Kosaki Shinya, Mino Hiroyuki
Division of Materials Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, 464-8602 Nagoya, Aichi, Japan.
J Phys Chem B. 2023 Jul 27;127(29):6441-6448. doi: 10.1021/acs.jpcb.3c01656. Epub 2023 Jul 18.
The high-spin S state of the photosynthetic oxygen-evolving cluster MnCaO, 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 (= HO), 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.
利用Q波段脉冲电子顺磁共振（EPR）研究了光合放氧簇MnCaO的高自旋S态，该态对应于X波段电子顺磁共振的g = 4.1信号，检测到一个位于g = 3.10的主峰和位于5.25、4.55及2.80的卫星峰。我们将自旋态评估为g = 0.465 cm⁻¹和D/g = 0.245且S = 5/2时的零场分裂。弛豫时间的温度依赖性表明，激发态能量比S = 5/2基态高28.7 cm⁻¹。通过比较当前的量子力学（QM）计算模型，具有两个氧原子W1（= OH）和W2（= HO）质子化状态的封闭立方烷结构是S = 5/2态最可能的结构。三脉冲电子自旋回波包络调制（ESEEM）检测到核信号，该信号被归属于与Mn1离子配位的His332中的氮。所获得的氮信号超精细常数与S = 1/2低自旋态相比显著降低。这些结果表明，S = 5/2自旋态源于封闭立方烷结构。
