Datensatz

Zusammenfassung

The EPR “split signals” represent key intermediates of the S-state cycle where the redox active D1-Tyr161 (Y_Z) has been oxidized by the reaction center of the photosystem II enzyme to its tyrosyl radical form, but the successive oxidation of the Mn₄CaO₅ cluster has not yet occurred (S_iY_Z˙). Here we focus on the S₂Y_Z˙ state, which is formed en route to the final metastable state of the catalyst, the S₃ state, the state which immediately precedes O–O bond formation. Quantum chemical calculations demonstrate that both isomeric forms of the S₂ state, the open and closed cubane isomers, can form states with an oxidized Y_Z˙ residue without prior deprotonation of the Mn₄CaO₅ cluster. The two forms are expected to lie close in energy and retain the electronic structure and magnetic topology of the corresponding S₂ state of the inorganic core. As expected, tyrosine oxidation results in a proton shift towards His190. Analysis of the electronic rearrangements that occur upon formation of the tyrosyl radical suggests that a likely next step in the catalytic cycle is the deprotonation of a terminal water ligand (W1) of the Mn₄CaO₅ cluster. Diamagnetic metal ion substitution is used in our calculations to obtain the molecular g-tensor of Y_Z˙. It is known that the g_x value is a sensitive probe not only of the extent of the proton shift between the tyrosine–histidine pair, but also of the polarization environment of the tyrosine, especially about the phenolic oxygen. It is shown for PSII that this environment is determined by the Ca²⁺ ion, which locates two water molecules about the phenoxyl oxygen, indirectly modulating the oxidation potential of Y_Z.