Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. I. Determination of the charge recombination pathway of D+QAQBand free energy and kinetic relations between QA-QB

The electron-transfer reactions and thermodynamic equilibria involving the quinone acceptor complex in bacterial reaction centers from R. sphaeroides were investigated. The reactions are described by the scheme: obs kD+ ~D hv kAa DQAQB ~'--~-- D +QAQn ~-~--~D +QAQa /CAD kBA We found that the charge recombination pathway of D+QAQ ~ proceeds via the intermediate state D + QA Qn, the direct pathway contributing less than approx. 5 % to the observed recombination rate. The method used to obtain this result was based on a comparison of the kinetics predicted for the indirect pathway (given by the product kAn-times the fraction of reaction centers in the QAQB state) with the observed recombination rate, k ~ ~ D " The kinetic measurements were used to obtain the pH dependence (6.1 ~< pH ~< 11.7) of the free energy difference between the states QAQB and QAQa. At low pH (less than 9) QAQn is stabilized relative to QAQa by 67 meV, whereas at high pH QAQB is energetically favored. Both QA and Qn associate with a proton, with pK values of 9.8 and 11.3, respectively. The stronger interaction of the proton with Q ~ provides the driving force for the forward electron transfer