An electrometrical technique was used to investigate proton-coupled electron transfer between the primary plastoquinone acceptor Q (A) (-) and the oxidized non-heme iron Fe(3+) on the acceptor side of photosystem II core particles incorporated into phospholipid vesicles. The sign of the transmembrane electric potential difference Deltapsi (negative charging of the proteoliposome interior) indicates that the iron-quinone complex faces the interior surface of the proteoliposome membrane. Preoxidation of the non-heme iron was achieved by addition of potassium ferricyanide entrapped into proteoliposomes. Besides the fast unresolvable kinetic phase (tau approximately 0.1 micro s) of Deltapsi generation related to electron transfer between the redox-active tyrosine Y(Z) and Q(A), an additional phase in the submillisecond time domain (tau approximately 0.1 ms at 23 degrees C, pH 7.0) and relative amplitude approximately 20% of the amplitude of the fast phase was observed under exposure to the first flash. This phase was absent under the second laser flash, as well as upon the first flash in the presence of DCMU, an inhibitor of electron transfer between Q(A) and the secondary quinone Q(B). The rate of the additional electrogenic phase is decreased by about one-half in the presence of D(2)O and is reduced with the temperature decrease. On the basis of the above observations we suggest that the submillisecond electrogenic reaction induced by the first flash is due to the vectorial transfer of a proton from external aqueous phase to an amino acid residue(s) in the vicinity of the non-heme iron. The possible role of the non-heme iron in cyclic electron transfer in photosystem II complex is discussed.

An electrometrical technique was used to investigate flash-induced electron transfer reactions between Mn-depleted spinach photosystem II core particles incorporated into liposomes and redox mediators. Besides the fast increase in the transmembrane electric potential difference associated with electron transfer between the redox active tyrosine (Y(Z)) and the primary quinone acceptor Q(A,) an additional electrogenic phase was observed in the presence of N,N,N'N'-tetramethyl-p-phenylenediamine and 2,6-dichlorophenol-indophenol. The latter phase is attributed to vectorial electron transfer from the redox dye(s) to the protein-embedded Y(Z). The data obtained suggest an electrically isolated location of the Y(Z) from the external water phase.

The generation of transmembrane electric potential difference (delta psi) in quinone acceptor complex of proteoliposomes containing core complexes of photosystem II from spinach was studied using for the measurements a direct electrometric technique. Besides the fast increase in the membrane potential associated with the electron transfer between the redox-active tyrosine 161 residue (Y(Z)) in D1 polypeptide and the primary quinone acceptor Q(A), an additional electrogenic phase with tau approximately 0.85 msec at pH 7.3 and the maximal relative amplitude of approximately 11% of the Y(Z)ox Q(A)- phase was observed after the second light flash. The sensitivity of this phase to diuron (an inhibitor of electron transfer between Q(A) and the secondary quinone acceptor Q(B)), the dependence of its amplitude on the light flash parity, and also a decrease in its rate constant with increase in pH indicated that it was due to dismutation of Q(A)- and Q(B)- with the subsequent protonation of a doubly reduced plastoquinone molecule: Q(A)- Q(B)- + 2H+ --> Q(A)Q(B)H2.