Fluorescence and Excited-State Conformational Dynamics of the Orange Carotenoid Protein

The orange carotenoid protein (OCP) mediates nonphotochemical quenching (NPQ) mechanisms in cyanobacteria. A bound ketocarotenoid serves as a sensor of midvisible light intensity and as a quencher of phycocyanobilin excitons in the phycobilisome. The photochemical mechanism that triggers conversion of the protein from a resting, orange state (OCP^O) to an active, red state (OCP^R) after optical preparation of the S₂ state of the carotenoid remains an open question. We report here that the fluorescence spectrum and quantum yield of the bound carotenoids in OCP^O report important details of the motions that follow optical preparation of the S₂ state. The fluorescence spectra from OCP^O preparations containing 3'-hydroxyechinenone (3hECN) or canthaxanthin (CAN) are markedly mirror asymmetric with respect to the absorption line shape and more than an order of magnitude more intense than for carotenoids in solution. Further, 3hECN exhibits a narrower fluorescence line shape and a larger quantum yield than CAN because its excited-state motions are hindered by a hydrogen bonding interaction between the 3'-hydroxyl group on its β2 ring and Leu37 in the N-terminal domain. These results show that large-amplitude motions of the carotenoid's β2-cyclohexene ring and of the conjugated polyene backbone initiate photochemistry in OCP^O.