Effects of Oxygen on the Dark Recombination between Photoreduced Secondary Quinone and Oxidized Bacteriochlorophyll in Rhodobacter sphaeroides Reaction Centers

P. P. Knox^*, E. P. Lukashev, K. N. Timofeev, and N. K. Seifullina

School of Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-1115; E-mail: knox@biophys.msu.ru

^* To whom correspondence should be addressed.

Received May 16, 2001; Revision received September 13, 2001

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The influence of duration of exposure to actinic light (from 1 sec to 10 min) and temperature (from 3 to 35°C) on the temporary stabilization of the photomobilized electron in the secondary quinone acceptor (Q_B) locus of Rhodobacter sphaeroides reaction centers (RC) was studied under aerobic or anaerobic conditions. Optical spectrophotometry and ESR methods were used. The stabilization time increased significantly upon increasing the exposure duration under aerobic conditions. The stabilization time decreased under anaerobic conditions, its dependence on light exposure duration being significantly less pronounced. Generation of superoxide radical in photoactivated aerobic samples was revealed by the ESR method. Possible interpretation of the effects is suggested in terms of interaction between the semiquinone Q_B with oxygen, the interaction efficiency being determined by the conformational transitions in the structure of RC triggered by actinic light on and off.

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KEY WORDS: purple bacteria, photosynthetic reaction center, ubiquinones, electron transport, oxygen

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