Redox and acid‐base characterization of cytochrome b‐559 in photosystem II particles

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Abstract
The redox and acid/base states and midpoint potentials of cytochrome b‐559 have been determined in oxygen‐evolving photosystem II (PS II) particles at room temperature in the pH range from 6.5 to 8.5. At pH 7.5 the fresh PS II particles present about 2/3 of their cytochrome b‐559 in its reduced and protonated (non‐auto‐oxidizable) high‐potential form and about 1/3 in its oxidized and non‐protonated low‐potential form. Potentiometric reductive titration shows that the protonated high‐potential couple is pH‐independent (E0, + 380 mV), whereas the low‐potential couple is non‐protonated and pH‐independent above pH 7.6 (E0, pH > 7.6, + 140 mV), but becomes pH‐dependent below this pH, with a slope of ‐72 mV/pH unit. Moreover, evidence is presented that in PS II particles cytochrome b‐559 can cycle, according to its established redox and acid/base properties, as an energy transducer at two alternate midpoint potentials and at two alternate pKa values. Red light absorbed by PS II induces reduction of cytochrome b‐559 in these particles at room temperature, the reaction being completely blocked by dichlorophenyldimethylurea.