Intermediates of a polynuclear manganese center involved in photosynthetic oxidation of water

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Abstract
EPR of spinach chloroplasts given a series of laser flashes, n = 0, 1, ..., 6, at room temperature and rapidly cooled to -140.degree. C reveals a signal possessing at least 16 and possibly 21 or more hyperfine lines when observed below 35 K. The spectrum is consistent with a pair of antiferromagnetically coupled Mn ions, or possibly a tetramer of Mn ions, in which Mn(III) and Mn(IV) oxidation states are present. The intensity of this signal peaks on the 1st and 5th flashes, suggesting a cyclic change in oxidation state of period 4. The multiline signal produced on the 1st flash is unaffected by the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea but is abolished by agents that influence the state of bound Mn such as incubation with alkaline Tris or dithionite, and by extraction with cholate detergent in the presence of ammonium sulfate. The paramagnetic signal is apparently monitoring oxidation state changes in the enzyme involved in oxidation of water.