DETECTION OF A PARAMAGNETIC INTERMEDIATE IN THE S1-STATE OF THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX

We report the detection of a new electron paramagnetic resonance (EPR) signal that demonstrates the presence of a paramagnetic intermediate in the resting (S1) state of the photosynthetic oxygen-evolving complex. The signal was detected using the method of parallel polarization EPR, which is sensitive to DELTA-m = 0 transitions in high spin systems. The properties of the parallel polarization EPR signal in the S1 state are consistent with an S = 1 spin state of an exchange-Coupled manganese center that corresponds to the reduced form of the species giving rise to the multiline EPR signal in the light-induced S2 state. The implications for the electronic structure of the oxygen-evolving complex are discussed.