Charge separation events in photosystem <scp>II</scp> during the <scp> S ₂ </scp> to <scp> S ₃ </scp> transition as revealed by <scp>EPR</scp> spectroscopy

Photosystem II (PSII) water oxidation includes proton and electron transfer pathways, occurring during the sequential S-state transitions of Mn₄CaO₆. Here, we investigate the charge separation events during the S₂ to S₃ transition that take place via the S₂TyrZ^• intermediate, utilizing electron paramagnetic resonance (EPR) spectroscopy. The increasing number of cycles of S₂TyrZ^• formation and decay results in a gradual diminution of the S₂TyrZ^• signal intensity which is proportional to the amount of S₃ state. Our results point to the progressive accumulation of a different configuration of the donor side of PSII at the S₂ state that allows the Mn₄CaO₆ to be oxidized. These results consolidate previous investigations supporting that, during the lifetime of the S₂TyrZ^•, a proton from Mn₄CaO₆ is removed, prior to the advancement to the S₃ state.