Time-Resolved ESR and ENDOR on Triplet States in Photosynthetic Antenna Complexes

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Angerhofer, AlexanderMCB-9983034Carotenoids are used in photosynthetic organisms to protect against the harmful photoinitiated sensitization of singlet oxygen by chlorophyll triplet states. The photoprotection mechanism of the carotenoids will be studied in bacterial, algal and higher plant photosynthetic antenna complexes using time-resolved ESR, ENDOR, and transient absorption techniques. The isotropic and anisotropic hyperfine interaction between the triplet electron spin and the carotenoid polyene chain protons will be analyzed by the time-resolved ENDOR. This will lead to the assignment of the triplet-carrying pigments and the determination of their electronic structures. The results of this research will lead to refinements of currently available X-ray structural data and/or the determination of structural changes that may occur in the excited triplet states. A detailed understanding of the temperature dependence of the triplet spin dynamics will be obtained by time-resolved ESR and will allow us to distinguish the different mechanisms of triplet energy transfer that take place in various antenna and reaction center complexes.Carotenoids are essential for the survival of photosynthetic organisms (plants, algae, and some bacteria). They serve an important protective role that allows the organism to get rid of excess energy without stimulating the production of singlet oxygen which is highly toxic. The process involves a transfer of a so-called triplet state from the chlorophylls to the carotenoids. This transfer of energy has so far been understood only in general terms and in many cases the identification of the actual pigments responsible for it is lacking. Furthermore, the details of the dynamic interaction between the triplet excited carotenoids or chlorophylls and their environment are not well understood. This work will fill this gap. This project is supported by the Molecular Biophysics Program in the Division of Molecular and Cellular Biosciences in the Directorate for Biological Sciences and the Physical Chemistry Program in the Division of Chemistry in the Mathematical and Physical Sciences Directorate.