Metalloradical EPR Signals from the Y-Z center dot 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 center dot in magnetic interaction with the CaMn4-cluster in the particular S-state, Y-Z center dot 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 center dot 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 center dot 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 CaMn4-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 pK (a)s found for decreased split signal induction were interpreted in the fate of the phenol proton.