Further evidence for dissipative energy migration via triplet states in photosynthesis. The protective mechanism of carotenoids in Rhodopseudomonas spheroides chromatophores

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The protective action of carotenoids against irreversible photodestruction was discovered in photosynthetic bacteria by Stanieda and coworkers. In green plant material it was found by Wolff and Witt ((1969) Z. Naturforsch. 24b, 1031–1037 and (1972) Proc. 2nd. Int. Congr. Photosynthesis Res. Stresa (Forti, G., Avron, M. and Melandri, A., eds.), Vol. 2, pp. 931–936, Dr. W. Junk, N. V. Publ. The Hague) that the formation of special carotenoid triplet states (via very rapid energy transfer from excited chlorophylls) and their fast radiationless decay in τ 1 2 ≈ 3 μ s is at least one mechanism for the protective action of carotenoids to irreversible photooxidation of the chlorophylls. Hence, it is anticipated that the same mechanism might be realized also in bacteria. The present study gives evidence for such a “triplet valve” to be established also in bacteria. This conclusion was derived from the following observations: 1. 1. The light-induced difference spectrum shows a bleaching of a carotenoid at three characteristic wavelength between 400 and 500 nm. A positive peak around 533 nm indicates the formation of a carotenoid triplet state. 2. 2. The absorption changes can be induced by red light which excites only bacteriochlorophyll. This indicates an energy transfer from bacteriochlorophyll to carotenoids. 3. 3. The light-induced carotenoid triplets decay radiationless in 3 μs in air-saturated aqueous suspensions of the chromatophores. 4. 4. The carotenoid triplet formation occurs only at actinic flash intensities where the photosynthesis becomes saturated. 5. 5. Addition of dithionite, which blocks photosynthesis, markedly increases the extent of carotenoid triplet formation. The different types of excilton migration within the photosynthetic unit are discussed, especially the routes leading to the dissipation of excess excitation energy.