A Novel Square-Planar Ni(II) Complex with an Amino鈥擟arboxamido鈥擠ithiolato-Type Ligand as an Active-Site Model of NiSOD

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• 作者：Daisuke Nakane ; Yuko Wasada-Tsutsui ; Yasuhiro Funahashi ; Tsubasa Hatanaka ; Tomohiro Ozawa ; Hideki Masuda
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：July 7, 2014
• 年：2014
• 卷：53
• 期：13
• 页码：6512-6523
• 全文大小：496K
• ISSN：1520-510X

文摘

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鈥揷arboxamido鈥揹ithiolato-type square-planar ligand (1, [Ni²⁺(L₁)]^{鈭?/sup>) 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 谓(C鈺怬) 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 (E_pa) 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 E_pa 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鈥搗is: 425, 525, and 650 nm; electron paramagnetic resonance (EPR) (4 K): g_// = 2.21, g = 2.01; resonance Raman: 谓(16O鈥?sup>16}O)/谓(¹⁸O鈥?sup>18O) = 1020/986 cm^鈥?), 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 (pK_a 4.65) added as mimic of superoxide (pK_a 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.