Parallel polarization electron paramagnetic resonance studies of the S-1-state manganese cluster in the photosynthetic oxygen-evolving system

Magnetic properties of the S-1-state manganese cluster in the oxygen-evolving photosystem II were studied by parallel polarization electron paramagnetic resonance spectroscopy. Dark minus light spectra gave rise to a broad S-1-state signal with a g value of about 4.9 [Dexheimer, S, L., Klein, M. P. (1992) J, Am. Chem. Sec. 114, 2821-2826]. Temperature variation of the signal intensity between 1.9 and 10 K observed in PS II with a sucrose buffer indicates that the signal originates from an excited state with a spin S of 1 with separation from the ground state (S = 0) of about 2.5 K. The S-1-state signal was also observed in the sucrose buffer supplemented by 50% glycerol, However, no S-1-state signal was detected by addition of 3% methanol or 30% ethylene glycol in the sucrose buffer, although illumination at 200 K in the presence of these alcohols induced the normal multiline St signal. Furthermore, modification of the Mn cluster by Cl- or Ca2+ depletion from PS II membranes failed to produce a detectable S-1-state signal. A possible magnetic structure of the Mn cluster responsible for the generation of the S-1-state signal is discussed on the basis of these observations.