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Q-Band EPR of the S₂ State of Photosystem II Confirms an S = 5/2 Origin of the X-Band g = 4.1 Signal

Alice Haddy ^*, K. V. Lakshmi , Gary W. Brudvig  and Harry A. Frank 

^* Department of Chemistry and Biochemistry, University of North Carolina, Greensboro, North Carolina; Department of Chemistry, Yale University, New Haven, Connecticut; and Department of Chemistry, University of Connecticut, Storrs, Connecticut

Correspondence: Address reprint requests to Alice Haddy, Dept. of Chemistry and Biochemistry, University of North Carolina, Greensboro, NC 27402. Tel.: 336-334-4605; Fax: 336-334-5402; E-mail: aehaddy{at}uncg.edu.

Disagreement has remained about the spin state origin of the g = 4.1 EPR signal observed at X-band (9 GHz) from the S₂ oxidation state of the Mn cluster of Photosystem II. In this study, the S₂ state of PSII-enriched membrane fragments was examined at Q-band (34 GHz), with special interest in low-field signals. Light-induced signals at g = 3.1 and g = 4.6 were observed. The intensity of the signal at g = 3.1 was enhanced by the presence of F^– and suppressed by the presence of 5% ethanol, indicating that it was from the same spin system as the X-band signal at g = 4.1. The Q-band signal at g = 4.6 was also enhanced by F^–, but not suppressed by 5% ethanol, making its identity less clear. Although it can be accounted for by the same spin system, other sources for the signal are considered. The observation of the signal at g = 3.1 agrees well with a previous study at 15.5 GHz, in which the X-band g = 4.1 signal was proposed to arise from the middle Kramers doublet of a near rhombic S = 5/2 system. Zero-field splitting values of D = 0.455 cm^–1 and E/D = 0.25 are used to simulate the spectra.