Q-band EPR of the S2 state of photosystem II confirms an S = 5/2 origin of the X-band g = 4.1 signal.

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Abstract

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. Copyright 2004 Biophysical Society

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Haddy, A., Lakshmi, K. V., Brudvig, G. W., & Frank, H. A. (2004). Q-band EPR of the S2 state of photosystem II confirms an S = 5/2 origin of the X-band g = 4.1 signal. Biophysical Journal, 87(4), 2885–2896. https://doi.org/10.1529/biophysj.104.040238

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