Electrogenicity at the secondary quinone acceptor site of cyanobacterial photosystem II

Flash-induced generation of the electric potential difference (delta psi) by a direct electrometrical method was studied in Anacystis nidulans photosystem II-containing proteoliposomes associated with a phospholipid-impregnated collodion film. Besides a rapid phase of delta psi generation corresponding to charge separation between P680 and QA, an additional electrogenic phase with a characteristic time of 0.27 ms at pH 7.0 was observed after the second laser flash. The maximal amplitude of this phase was approx. 4% of that related to the P680+QA- formation. The sensitivity of this phase to DCMU, the flash-number dependence of its amplitude as well as the amplitude and the rate constant pH-dependences, indicate that it is due to the dismutation of QA- and QB- and to subsequent protonation of a doubly reduced plastoquinone QB2-.