Influence of Thiolate Ligands on Reductive N鈭扥 Bond Activation. Probing the O2鈭?/sup> Binding Site of a Biomimetic Superoxide Reductase Analogue and Examining the Proton-Dependent Redu

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• 作者：Gloria Villar-Acevedo ; Elaine Nam ; Sarah Fitch ; Jason Benedict ; John Freudenthal ; Werner Kaminsky ; Julie A. Kovacs
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：February 9, 2011
• 年：2011
• 卷：133
• 期：5
• 页码：1419-1427
• 全文大小：983K
• 年卷期：v.133,no.5(February 9, 2011)
• ISSN：1520-5126

文摘

Nitric oxide (NO) is frequently used to probe the substrate-binding site of 鈥渟pectroscopically silent鈥?non-heme Fe²⁺ sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate-ligated biomimetic SOR model [Fe^II(S^Me₂N₄(tren))]⁺ (1). Like NO-bound trans-cysteinate-ligated SOR (SOR-NO), the rhombic S = 3/2 EPR signal of NO-bound cis-thiolate-ligated [Fe(S^Me₂N₄(tren)(NO)]⁺ (2; g = 4.44, 3.54, 1.97), the isotopically sensitive 谓_NO(谓_¹⁵NO) stretching frequency (1685(1640) cm^鈭?), and the 0.05 脜 decrease in Fe鈭扴 bond length are shown to be consistent with the oxidative addition of NO to Fe(II) to afford an Fe(III)鈭扤O^{鈭?/sup> {FeNO}7 species containing high-spin (S = 5/2) Fe(III) antiferromagnetically coupled to NO鈭?/sup> (S = 1). The cis versus trans positioning of the thiolate does not appear to influence these properties. Although it has yet to be crystallographically characterized, SOR-NO is presumed to possess a bent Fe鈭扤O similar to that of 2 (Fe鈭扤鈭扥 = 151.7(4)掳). The N鈭扥 bond is shown to be more activated in 2 relative to N- and O-ligated {FeNO}7 complexes, and this is attributed to the electron-donating properties of the thiolate ligand. Hydrogen-bonding to the cysteinate sulfur attenuates N鈭扥 bond activation in SOR, as shown by its higher 谓_NO frequency (1721 cm鈭?). In contrast, the 谓_O鈭扥 frequency of the SOR peroxo intermediate and its analogues is not affected by H-bonds to the cysteinate sulfur or other factors influencing the Fe鈭扴R bond strength; these only influence the 谓_Fe鈭扥 frequency. Reactions between 1 and NO₂鈭?/sup> are shown to result in the proton-dependent heterolytic cleavage of an N鈭扥 bond. The mechanism of this reaction is proposed to involve both FeII鈭扤O₂鈭?/sup> and {FeNO}6 intermediates similar to those implicated in the mechanism of NiR-promoted NO₂鈭?/sup> reduction.}