The reaction between spinach photosystem II (PSII) membranes and hydroxylamine has been investigated by equilibrium titrations and flash-induced reactions with electron paramagnetic resonance (EPR) spectroscopy to monitor the odd-electron species, O2evolution rate, and manganese binding. Two high-affinity sites for NH2OH reaction have been characterized. Binding to the first site occurs within the water-oxidizing complex (WOC) and produces the well-known two flash shift in O2evolution. The usual two-electron shift in O2yield is accompanied by a parallel two-electron shift in the yield of the S2multiline EPR signal. This reaction occurs in two steps—an initial reversible reduction of manganese by two electrons at low concentrations (≤5 NH2OH/PSII) followed by, at higher concentrations, further reduction that is irreversible due to the release of 3 out of 4 Mn/PSII. The titration curve suggests that 2–3 Mn/PSII are released cooperatively, presumably from a common site. Binding to the second high-affinity site (≤6 NH2OH/PSII) produces a structural change in the ferrosemiquinone electron acceptor that is characterized by the conversion of the normal form of its EPR signal from g = 1.9 to a new form having g = 2.1. This structural change is blocked by herbicides, such as 3-(3,4-dichlorophenyl)-l,l-dimethylurea, which block access to the QBacceptor site. The two flash delay in turnover seen at room temperature is lost at low temperatures (150–500 K) due to a block in multiple turnovers caused by NH2OH. The site for the low-temperature blockage is undetermined but correlates with the structural change at the ferroquinone site. This suggests that the reoxidation of QA-by QBfollowing turnover is blocked, resulting instead in recombination upon warming. The reversible loss of both of the S2-state EPR signals, the multiline and the g = 4.1 signals, caused by NH2OH, titrated with identical curves, suggesting a common chemical reactivity and hence origin for these signals. The reaction between the S2state and NH2OH occurs in less than 10 s and is considerably faster than binding to the (dark) S1state. The reversible binding of NH2OH produces no stable paramagnetic products in the dark. The release of Mn by NH2OH is followed by reduction of the oxidized donor D+responsible for EPR signal IIslowand signal IIdark, confirming earlier work establishing the accessibility of this donor to the aqueous phase through the Mn binding site [Ghanotakis, D. F., & Babcock, G. T. (1983) FEBS Lett. 153, 231–234].
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