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

► Active and inactive forms of OCP coexist in OCP sample at daylight. ► Hydroxyechinenone S1 energy in inactive OCP is higher than in solution. ► Activation of OCP lowers the S1 energy of hydroxyechinenone bound to OCP.