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

Abstract

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 S1 energy of hECN in organic solvent and compared it with the S1 energy of hECN bound to the OCP. In methanol and n-hexane, hECN has an S1 energy of 14,300cm−1, slightly higher than carotenoids with shorter conjugation lengths such as zeaxanthin or β-carotene; this is consistent with the proposal that the presence of the conjugated carbonyl group in hECN increases its S1 energy. The S1 energy of hECN in organic solvent is independent of solvent polarity. Upon binding to the OCP, the S1 energy of hECN is further increased to 14,700cm−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 S1 energy of 14,000cm−1, indicating that significant changes in the vicinity of the conjugated carbonyl group occur upon activation.