Electron Transfer in Deuterated Reaction Centers of Rhodobacter sphaeroides at 90 K According to Femtosecond Spectroscopy Data

A. G. Yakovlev^1* and V. A. Shuvalov^1,2

¹Department of Photobiophysics, Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow 119992, Russia; fax: (095) 939-3181; E-mail: yakov@genebee.msu.su

²Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region 142292, Russia; fax: (277) 90532; E-mail: shuvalov@issp.serpukhov.su

^* To whom correspondence should be addressed.

Received January 4, 2003; Revision received February 26, 2003

---

The primary act of charge separation was studied in P⁺B_A^- and P⁺H_A^- states (P, primary electron donor; B_A and H_A, 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 B_A^- (1020 nm) and of absorption band of H_A (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 P_B and primary electron acceptor B_A. 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 B_A. The results allow for tracing of the possible pathway of electron transfer from P* to B_A along a chain consisting of polar atoms according to the Brookhaven Protein Data Bank (1PRC): Mg(P_B)-N-C-N(His M200)-HOH-O = B_A. We assume that the role of 32-cm^-1 modulation in electron transfer along this chain consist of a fixation of electron density at B_A^- during a reversible electron transfer, when populations of P* and P⁺B_A^- states are approximately equal.

---

KEY WORDS: photosynthesis, reaction center, electron transfer, wavepacket, femtosecond spectroscopy

---