Binding of ubiquinone to photosynthetic reaction centers. 2: Determination of enthalpy and entropy changes for the binding to the QA site in reverse micelles

The Q(A) Site binding properties of the purple non-sulfur bacterium Rhodobacter sphaeroides reaction centers solubilized in phospholipid-based reverse micelles have been determined. By means of time-resolved absorbance measurements, the binding of the ubiquinone-10 to the Q(A) Site has been followed at different temperatures and quinone concentrations yielding the relative binding constants. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. Enthalpy and entropy changes obtained for the binding at the Q(A) Site (Delta H degrees(bind) = -75.3 +/- 3.4 kJ mol(-1) and Delta S degrees(bind) = -181 +/- 11 J mol(-1) K-1) confirm that the quinone binding to the primary site is stronger with respect to that at the Q(B) site. A Monte Carlo simulation of both the classical Van't Hoff and global analysis approaches is also presented, showing the higher reliability of the thermodynamic parameters derived with the latter method (uncertainty less than 1% with respect to more than 40% of the Van't Hoff analysis). Such an analysis indicates also that the enthalpy-entropy compensation previously observed through the ubiquinone series is likely due to a statistical artifacts.