EPR evidence for the primary water oxidation step upon the S2 → S3 transition in the Joliot-Kok cycle of plant photosystem II

A new type of 90–240 G wide EPR signal from the modified S3 state of Ca2+-depleted photosystem II (PSII) is concluded to arise from a partially oxidized water radical with spin S = 1 2 interacting with the S = 1 2 S2-state manganese tetramer (‘Mn4’). This is based exclusively on the fact that the average g value of the radical is ≈ 2.010–2.012, a value close to that of OH radical (2.011) and significantly larger than either one of an oxidized imidazole (2.00226) or an oxidized tyrosine (2.0046), indicating that the radical may be (HOOH)−, the most probable intermediate produced by abstracting two protons and one electron from a bound water dimer. The effective interactions between the Mn4 and radical spins (S1 and S2, respectively) of the form H int = J 12 S 1 · S 2 + S 1 · D 12 · S 12 have been thoroughly investigated to find which Mn4-radical complex can reasonably make both J 12 and D 12 as small as ≈ 100 G in magnitude and can, simultaneously, yield an X-ray absorption Mn K-edge energy 0.7 ± 0.3 eV higher than that in the modified S2 state. As the most probable model, we propose that the radical must form the third bridging ligand between di-μ-oxo or μ2-oxo-(μ3-oxo) bridged Mna(III) and Mnb(IV) ions on the opposite side of mono-μ2-oxo bridged Mnc and Mnd ions.