Energy transfer in the major intrinsic light-harvesting complex from Amphidinium carterae

Carbonyl carotenoids are important constituents of the antenna complexes of marine organisms. These carotenoids possess an excited state with a charge-transfer character (intramolecular charge transfer state, ICT), but many details of the carotenoid to chlorophyll energy transfer mechanisms are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the intrinsic light-harvesting complex (LHC) of dinoflagellates, which contains the carbonyl carotenoid peridinin. Carotenoid to chlorophyll energy transfer efficiency is about 90% in the 530-550 nm region, where the peridinin S₂ state transfers energy with an efficiency of 25-50%. The rest proceeds via the S₁/ICT channel, and the major S₁/ICT-mediated energy transfer pathway utilizes the relaxed S₁/ICT state and occurs with a time constant of 2.6 ps. Below 525 nm, the overall energy transfer efficiency drops because of light absorption by another carotenoid, diadinoxanthin, that contributes only marginally to energy transfer. Instead, its role is likely to be photoprotection. In addition to the peridinin-Chl-a energy transfer, it was shown that energy transfer also occurs between the two chlorophyll species in LHC, Chl-c₂, and Chl-a. The time constant characterizing the Chl-c₂ to Chl-a energy transfer is 1.4 ps. The results demonstrate that the properties of the S₁/ICT state specific for carbonyl carotenoids is the key to ensure the effective harvesting of photons in the 500-600 nm region, which is of vital importance to underwater organisms.