Triplet-triplet energy transfer in the major intrinsic light-harvesting complex of Amphidinium carterae as revealed by ODMR and EPR spectroscopies.

We present an optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectroscopic study on the quenching of photo-induced chlorophyll triplet states by carotenoids, in the intrinsic light-harvesting complex (LHC) from the dinoflagellate Amphidinium carterae. Two carotenoid triplet states, differing in terms of optical and magnetic spectroscopic properties, have been identified and assigned to peridinins located in different protein environment. The results reveal a parallelism with the triplet-triplet energy transfer (TTET) process involving chlorophyll a and luteins observed in the LHC-II complex of higher plants. Starting from the hypothesis of a conserved alignment of the amino acid sequences at the cores of the LHC and LHC-II proteins, the spin-polarized time-resolved EPR spectra of the carotenoid triplet states of LHC have been calculated by a method which exploits the conservation of the spin momentum during the TTET process. The analysis of the spectra shows that the data are compatible with a structural model of the core of LHC which assigns the photo-protective function to two central carotenoids surrounded by the majority of Chl a molecules present in the protein, as found in LHC-II. However, the lack of structural data, and the uncertainty in the pigment composition of LHC, leaves open the possibility that this complex posses a different arrangement of the pigments with specific centers of Chl triplet quenching.