When acetate-inhibited photosystem II (PSII) membranes are illuminated at temperatures above 250 K and quickly cooled to 77 K, a 240 G-wide electron paramagnetic resonance (EPR) signal is observed at 10 K. This EPR signal arises from a reciprocal interaction between the spin 1/2 ground state of the S2 state of the Mn4 cluster, for which a multiline EPR signal with shifted 55Mn hyperfine peaks is observed, and the oxidized tyrosine residue, Y(Z)·, for which a broadened Y(Z)· EPR spectrum is observed. The S2Y(Z) EPR signal in acetate-inhibited PSII is the first in which characteristic spectral features from both paramagnets can be observed. The observation of distinct EPR signals from each of the paramagnets together with the lack of a half-field EPR transition indicates that the exchange and dipolar couplings are weak. Below 20 K, the S2Y(Z)· EPR signal in acetate-inhibited PSII is in the static limit. Above 20 K, the line width narrows dramatically as the broad low-temperature S2Y(Z)· EPR signal is converted to a narrow Y(Z) EPR signal at room temperature. The line width narrowing is interpreted to be due to averaging of the exchange and dipolar interactions between Y(Z)· and the S2 state of the Mn4 cluster by rapid spinlattice relaxation of the Mn4 cluster as the temperature is increased. Decay of the S2Y(Z)· intermediate at 200 K shows that the g = 4.1 form of the S2 state is formed and that a noninteracting S2-state multiline EPR signal is not observed as an intermediate in the decay. This result shows that a change in the redox state of Y(Z) induces a spin-state change in the Mn4 cluster in acetate-inhibited PSII. The interconversion between spin states of the Mn4 cluster in acetate- inhibited PSII supports the idea that Y(Z) oxidation or Y(Z)· reduction is communicated to the Mn4 cluster through a direct hydrogen-bonding pathway, possibly involving a ligand bound to the Mn4 cluster.
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