Electronic Structure of the S₁ State Manganese Cluster in Photosystem II Investigated Using Q‑Band Selective...

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 Y_D ^• 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 = O^2-, and O5 = O^2- 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.