Insights into the Structural Changes Occurring upon Photoconversion in the Orange Carotenoid Protein from Broadband Two-Dimensional Electronic Spectroscopy

Carotenoids play an essential role in photo­protection, interacting with other pigments to safely dissipate excess absorbed energy as heat. In cyano­bacteria, the short time scale photo­protective mechanisms involve the photo­active orange carotenoid protein (OCP), which binds a single carbonyl carotenoid. Blue-green light induces the photoswitching of OCP from its ground state form (OCPO) to a metastable photoproduct (OCPR). OCPR can bind to the phyco­bilisome antenna and induce fluorescence quenching. The photoswitching is accompanied by structural and functional changes at the level of the protein and of the bound carotenoid. Here, we use broadband two-dimensional electronic spectroscopy to study the differences in excited state dynamics of the carotenoid in the two forms of OCP. Our results provide insight into the origin of the pronounced vibrational lineshape and oscillatory dynamics observed in linear absorption and 2D electronic spectroscopy of OCPO and the large inhomo­geneous broadening in OCPR, with consequences for the chemical function of the two forms