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

Photosystem II (PSII) water oxidation includes proton and electron transfer pathways, occurring during the sequential S‐state transitions of Mn 4 CaO 6 . Here, we investigate the charge separation events during the S 2 to S 3 transition that take place via the S 2 TyrZ • intermediate, utilizing electron paramagnetic resonance (EPR) spectroscopy. The increasing number of cycles of S 2 TyrZ • formation and decay results in a gradual diminution of the S 2 TyrZ • signal intensity which is proportional to the amount of S 3 state. Our results point to the progressive accumulation of a different configuration of the donor side of PSII at the S 2 state that allows the Mn 4 CaO 6 to be oxidized. These results consolidate previous investigations supporting that, during the lifetime of the S 2 TyrZ • , a proton from Mn 4 CaO 6 is removed, prior to the advancement to the S 3 state.