The Orange Carotenoid Protein (OCP) mediates nonphotochemical quenching (NPQ) mechanisms in cyanobacteria using a bound ketocarotenoid as a sensor of light intensity and as a quencher of phycocyanobilin excitons in the phycobilisome. During periods of high light intensity, the protein undergoes a conversion from a resting, orange state (OCPO) to an active, red state (OCPR) after optical preparation of the S2 state of the ketocarotenoid. The mechanism by which this conversion is photochemically initiated remains an open question. We determined that the spectrum, polarization, and quantum yield of fluorescence from two examples of ketocarotenoids bound in OCPO, 3'-hydroxyechinenone and canthaxanthin, reveal important details of the dynamics that follow optical preparation of the S2 state. Additionally, by measuring the fluorescence emission anisotropy of a prototype carotenoid, β-carotene, we observed large-amplitude distortions after the S2 state passes over a low activation-energy barrier that are consistent with twisting and pyramidalization near the center of the isoprenoid backbone. These results indicate that the ketocarotenoid is poised in OCPO in a distorted conformation from which the photoactivation reaction is triggered by further excited-state torsional distortions of the conjugated polyene backbone.
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