Electronic Structure of the S₁ State Manganese Cluster in Photosystem II Investigated Using Q-Band Selective Hole-Burning

The electronic structure of the S1 state of photosystem II (PSII) was investigated using selective hole burning of Q-band pulsed electron paramagnetic resonance. The free induction decay and spin–echo signals of the tyrosine radical YD• in the plant PSII oscillated because of the magnetic dipole–dipole interaction with the S1 state Mn cluster. The initial period was 410 ns (2.44 MHz) and was assigned to the S = 1 spin state. Based on the oscillation analysis, both Mn1 and Mn4 and both Mn2 and Mn3 were assigned as Mn(III) and Mn(IV), respectively, which is consistent with the quantum chemical calculations. The 410 ns period was accounted for in the simplified model using the isotropic spin density distribution ratio [1.6:–1.1:–1.1:1.6] for Mn1–4 ions. This oscillation was identical with that observed in the presence of methanol. The oscillation decreased in PsbP/Q- and PsbO/P/Q-depleted PSII. In Thermosynechococcus vulcanus, two periods, 390 ns (2.56 MHz) and 630 ns (1.59 MHz), were detected, indicating that the cyanobacterial S1 state includes two isomers, S = 1 and S ≥ 2 spins. The S ≥ 2 spin was not detected in PsbO/U/V-depleted PSII without polyethylene glycol. The S ≥ 2 state was consistent with the reported quantum chemical calculation using S = 3. A simplified model accounted for the S = 1 state as the spin density distribution [1.8:–1.3:–1.3:1.8] and for the S ≥ 2 state as the isotropic spin density distribution [−0.5:0.5:0.5:0.5] for Mn1–4 ions. In combination with quantum chemical calculations, the most probable protonated structure is W1 = H2O, W2 = H2O, O4 = O2–, and O5 = O2– for the S1 state. These results demonstrate that the selective hole burning method is a powerful tool to complement X-ray studies to determine the valence and protonation structure of manganese clusters, not only in the S1 state but also in higher S-states and general metal clusters, which would provide important insights into the water oxidation mechanism.