Oxidation‐Reduction Physical Chemistry of the Acceptor Quinone Complex in Bacterial Photosynthetic Reaction Centers: Evidence for a New Model of Herbicide Activity

The oxidation‐reduction midpoint potential ( E m ) of the primary quinone (Q A ) of the acceptor quinone complex of bacterial photosynthetic reaction centers has been measured as a function of pH in the presence and absence of ubiquinone and o ‐phenanthroline ( o ‐Phen). Reaction centers, isolated from Rhodopseudomonas sphaeroides , were incorporated into egg phosphatidylcholine vesicles. Contrary to earlier reports, the E m was found to exhibit a pH‐dependence very similar to that observed in chromatophores, with a slope of ‐ 60 mV/pH up to a p K for Q − A /Q − A (H + ) at pH 9.5–10.0. In the presence of ubiquinone to reconstitute the secondary quinone (Q B ), the E m /pH curve of Q A was shifted to lower potentials, indicating that the binding of Q n (actually Q B H 2 ) was suppressed by reduction of Q A . o ‐Phen, an inhibitor of electron transport from Q A to Q B , raised the p K of Q − A /Q − A (H + ) and, at pH‐values below but not above this p K , reversed the effects of Q B . In the absence of Q B , o ‐Phen lowered the E m of Q A above the p K but had no effect below it. These results are discussed in terms of interactions between the binding sites for Q A and Q B (A‐ and B‐sites). It is suggested that ubiquinone and o ‐Phen compete for the B‐site in a mutually exclusive fashion, and that their relative binding strengths are modulated by the redox and protonation state of Q A . In preparations with low quinone content, o ‐Phen inhibits photochemistry suggesting that it can also compete with ubiquinone at the A‐site. Competitive displacement of quinone from the B‐site by o ‐Phen and other inhibitors is suggested as the primary mode of action of a broad class of herbicides active in Photosystem II of plants. The relative binding affinities of the various redox states of Q B are also discussed and it is concluded that the order of binding strength is: Q − B → Q B → Q B H 2 . This accounts for the atypical stability of the semiquinone and the lower average E m for reduction to the quinol, compared to free ubiquinone in the quinone pool. It may also be significant in the functioning of quinones in communicating reducing equivalents from the reaction center to other electron transport complexes in the intact membrane.