In cyanobacteria, the orange carotenoid protein (OCP) mediates nonphotochemical quenching of bilin excited states in the phycobilisome. An intrinsic ketocarotenoid serves in OCP as an ambient light-intensity sensor by triggering the conversion of the resting, orange form (OCPO) to the active, red form (OCPR), which can then bind to the phycobilisome's core. We have performed fluorescence anisotropy measurements over the 80–296 K temperature range to determine the conformational motions of the ketocarotenoid canthaxanthin (CAN) that initiate the photoactivation response. The results reveal that even at cryogenic temperatures, CAN undergoes twisting and bending displacements of a C=C bond near the middle of the isoprenoid backbone during the nonradiative decay pathway from the optically prepared S2 state via the Sx intermediate to the dark S1 state. The anisotropy values are consistent with preparation of a pyramidal structure with significant delocalized intramolecular charge-transfer character near a seam of conical intersections with the S1 state. At low temperatures in 2-MTHF solution, however, the frozen solvent cavity restricts the motions of CAN in the S2 and Sx states to volume-conserving, concerted motions of more than one C=C bond, such as those of the "hula-twist" structure. These findings indicate definitively that the photoactivation reaction is initiated by conformational motions of the ketocarotenoid inside the OCP structure in the OCPO state and that these motions are not hindered at cryogenic temperatures. We suggest that these light-driven motions of the ketocarotenoid provide the photophysical trigger for the photoactivation reaction by initiating providing strain and electrostatic driving forces for the displacement of the C-terminal domain.

Detection of light-driven conformational motions of canthaxanthin inside the orange carotenoid protein

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