Carotenoid-protein interaction alters the S1 energy of hydroxyechinenone in the Orange Carotenoid Protein

The Orange Carotenoid Protein (OCP) is a photoactive water soluble protein that is crucial for photoprotection in cyanobacteria. When activated by blue-green light, it triggers quenching of phycobilisome fluorescence and regulates energy flow from the phycobilisome to the reaction center. The OCP contains a single pigment, the carotenoid 3'-hydroxyechinenone (hECN). Binding to the OCP causes a conformational change in hECN leading to an extension of its effective conjugation length. We have determined the S-1 energy of hECN in organic solvent and compared it with the S-1 energy of hECN bound to the OCP. In methanol and n-hexane, hECN has an S-1 energy of 14,300 cm(-1), slightly higher than carotenoids with shorter conjugation lengths such as zeaxanthin or beta-carotene; this is consistent with the proposal that the presence of the conjugated carbonyl group in hECN increases its Si energy. The S-1 energy of hECN in organic solvent is independent of solvent polarity. Upon binding to the OCP, the S-1 energy of hECN is further increased to 14,700 cm(-1), underscoring the importance of protein binding which twists the conjugated carbonyl group into s-trans conformation and enhances the effect of the carbonyl group. Activated OCP, however, has an S-1 energy of 14,000 cm(-1), indicating that significant changes in the vicinity of the conjugated carbonyl group occur upon activation. (C) 2012 Elsevier B.V. All rights reserved.