The S3 state of the oxygen-evolving complex in photosystem II is converted to the S2YZ• state at alkaline pH

Here we report an EPR signal that is induced by a pH jump to alkaline pH in the S-3 state of the oxygen-evolving complex in photosystem II. The S-3 state is first formed with two flashes at pH 6. Thereafter, the pH is changed in the dark prior to freezing of the sample. The EPR signal is 90-100 G wide and centered around g=2. The signal is reversibly induced with a pK=8.5 +/-0.3 and is very stable with a decay half-time of 5-6 min. If the pH is changed in the dark from pH 8.6 to 6.0, the signal disappears although the S-3 state remains. We propose that the signal arises from the interaction between the Mn cluster and Y-Z, resulting in the spin-coupled S2YZcircle signal. Our data suggest that the potential of the Y-Z(circle)/Y-Z redox couple is sensitive to the ambient pH in the S-3 state. The alkaline pH decreases the potential of the Y-Z(circle)/Y-Z couple so that Y-Z can give back an electron to the S-3 state, thereby obtaining the S2YZcircle EPR signal. The tyrosine oxidation also involves proton release from Y-Z, and the results support a mechanism where this proton is released to the bulk medium presumably via a close-lying base. Thus, the equilibrium is changed from S3YZ to S2YZcircle by the alkaline pH. At normal pH (pH 5.5-7), this equilibrium is set strongly to the S3YZ state. The results are discussed in relation to the present models of water oxidation. Consequences for the relative redox potentials of Y-z(circle)/Y-z and S-3/S-2 at different pH values are discussed. We also compare the pH-induced S2YZcircle signal with the S2YZcircle signal from Ca2+- depleted photosystem II.