Excited state properties of 3'-hydroxyechinenone in solvents and in the orange carotenoid protein from Synechocystis sp. PCC 6803.

The orange carotenoid protein (OCP) is a 35 kDa water-soluble protein involved in a photoprotective mechanism of the photosynthetic apparatus of cyanobacteria. The OCP protein contains a single molecule of the carotenoid 3'-hydroxyechinenone (3'-hECN). We have performed transient absorption studies at 77 K in the visible and near-infrared spectral ranges on 3'-hECN in solvent glass and in both inactive (orange) and active (red) forms of OCP. In the OCP the cryogenic temperature prohibited the protein from spontaneous conversion between activity stages and allowed us to study well-defined spectral forms of the protein. The studies show that each form of the OCP consists of two protein subpopulations having different photophysical properties of the bound 3'-hECN. At 77 K the inactive OCP reveals two lifetimes of the first excited state of 3'-hECN of 5.2 and 11 ps while in the active form of OCP these are 3.2 and 7.1 ps. We have also determined the energy of the first excited singlet state of 3'-hECN in long-lived subpopulations of both OCP forms at 77 K. This is 13,750 cm(-1) in the inactive OCP and 12,300 cm(-1) in the active OCP. Shortening of the lifetime and decrease of the energy of the first excited singlet state of 3'-hECN confirm the lengthening of the effective conjugation of the carotenoid upon the inactive-to-active conversion of OCP.