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Pure Appl. Chem., 2006, Vol. 78, No. 8, pp. 1505-1518

Structures and functions of carotenoids bound to reaction centers from purple photosynthetic bacteria

Hideki Hashimoto1,2, Ritsuko Fujii1, Kazuhiro Yanagi1, Toshiyuki Kusumoto1, Alastair T. Gardiner3, Richard J. Cogdell3, Aleksander W. Roszak4, Neil W. Issacs4, Zeus Pendon5, Dariusz Niedzwiedski5 and Harry A. Frank5

1 Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
2 "Light and Control", PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
3 Division of Biochemistry and Molecular Biology, IBLS, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
4 Department of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
5 Department of Chemistry, U-3060, University of Connecticut, Storrs, CT 06269-3060, USA

Abstract: The photoprotective function of 15,15'-cis-carotenoids bound to the photosynthetic reaction centers (RCs) of purple bacteria has been studied using carotenoids reconstituted into carotenoidless RCs from Rhodobacter sphaeroides strain R26.1. The triplet-energy level of the carotenoid has been proposed to affect the quenching of the triplet state of special-pair bacteriochlorophyll (P). This was investigated using microsecond flash photolysis to detect the carotenoid triplets as a function of the number of conjugated double bonds, n. The carotenoid triplet signals were extracted by using singular-value decomposition (SVD) of the huge matrices data, and were confirmed for those having n = 8 to 11. This interpretation assumes that the reconstituted carotenoids occupy the same binding site in the RC. We have been able to confirm this assumption using X-ray crystallography to determine the structures of carotenoidless, wild-type carotenoid-containing, and 3,4-dihydro-spheroidene-reconstituted RCs. The X-ray study also emphasized the importance of the methoxy group of the carotenoids for binding to the RCs. Electroabsorption (Stark) spectroscopy was used to investigate the effect of the carotenoid on the electrostatic field around P. This electrostatic field changed by 10 % in the presence of the carotenoid.