Interactions of photosynthetic reaction center with 2,3-dimethoxy-5-methyl-1,4-benzoquinone in reverse micelles

The photosynthetic reaction center from the bacterium Rhodobacter sphaeroides has been depleted of native quinone and solubilized in reverse micelles of phospholipids. The kinetics of the charge recombination from the secondary quinone acceptor (QB) to the bacteriochlorophyll dimer (P) has been investigated, by flash absorption spectroscopy, as a function of the concentration of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Q0) at different temperatures. The dependence of the maximum bleaching amplitude after a light flash on Q0 concentration and temperature leads to the determination of the enthalpy and entropy changes of binding to the QA site (ΔHQA°=−67±5 kJ mol−1, ΔSQA°=−156±15 J K−1 mol−1). Deconvolution of P+ decay shows that, in reverse micellar solutions, Q0 molecules are in fast exchange between the QB site of the protein and the organic bulk. Global analysis of the P+ decays allows proper separation of the contribution of the binding at QB from that of the P+QA−QB→P+QAQB− electron transfer. The enthalpy and entropy changes obtained for the binding at the QB site (ΔHQB°=−39±3 kJ mol−1, ΔSQB°=−72±9 J K−1 mol−1) and for the P+QA−QB⇄P+QAQB− equilibrium (ΔHAB°=−21.6±1.5 kJ mol−1, ΔSAB°=−55±6 J K−1 mol−1) are, evaluated. A comparison of the thermodynamic parameters determined for Q0 with those previously found for the native ubiquinone-10 (Q10) indicates that the isoprenyl tail of Q10 is strongly involved in QB binding and function, and that both processes are strongly affected by entropy changes, likely due to tail–protein interaction.