Dual-mode EPR study of new signals from the S-3-state of oxygen-evolving complex in photosystem II

The light-induced new EPR signals at g = 12 and 8 were observed in photosys tem II (PS II) membranes by parallel polarization EPR. The signals were gen erated after two flashes of illumination at room temperature, and the signa l intensity had four flashes period oscillation, indicating that the signal origin could be ascribed to the S-3-state, Successful simulations were obt ained assuming S = 1 spin for the values of the zero-field parameters, D = +/-0.435 +/- 0.005 cm(-1) and E/D = -0.317 +/- 0.002, Orientation dependenc e of the g = 12 and 8 signal intensities shows that the axial direction of the zero-field interaction of the manganese cluster is nearly parallel to t he membrane normal.