BINDING OF UBIQUINONE TO PHOTOSYNTHETIC REACTION CENTERS - 2 - DETERMINATION OF ENTHALPY AND ENTROPY CHANGES FOR THE BINDING TO THE Q(A) SITE IN REVERSE MICELLES

The Q(A) Site binding properties of the purple non-sulfur bacterium Rh odobacter sphaeroides reaction centers solubilized in phospholipid-bas ed reverse micelles have been determined. By means of time-resolved ab sorbance measurements, the binding of the ubiquinone-10 to the Q(A) Si te has been followed at different temperatures and quinone concentrati ons yielding the relative binding constants. A global fit of the exper imental data allowed us to get quite reliable values of the thermodyna mic parameters joined to the binding process. Enthalpy and entropy cha nges 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 m ol(-1) K-1) confirm that the quinone binding to the primary site is st ronger 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 al so presented, showing the higher reliability of the thermodynamic para meters derived with the latter method (uncertainty less than 1% with r espect to more than 40% of the Van't Hoff analysis). Such an analysis indicates also that the enthalpy-entropy compensation previously obser ved through the ubiquinone series is likely due to a statistical artif acts.