The photochemistry in photosystem II of spinach has been characterized by electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 77-235 K, and the yields of the photooxidized species have been determined by integration of their EPR signals. In samples treated with 3-(3,4-di-chlorophenyl)-l,l-dimethylurea (DCMU), a single stable charge separation occurred throughout the temperature range studied as reflected by the constant yield of the Fe(II)-QA- EPR signal. Three distinct electron donation pathways were observed, however. Below 100 K, one molecule of cytochrome b559 was photooxidized per reaction center. Between 100 and 200 K, cytochrome b559 and the S1 state competed for electron donation to P680+. Photooxidation of the S1 state occurred via two intermediates: the g = 4.1 EPR signal species first reported by Casey and Sauer [Casey, J. L., & Sauer, K. (1984) Biochim. Biophys. Acta 767, 21-28] was photooxidized between 100 and 160 K, and upon being warmed to 200 K in the dark, this EPR signal yielded the multiline EPR signal associated with the S2 state. Only the S1 state donated electrons to P680+ at 200 K or above, giving rise to the light-induced S2-state multiline EPR signal. These results demonstrate that the maximum S2-state multiline EPR signal accounts for 100% of the reaction center concentration. In samples where electron donation from cytochrome b559 was prevented by chemical oxidation, illumination at 77 K produced a radical, probably a chlorophyll cation, which accounted for 95% of the reaction center concentration. This electron donor competed with the S1state for electron donation to P680+ below 100 K. Chemical oxidation of cytochrome b559, however, had no effect on the photooxidation of the g = 4.1 or multiline EPR signal species. Quantitation of the cytochrome b559 EPR signal produced by chemical oxidation showed that two molecules of cytochrome b559 are present per reaction center. The S2 → S3 transition occurred in samples illuminated above 190 K. The g = 4.1 EPR signal was not detected, however, as an intermediate in the S2 →S3 transition. We propose that the g = 4.1 and multiline EPR signals both arise from the same site in the S2 oxidation state and the spectroscopic differences reflect temperature-dependent structural changes in the Mn active site.
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