2Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142292, Russia; fax: (277) 90532; E-mail: email@example.com
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Received January 4, 2003; Revision received February 26, 2003
The primary act of charge separation was studied in P+BA- and P+HA- states (P, primary electron donor; BA and HA, primary and secondary electron acceptor) of native reaction centers (RCs) of Rhodobacter sphaeroides R-26 using femtosecond absorption spectroscopy at low (90 K) and room temperature. Coherent oscillations were studied in the kinetics of the stimulated emission band of P* (935 nm), of absorption band of BA- (1020 nm) and of absorption band of HA (760 nm). It was found that in native RCs kept in heavy water (D2O) buffer the isotopic decreasing of basic oscillation frequency 32 cm-1 and its overtones takes place by the same factor ~1.3 in the 935, 1020, and 760 nm bands in comparison with the samples in ordinary water H2O. This suggests that the femtosecond oscillations in RC kinetics with 32 cm-1 frequency may be caused by rotation of hydrogen-containing groups, in particular the water molecule which may be placed between primary electron donor PB and primary electron acceptor BA. This rotation may appear also as high harmonics up to sixth in the stimulated emission of P*. The rotation of the water molecule may modulate electron transfer from P* to BA. The results allow for tracing of the possible pathway of electron transfer from P* to BA along a chain consisting of polar atoms according to the Brookhaven Protein Data Bank (1PRC): Mg(PB)-N-C-N(His M200)-HOH-O = BA. We assume that the role of 32-cm-1 modulation in electron transfer along this chain consist of a fixation of electron density at BA- during a reversible electron transfer, when populations of P* and P+BA- states are approximately equal.
KEY WORDS: photosynthesis, reaction center, electron transfer, wavepacket, femtosecond spectroscopy