pH-dependent oxidation of Cytochrome b559 is different in the different S-states

We have studied the state of Cyt b559 and the Mn cluster by EPR and optical spectroscopy in the different S-states between pH 4.0-10.0. In these experiments, the pH was changed after the OEC turnover to the appropriate S-state (at pH 6.0) by 0-3 saturating flashes. Furthermore, the state of Cyt b559 after a subsequent flash given after the pH modification was also investigated.
The results show that low (< pH 6.0) and high pH (> pH 9.0) resulted in an increase in the oxidized form of Cyt b559 in the S2 and S3 states. The effect was more pronounced at low than high pH. There was no further oxidation after the subsequent flash. Different to this, the pH induced oxidized Cyt b559 population in the S0 and S1 states was significantly smaller. In the S1 state, the pH dependent oxidation of Cyt b559 was very weak, with a slight increase from pH 10.0 to 4.0. A subsequent flash in S0 or S1 resulted in a significant Cyt b559 oxidation at both low and high pH’s.
The ratio of the high and low potential forms of Cyt b559 at different pH’s was also different in different S-states. At low and normal pH’s both oxidized LP and HP forms were observed, while at high pH’s, only the LP form was oxidized. The possible involvement of Cyt b559 in electron donation to the Mn cluster during S-cycle at extreme pH is discussed.