Molecular Structure Related to an S =5/2 High-Spin S2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy

The high-spin S-2 state of the photosyntheticoxygen-evolvingcluster Mn4CaO5, corresponding to the g = 4.1 signal for X-band electron paramagnetic resonance(EPR), was investigated using Q-band pulsed EPR, which detected amain peak at g = 3.10 and satellite peaks at 5.25,4.55, and 2.80. We evaluated the spin state as the zero-field splittingof D = 0.465 cm(-1) and E/D = 0.245 with S =5/2. The temperature dependence of the T (1) relaxation time revealed that the excited-state energy was 28.7cm(-1) higher than that of the S = 5/2 ground state. By comparing present quantum mechanical(QM) calculation models, a closed-cubane structure with the protonationstate of two oxygens, W1 (= OH-) and W2 (= H2O), was the most probable structure for the S = 5/2 state. The three-pulse electron spin-echoenvelope modulation (ESEEM) detected the nuclear signal, which wasassigned to nitrogen as His332 ligated to the Mn1 ion. The obtainedhyperfine constant for the nitrogen signal was significantly reducedfrom that in the S = 1/2 low-spin state.These results indicate that the S =5/2 spin state arises from the closed-cubane structure.

Automated summary

The high-spin S-2 state of Mn4CaO5 photosyntheticoxygen-evolving cluster was investigated using Q-band pulsed EPR, which showed main and satellite peaks. The spin state was evaluated with a zero-field splitting of D=0.465 cm(-1) and E/D=0.245, corresponding to S=5/2. The temperature dependence of the T (1) relaxation time revealed a 28.7 cm(-1) higher excited-state energy compared to the S=5/2 ground state. By comparing quantum mechanical models, a closed-cubane structure was determined to be the most probable for the S=5/2 state. The three-pulse ESEEM detected a nuclear signal assigned to nitrogen ligated to the Mn1 ion.