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 sphaeroid es has been depleted of native quinone and solubilized in reverse micelles of phospholipids. The kinetics of the charge recombination from the seconda ry quinone acceptor (Q(B)) to the bacteriochlorophyll dimer (P) has been in vestigated, by flash absorption spectroscopy, as a function of the concentr ation of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Q(0)) at different temper atures. The dependence of the maximum bleaching amplitude after a light fla sh on Q(0) concentration and temperature leads to the determination of the enthalpy and entropy changes of binding to the Q(A) site (Delta H-QA(degree s) = -67 +/- 5 kJ mol(-1), Delta S-QA(degrees) = -156 +/- 15 J K-1 mol(-1)) . Deconvolution of P+ decay shows that, in reverse micellar solutions, Q(0) molecules are in fast exchange between the Q(B) site of the protein and th e organic bulk. Global analysis of the P+ decays allows proper separation o f the contribution of the binding at Q(B) from that of the P(+)Q(A)(-)Q(B) --> P(+)Q(A)Q(B)(-) electron transfer. The enthalpy and entropy changes obt ained for the binding at the Q(B) site (Delta H-QB(degrees) = -39 +/- 3 kJ mol(-1), Delta S-QB(degrees) = -72 +/- 9 J K-1 mol(-1)) and for the P(+)Q(A )(-)Q(B) reversible arrow P(+)Q(A)Q(B)(-) equilibrium (Delta H-AB(degrees) = -21.6 +/- 1.5 kJ mol(-1), Delta S-AB(degrees) = -55 +/- 6 J K-1 mol(-1)) are, evaluated. A comparison of the thermodynamic parameters determined for Q(0) with those previously found for the native ubiquinone-10 (Q(10)) indi cates that the isoprenyl tail of Q(10) is strongly involved in Q(B) binding and function, and that both processes are strongly affected by entropy cha nges, likely due to tail-protein interaction.