Triplet-triplet energy transfer in the major intrinsic light-harvesting complex of Amphidinium carterae as revealed by ODMR and EPR spectroscopies |
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Authors: | Marilena Di Valentin Giancarlo Agostini Stefano Ceola Giorgio Mario Giacometti |
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Affiliation: | a Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italyb CNR, Inst Biomol Chem, Padova Unit, I-35131, via Marzolo 1, 35131 Padova, Italyc Department of Biological Sciences, Macquarie University, 2109 NSW, Australiad Department of Biology, University of Padova, via U. Bassi 58/B, 35121 Padova, Italy |
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Abstract: | 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. |
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Keywords: | LHC, light-harvesting complex Car, carotenoid Chl, chlorophyll Cla, chlorophyll a ZFS, zero field splitting ISC, intersystem crossing ODMR, optically detected magnetic resonance FDMR, fluorescence detected magnetic resonance ADMR, absorption detected magnetic resonance T-S, triplet minus singlet ESE, electron spin echo FSE, field-swept echo TR-EPR, time-resolved electron paramagnetic resonance TTET, triplet-triplet energy transfer |
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