey labels indicate time points when I1–I4 intermediates have been cryotrapped. Source data are provided as a Source Data file. c Difference electron density (DED) maps were obtained for the four states: (I1) 1 min (F_I1 – F_Dark), (I2) 2 min (F_I2 – F_Dark), (I3) 5 min (F_I3 – F_Dark), (I4)10 min (F_I4 – F_Dark). Red: negative DED, blue: positive DED on the −3σ/3σ contour levels, respectively. Carotenoid coordinates of dark (OCP^O) and light states (I1-I4) are represented by cyan and orange lines respectively. Illuminated coordinates were obtained from extrapolated electron densities. Populations of the illuminated states obtained from the occupancy refinement (see “Methods”) are 15% (I1), 22 % (I2), 25 % (I3), and 32% (I4). Arrow in the insert in l1 state indicates carotenoid movement during illumination. d CAN in C9’-C8’ trans/ C7’-C6’ cis state is present in OCP^O while C9’-C8’ cis/ C7’-C6’ trans conformation (bicycle pedal isomerization) is observed in I1. The latter thermally recovers back to OCP^O CAN state with prolonged illumination time (after 2–5 min).