Five distinct EPR signals which originate from the S2 state of the O2-evolving complex of photosystem II are generated by low-temperature illumination and observed at liquid helium temperatures. The magnetic properties of these five S2 state species are interpreted in terms of a model consisting of an exchange coupled Mn tetramer, where both ferromagnetic and antiferromagnetic exchange occur simultaneously to give an s = 3/2 ground state and an s = 1/2 first excited state. The multiline EPR signals from the S2 state of resting, active, and NH4Cl-treated photosystem II membranes are proposed to arise from the excited s = 1/2 state. The different magnetic properties of these S2 state species are explained by changes in the exchange coupling constants which modulate the energy spacing between the ground s = 3/2 state and the excited s = 1/2 state. The g = 4.1 S2 state EPR signal exhibits Curie behavior in the 4.0-25.0 K range and is proposed to arise from the s = 3/2 ground state of the Mn complex, in a configuration where the s = 1/2 excited state is thermally inaccessible. The Mn tetramer model is successful in predicting the 55Mn nuclear hyperfine interaction in the S2 state multiline EPR spectrum from untreated resting state samples and also the reduced 55Mn nuclear hyperfine coupling in the S2 state multiline EPR spectrum from NH4Cl-treated samples relative to untreated resting state samples. The combination of ferromagnetic and antiferromagnetic exchange coupling in the Mn site of the O2-evolving complex is analogous to the case encountered in Fe4S4 proteins and Cu4O4 cubane-like complexes. On the basis of this analogy, we postulate a Mn4O4 cubane-like structure for the active site of water oxidation in the S2 state.
|Number of pages||8|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1986|
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