Photosystem II oxidation of charged electron donors. Surface charge effects

Reactions occurring on the oxidizing side of Photosystem II have been studied in Tris-washed chloroplasts by monitoring the decay kinetics of EPR signal IIf, arising from the photoinduced oxidation of Z, an intermediate in the electron transport chain between P-680 and the water-splitting enzyme. Upon addition of electron donors, signal IIf follows pseudo-first order decay kinetics with rates dependent on the chemical nature of the donor. Negatively charged donors (I⁻, Fe(CN)⁴⁻₆, W(CN)⁴⁻₈) are poor reducing agents for Z⁺· whereas neutral donors (benzidine, hydroquinone, diphenylcarbazide) are more efficient, their effectiveness paralleling their lipophilicity. The slow signal IIf reduction observed with the charged donors is consistent with the non-polar nature of the thylakoid membrane and a location for Z toward the inner membrane surface. It most probably exists in a hydrophobic site as indicated by the positive correlation between rate constant and lipophilicity for the neutral donors.

A detailed study of the mechanism of Photosystem II reduction by ascorbic acid has been carried out. The pH dependence of the decay kinetics of signal IIf in the presence of this donor is consistent with a model in which both the neutral acid and the ascorbate mono-anion serve as reducing agents to Z⁺·. The second-order rate constant for reduction by the mono-anion is less than that of the neutral acid and is found to vary with the suspension pH. This observation is interpreted to indicate the occurrence of negative charge on the inner membrane surface in the vicinity of Z. Additional experiments, which assessed the effect of mono- and divalent cations and of cationic detergents on the signal IIf reaction rate constants, support both the presence of negative surface charge and its location on the membrane inner surface.