Proton equilibria in the manganese cluster of photosystem II control the intensities of the S0 and S2 state g ≈ 2 electron paramagnetic resonance signals

We have studied the pH effect on the So and S-2 multiline electron paramagnetic resonance (EPR) signals from the water-oxidizing complex of photosystem II. Around pH 6, the maximum signal intensities were detected. On both the acidic and alkaline sides of pH 6, the intensities of the EPR signals decreased. Two pKs were determined for the So multiline signal; pK(1) = 4.2 +/- 0.2 and pK(2) = 8.0 +/- 0.1, and for the S2 multiline signal the pKs were pK(1) = 4.5 +/- 0.1 and pK(2) = 7.6 +/- 0.1. The intensity of the So-state: EPR signal was partly restored when the pH was changed from acidic or alkaline pH back to pH approximate to 6. In the S2 State we observed partial recovery of the multiline signal when going from alkaline pH back to pH approximate to 6, whereas no significant recovery of the S2 multiline signal was observed when the pH was changed from acidic pH back to pH approximate to 6. Several possible explanations for the intensity changes as a function of pH are discussed. Some are ruled out, such as disintegration of the Mn cluster or decay of the S states and formal Cl- and Ca2+ depletion. The altered EPR signal intensities probably reflect the protonation/deprotonation of ligands to the Mn cluster or the oxo bridges between the Mn ions. Also, the possibility of decreased multiline signal intensities at alkaline pH as an effect of changed redox potential of Yz is put forward.