Metalloradical EPR Signals from the YZ·S-State Intermediates in Photosystem II

The redox-active tyrosine residue (Y Z ) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced Y Z · in magnetic interaction with the CaMn 4 -cluster in the particular S -state, Y Z ·S X intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cryogenic temperatures and provide means to both study the otherwise transient Y Z · and to probe the S -states with EPR spectroscopy. The illumination used for signal induction grouped the observed split EPR signals in two categories: (i) Y Z in the lower S -states was oxidized by P680 + formed via charge separation, while (ii) Y Z in the higher S -states was oxidized by an excited, highly oxidizing Mn species. Applied mechanistic studies of the Y Z ·S X intermediates in the different S -states are reviewed and compared to investigations in photosystem II at physiological temperature. Addition of methanol induced S -state characteristic changes in the split signals’ formation which reflect changes in the magnetic coupling within the CaMn 4 -cluster due to methanol binding. The pH titration of the split EPR signals, on the other hand, could probe the proton-coupled electron transfer properties of the Y Z oxidation. The apparent p K a s found for decreased split signal induction were interpreted in the fate of the phenol proton.