We have characterized the electron donors to Photosystem II (PS II) in an O2‐evolving reaction center core preparation from spinach (Ghanotakis, D.F.; Yocum, C.F. FEBS Lett., 1986, 197: 244–248) using EPR spectroscopy of the manganese center involved in water oxidation and of the tyrosine donor responsible for signal II. Both the 16‐line and the 19‐line form of the S2 multiline EPR signal can be observed in the core particles under conditions similar to those which produce these signals in PS II membranes. Consequently, the structure of the coupled cluster of three or four Mn ions remains intact in the core particles. The Kok parameters which characterize the number of reaction centers capable of advancing by 0, 1, or 2 equivalents in response to a short laser pulse are found to be the same in PS II membranes and in the core particles. Also, there is no evidence for partially inactivated centers which reach the S2 or S3 states, but which do not advance to O2 release. Thus the new class of highly resolved O2‐evolving particles appears to be well suited for biophysical studies directed at the mechanism of water oxidation.
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