Effect of Oxygen on Temporary Stabilization of Photoreduced Quinone Acceptors in Rhodobacter sphaeroides Reaction Centers

P. P. Knox^1*, M. Heinnickel², and A. B. Rubin¹

¹Faculty of Biology, Lomonosov Moscow State University, Moscow 119899, Russia; fax: (7-095) 939-1115; E-mail: knox@biophys.msu.ru

²Department of Biochemistry and Molecular Biology, Pennsylvania State University, S-310 Frear Building, University Park, PA 16802, USA; fax: 814-863-7405; E-mail: mlh300@psu.edu

^* To whom correspondence should be addressed.

Received January 27, 2003; Revision received March 13, 2003

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The effect of molecular oxygen on the photochemical activity of the Rhodobacter sphaeroides reaction centers frozen to 160 K under actinic illumination was investigated by the ESR method. About 90% of initially photochemically active bacteriochlorophyll (P) were fixed at 160 K for a long time in aerobic samples in an inactive form. In anaerobic samples, not more than 65% were fixed in an inactive form under the same conditions. In aerobic preparations, a small portion of photochemically active bacteriochlorophyll (about 10%) that retains its photochemical activity at 160 K after freezing under illumination has dark reduction kinetics similar to that of samples at room temperature after several seconds of actinic illumination. In anaerobic samples frozen under illumination, the remaining photochemically active reaction centers (35%) have the same dark reduction kinetics as samples illuminated at 295 K for 1-2 min. The conclusion is that the irreversible stabilization of bacteriochlorophyll P in the oxidized inactive state formed in the reaction centers frozen under illumination is brought about by light-induced conformational changes fixed under low temperatures.

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KEY WORDS: purple bacteria, photosynthetic reaction center, electron transport, conformational changes, quinones, oxygen

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