Nakane Daisuke, Wasada-Tsutsui Yuko, Funahashi Yasuhiro, Hatanaka Tsubasa, Ozawa Tomohiro, Masuda Hideki
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology , Gokiso, Showa, Nagoya 466-8555, Japan.
Inorg Chem. 2014 Jul 7;53(13):6512-23. doi: 10.1021/ic402574d. Epub 2014 Jun 18.
To understand the role of the unique equatorial coordination environment at the active center in nickel superoxide dismutase (NiSOD), we prepared a novel Ni(II) complex with an amino-carboxamido-dithiolato-type square-planar ligand (1, Ni(2+)(L1)) as a model of the NiSOD active site. Complex 1 has a low-spin square-planar structure in all solvents. Interestingly, the absorption wavelength and ν(C═O) stretching vibrations of 1 are affected by solvents. This provides an indication that the carbonyl oxygens participate in hydrogen-bonding interactions with solvents. These interactions are reflected in the redox potentials; the peak potential of an anodic wave (Epa) values of Ni(II)/Ni(III) waves for 1 are shifted to a positive region for solvents with higher acceptor numbers. This indicates that the disproportionation of superoxide anion by NiSOD may be regulated by hydrogen-bonding interactions between the carboxamido carbonyl and electrophilic molecules through fine-tuning of the redox potential for optimal SOD activity. Interestingly, the Epa value of the Ni(III)/Ni(II) couple in 1 in water (+0.303 V vs normal hydrogen electrode (NHE)) is similar to that of NiSOD (+0.290 V vs NHE). We also investigated the superoxide-reducing and -oxidizing reactions of 1. First, 1 reacts with superoxide to yield the superoxide-bound Ni(II) species (UV-vis: 425, 525, and ∼650 nm; electron paramagnetic resonance (EPR) (4 K): g// = 2.21, g⊥ = 2.01; resonance Raman: ν((16)O-(16)O)/ν((18)O-(18)O) = 1020/986 cm(-1)), which is then oxidized to Ni(III) state only in the presence of both a proton and 1-methylimidazole, as evidenced by EPR spectra. Second, EPR spectra indicate that the oxidized complex of 1 with 1-methylimidazole at the axial site can be reduced by reaction with superoxide. The Ni(III) complex with 1-methylimidazole at the axial site does not participate in any direct interaction with azide anion (pKa 4.65) added as mimic of superoxide (pKa 4.88). According to these data, we propose the superoxide disproportionation mechanism in superoxide-reducing and -oxidizing steps of NiSOD in both Ni(II) and Ni(III) states.
为了理解镍超氧化物歧化酶（NiSOD）活性中心独特的赤道配位环境所起的作用，我们制备了一种新型的Ni(II)配合物，其配体为氨基 - 羧酰胺 - 二硫醇盐型平面四边形配体（1，[Ni(2 +)(L1)]⁻），作为NiSOD活性位点的模型。配合物1在所有溶剂中均具有低自旋平面四边形结构。有趣的是，1的吸收波长和ν(C═O)伸缩振动受溶剂影响。这表明羰基氧与溶剂参与了氢键相互作用。这些相互作用反映在氧化还原电位上；对于1，Ni(II)/Ni(III)波的阳极波峰电位（Epa）值在接受数较高的溶剂中向正区域移动。这表明NiSOD对超氧阴离子的歧化作用可能通过羧酰胺羰基与亲电分子之间的氢键相互作用来调节，通过微调氧化还原电位以实现最佳的SOD活性。有趣的是，1在水中的Ni(III)/Ni(II)电对的Epa值（相对于标准氢电极（NHE）为 +0.303 V）与NiSOD的Epa值（相对于NHE为 +0.290 V）相似。我们还研究了1的超氧阴离子还原和氧化反应。首先，1与超氧阴离子反应生成超氧阴离子结合的Ni(II)物种（紫外可见光谱：425、525和～650 nm；电子顺磁共振（EPR）（4 K）：g// = 2.21，g⊥ = 2.01；共振拉曼光谱：ν((¹⁶)O-(¹⁶)O)/ν((¹⁸)O-(¹⁸)O) = 1020/986 cm⁻¹），然后仅在质子和1 - 甲基咪唑同时存在的情况下被氧化为Ni(III)态，EPR光谱证明了这一点。其次，EPR光谱表明，轴向位点带有1 - 甲基咪唑的1的氧化配合物可通过与超氧阴离子反应而被还原。轴向位点带有1 - 甲基咪唑的Ni(III)配合物不与作为超氧阴离子模拟物添加的叠氮阴离子（pKa 4.65）发生任何直接相互作用。根据这些数据，我们提出了NiSOD在Ni(II)和Ni(III)态下超氧阴离子还原和氧化步骤中的超氧阴离子歧化机制。
