How Orange Carotenoid Protein controls the excited state dynamics of Canthaxanthin

Orange Carotenoid Protein (OCP) is a ketocarotenoid-binding protein essential for photoprotection in cyanobacteria. The main steps of the photoactivated conversion which converts OCP from its resting state to the active one have been extensively investigated. However, the initial photochemical event in the ketocarotenoid which triggers the large structural changes finally leading to the active state is still not understood. Here we employ QM/MM surface hopping nonadiabatic dynamics to investigate the excited-state decay of canthaxanthin in OCP, both in the ultrafast S2 to S1 internal and the slower decay leading back to the ground state. For the former step we show the involvement of an additional excited state, which has been often named SX state, and we characterize its nature. For the latter step, we reveal an excited state decay characterized by multiple timescales, which are related to the ground-state conformational het- erogeneity of the carotenoid. We assigned the slowly decaying population to the so-called S∗ state.Finally, we identify a minor decay pathway involving the photoisomerization which could be the initial trigger to photoactivation of OCP.