The Multiple Roles of the Protein in the Photoactivation of Orange Carotenoid Protein

Orange carotenoid protein (OCP) is a carotenoid-binding protein involved in photo-protection mechanisms of cyanobacteria. Upon exposure to high light, OCP interconverts from an orange resting form (OCP^O) to a red active one (OCP^R); the mechanism of this interconversion, even if extensively studied, has still not been fully elucidated. In this work, we use multiscale atomistic models ranging from classical to hybrid quantum-classical (QM/MM) molecular dynamics to give a comprehensive molecular explanation of the drastic spectroscopic changes observed upon interconversion. The findings are finally used to formulate a new hypothesis on the role of the protein in the photoactivation mechanism.