Carotenoids are well-known as light-harvesting pigments. They also play important roles in protecting the photosynthetic apparatus from damaging reactions of chlorophyll triplet states and singlet oxygen in both plant and bacterial photosynthesis. Recently, it has been found that β-carotene functions as a redox intermediate in the secondary pathways of electron transfer within photosystem II and that carotenoid cation radicals are transiently formed after photoexcitation of bacterial light-harvesting complexes. The redox role of β-carotene in photosystem II is unique among photosynthetic reaction centers and stems from the very strongly oxidizing intermediates that form in the process of water oxidation. Because of the extended π-electron-conjugated system of carotenoid molecules, the cation radical is delocalized. This enables β-carotene to function as a "molecular wire", whereby the centrally located oxidizing species is shuttled to peripheral redox centers of photosystem II where it can be dissipated without damaging the system. The physiological significance of carotenoid cation radical formation in bacterial light-harvesting complexes is not yet clear, but may provide a novel mechanism for excitation energy dissipation as a means of photoprotection. In this paper, the redox reactions of carotenoids in photosystem II and bacterial light-harvesting complexes are presented and the possible roles of carotenoid cation radicals in photoprotection are discussed.
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