Light-induced high-spin signals from the oxygen evolving center in Ca2+-depleted photosystem II studied by dual mode electron paramagnetic resonance spectroscopy.

Illuminating Ca2+-depleted photosystem (PS) II membranes generated two new EPR signals at g = 11 and 15 by perpendicular and parallel polarization modes, respectively. Two turnovers of the oxygen evolving center (OEC) beyond the modified S2' state are required for the appearance of these signals. The formation of the signals correlated with that of an asymmetric (singlet-like) EPR signal observed at g approximately 2. Spectral simulation indicated that both signals arose from a transition between |2(+/-)> levels with intradoublet splitting of Delta = 0.276 cm-1 in an S = 2 spin system. Furthermore, the two signals in parallel and perpendicular modes were formed at the same time, indicating that the same metal center was responsible. The molecular z-axis of the S = 2 spin system for the signals was almost parallel to the plane of thylakoid membranes. These results indicate that the Mn cluster in the photosynthetic oxygen evolving center is the source of the new EPR species which may be a Mn(IV)-Mn(IV) or Mn(III)-Mn(III) dimer or a Mn(III) monomer. Redox events of the Mn cluster in the Ca2+-depleted PS II are discussed based on these observations.