Illumination of PSII core preparations can cause the production of H2O2at rates which approach 60 µmol of H2O2(mg of Chl·h)-1. The rate of peroxide production is maximal at pH 7.2 at low sucrose concentrations and at concentrations of Cl-(1.5-3.0 mM) that limit the rate of the oxidation of water to O2. The rate of H2O2production increased with pH from pH 6.8 to 7.2 and was inversely proportional to the oxidation of water to O2from pH 6.8 to 7.5. While EDTA does not inhibit H2O2production, this reaction is abolished by 5 mM NH2OH and inhibited by the same concentrations of NH3 that affect water oxidation which indicates that the oxygen-evolving complex is responsible for the production of peroxide generated upon illumination of PSII core preparations. These results support a mechanism in which bound Cl-in the S2state is displaced by OH-ions which are then oxidized by the OEC to form H2O2. Thus, the OEC requires Cl- to prevent access to the active site of the OEC until four oxidizing equivalents can be generated to allow the oxidation of water to O2.
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