Export or recombination of charges in reaction centers in intact cells of photosynthetic bacteria

The kinetics and thermodynamics of forward and reverse electron transfer around the reaction center of purple bacterium Rhodobacter sphaeroides were studied in vivo by flash-excited delayed fluorescence, prompt fluorescence (induction) and kinetic difference absorption. By protection of the photomultiplier from intense bacteriochlorophyll prompt fluorescence evoked by laser excitation, the time resolution of the fluorometer was reduced typically 10 μs. Two precursor states of the delayed fluorescence were identified: P+QA− and cyt c23+QA− whose enthalpy levels were 340 meV and 1020 meV below A⁎, respectively. The free energy of the P+QA− state relative to A⁎ was − 870 meV in whole cells. Similar values were obtained earlier for isolated reaction center and chromatophore. The free energies of cyt c23+QA− and P+QA− states showed no or very weak (− 6 meV/pH unit) pH-dependence, respectively, supporting the concept of pH-independent redox midpoint potential of QA/QA− in intact cells. In accordance with the multiphasic kinetics of delayed fluorescence, the kinetics of re-opening of the closed reaction center is also complex (it extends up to 1 s) as a consequence of acceptor and donor-side reactions. The control of charge export from the reaction center by light regime, redox agents and inhibitors is investigated. The complex kinetics may arise from the distribution of quinones in different redox states on the acceptor side (QB binding site and pool) and from organization of electron transfer components in supercomplexes.