Stabilization of charge separation and cardiolipin confinement in antenna reaction center complexes purified from Rhodobacter sphaeroides

The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter spaheroides has been studied with respect to the kinetics of charge recombination and to the pospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5<pH<9.5, P+QB- recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH>11. In both systems kinetics are described by a continuous rate distribution, which broadens at pH>9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the QAQB- state is stabilized by about 40 meV at 6.5<pH<9.5, while it is destabilized by approx. 50 meV at pH>11. The phospholipids/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospolipid and 10-15 ubiquinone molecules. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and 31P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, less than 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflect the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.