High Power RYDMR Spectra of P+H− in Reaction Centers of Photosynthetic Bacteria

Abstract

Among the problems related to the function of photosynthetic reaction centers (RCs) the mechanism of the fast electron transfer (ET) from the primary donor 1P* to bacteriopheo-phytin (H) constitutes one of the most challenging questions. It pertains to the involvement of a bacteriochlorophyll monomer (B) which in the X-ray structural analysis [1,2] has been shown to connect P and H. In this paper we compare the recombination dynamics of the radical ion pair (RP) P+H− in Rb.sphaeroides R-26, Chloroflexus aurantiacus, Rb.capsulatus wild-type and mutants of the latter employing as a most sensitive technique Reaction Yield Detected Magnetic Resonance (RYDMR [3]). The method rests on the observation that in quinone(Q)-depleted RCs the lifetime of the RP state is sufficiently long to allow the hyperfine interaction (HFI) to induce transitions between the initially generated singlet 1(P+H−) and the triplet state 3(P+H−) and to open thereby the triplet recombination channel 3(P+H−)→3P* characterized by the rate kT. In a simple picture (Fig.l) the RP singlet and triplet states are separated by an effective exchange interaction J between the unpaired electron spins. The splitting within the three triplet subleveis due to magnetic spin dipole interaction is negligible [4].