The manganese cluster of the oxygen-evolving enzyme of photosystem II is chemically reduced upon interaction with nitric oxide at -30 °C. The state formed gives rise to an S = 1/2 multiline EPR signal [Goussias, Ch., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261] that is attributed to a Mn(II)-Mn(III) dimer [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581]. In this work, we sought to establish whether the state could be assigned to a specific, reduced S state by using flash oxymetry, chlorophyll a fluorescence, and electron paramagnetic resonance spectroscopy. With the Joliot-type O2 electrode, the first maximum of oxygen evolution was observed on the sixth or seventh flash. Three saturating preflashes were required to convert the flash pattern characteristic of NO-reduced samples to that of the untreated control (i.e., O2 evolution maximum on the third flash). Measurements of the S state-dependent level of chlorophyll fluorescence in NO-treated PSII showed a three-flash downshift compared to untreated controls. In the EPR study, the maximum S2 multiline EPR signal was observed after the fourth flash. The results from all three methods are consistent with the Mn cluster being in a redox state corresponding to an S-2 state in a majority of centers after treatment with NO. We were unable to generate the Mn(II)-Mn(III) multiline signal using hydrazine as a reductant; it appears that the valence distribution and possibly the structure of the Mn cluster in the S-2 state are dependent on the nature of the reductant that is used.
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