Q-band Electron Paramagnetic Resonance Studies of the S3 State of the OEC of Photosystem II

Abstract

We have applied Q-band EPR spectroscopy to study the S3 state of the Mn cluster in untreated and in methanol treated PSII membranes, extending earlier bimodal studies at X band EPR. Prominent EPR signals are observed in both sets of samples. A simultaneous analysis of the spectra at both frequency bands indicates that the spin associated with the signals of S3 is S = 3 rather than S = 1. An S = 3 spin state is also indicated in the presence of methanol, but with significantly different zero field splitting parameters, which explain the absence of X-band signals in this case.