Molecular Structure Related to an S = 5/2 High-Spin S 2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy

The high-spin S state of the photosynthetic oxygen-evolving cluster Mn CaO , corresponding to the = 4.1 signal for X-band electron paramagnetic resonance (EPR), was investigated using Q-band pulsed EPR, which detected a main peak at = 3.10 and satellite peaks at 5.25, 4.55, and 2.80. We evaluated the spin state as the zero-field splitting of = 0.465 cm and / = 0.245 with = 5/2. The temperature dependence of the relaxation time revealed that the excited-state energy was 28.7 cm higher than that of the = 5/2 ground state. By comparing present quantum mechanical (QM) calculation models, a closed-cubane structure with the protonation state of two oxygens, W1 (= OH ) and W2 (= H O), was the most probable structure for the = 5/2 state. The three-pulse electron spin-echo envelope modulation (ESEEM) detected the nuclear signal, which was assigned to nitrogen as His332 ligated to the Mn1 ion. The obtained hyperfine constant for the nitrogen signal was significantly reduced from that in the = 1/2 low-spin state. These results indicate that the = 5/2 spin state arises from the closed-cubane structure.