Proton equilibria in the manganese cluster of photosystem II control the intensities of the S-0 and S-2 state g approximate to 2 electron paramagnetic resonance signals

We have studied the pH effect on the So and S-2 multiline electron paramagn etic resonance (EPR) signals from the water-oxidizing complex of photosyste m II. Around pH 6, the maximum signal intensities were detected. On both th e acidic and alkaline sides of pH 6, the intensities of the EPR signals dec reased. 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 alk aline pH back to pH approximate to 6. In the S2 State we observed partial r ecovery of the multiline signal when going from alkaline pH back to pH appr oximate to 6, whereas no significant recovery of the S2 multiline signal wa s 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 o f pH are discussed. Some are ruled out, such as disintegration of the Mn cl uster or decay of the S states and formal Cl- and Ca2+ depletion. The alter ed EPR signal intensities probably reflect the protonation/deprotonation of ligands to the Mn cluster or the oxo bridges between the Mn ions. Also, th e possibility of decreased multiline signal intensities at alkaline pH as a n effect of changed redox potential of Yz is put forward.