Binding of Ubiquinone to Photosynthetic Reaction Centers: Determination of Enthalpy and Entropy Changes in Reverse Micelles

Abstract

The photosynthetic reaction center from the purple non-sulfur bacterium Rhodobacter sphaeroides has been solubilized in reverse micelles of phospholipids in n-hexane. The rate 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 ubiquinone-10 at eight temperatures between 37 and 6 °C. Deconvolution of P+ decay shows that, in reverse micellar solutions, quinone molecules are in fast exchange between the QB site of the protein and the bulk organic phase, with the exception of a small fraction of reaction centers lacking the QB functionality. Global analysis of the kinetics of charge recombination permits proper separation of the contribution of the binding at QB from that of the P+QA-QB→ P+QAQB- electron transfer. Enthalpy and entropy changes obtained for the electron transfer process (ΔHAB° = −0.140 ± 0.016 meV; ΔSAB° = −2.01 × 10-4 ± 6.7 × 10-5 meV/K) are in agreement with previous evaluations in detergent suspensions of reaction centers. Thermodynamic parameters of the enthalpically driven quinone binding at QB (ΔHbind° = −0.526 ± 0.058 meV; ΔSbind° = −1.36 × 10-3 ± 2.3 × 10-4 meV/K) compare favorably with data in aqueous systems when considering a free energy change of about −4.77 kJ/mol for the transfer of ubiquinone-10 from a direct detergent micellar phase to a n-hexane reverse micellar solution. This comparison indicates that the quinone affinity for the QB site is approximately the same in the (dark) PQA and (light) P+QA- states of the reaction center.