Mechanism of N–N Bond Formation by Transition Metal–Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases,Inorganic Chemistry

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Mechanism of N–N Bond Formation by Transition Metal–Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases
Inorganic Chemistry ( IF 4.3 ) Pub Date : 2018-04-02 00:00:00 , DOI: 10.1021/acs.inorgchem.7b02333
Casey Van Stappen ₁ , Nicolai Lehnert ₁

Affiliation

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N₂O. The well-characterized diiron model complex [Fe₂(BPMP)(OPr)(NO)₂]²⁺ (BPMP⁻ = 2,6-bis[(bis(2- pyridylmethyl)amino)methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N–N bond formation and N₂O generation, while the parent complex remains stable. The initial formation of a N–N bond to generate hyponitrite (N₂O₂^2–) follows a radical-type coupling mechanism, which requires strong Fe–NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with ΔG^‡ = 7–8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N₂O₂^2– complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N₂O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N₂O formation alone. Surprisingly, the one-electron “semireduced” mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP^– ligand, allowing a one-electron reduction to overcome one of the primary Fe–NO π-interactions. Generally, mediation of N₂O formation by high-spin [{M-NO⁻}]₂ cores depends on the ease of oxidizing the M centers and breaking of the M–NO π-bonds to formally generate a “full” ³NO^– unit, allowing for the critical step of N–N bond formation to proceed (via a radical-type coupling mechanism).

中文翻译：

过渡金属-亚硝酰基配合物形成N–N键的机理：模拟黄酮二铁一氧化氮还原酶

一氧化氮（NO）具有许多重要的生物学功能，包括神经信号转导，血压控制以及更高浓度的免疫防御。一些病原性细菌的已开发通过使用flavodiiron氧化氮还原酶（FNORs），其利用非血红素二铁的活性位点，以减少NO→N中的劣化这种有毒分子的方法₂的Fe O.充分表征的模型二铁配合物[ ₂（的Bpmp）（OPR）（NO）₂ ] ²⁺（的Bpmp ^-= 2，6-双[（双（2-吡啶基甲基）氨基）甲基] -4-甲基酚盐），模仿了这些酶的活性位点结构和反应性，为FNOR的机理提供了重要的见识。目前，我们已使用计算方法阐明了一种相干反应机理，该机理表明该配合物的一电子和二电子还原如何诱导N–N键形成和N ₂ O生成，而母体配合物保持稳定。N-N键的初始形成会生成亚硝酸盐（N ₂ O ₂^2–）遵循自由基类型的耦合机制，这就需要克服强的Fe– NOπ相互作用才能有效地氧化铁中心。次黄铁矿的形成提供最大的激活屏障，ΔG ^‡在双电子超还原机理中= 7–8 kcal / mol（几种功能的平均值）。这之后是形成的具有N ₂ ö ₂^2-配合物与用于非血红素二铁系统一个新的结合模式，允许对于N的容易释放₂ ö与羧酸移位的协助。这为反应通过单独的N ₂ O形成提供了足够的热力学驱动力。出乎意料的是，单电子“半还原”机理被认为与超还原机理具有竞争性。这是由于BPMP ^–配体赋予的不对称性，允许单电子还原克服了主要的Fe– NOπ相互作用之一。通常，N _2的介导通过高自旋澳组[{M-NO ^- }] ₂芯依赖于易于氧化的M个中心和破M-NOπ-键到的正式产生一个“完整” ³ NO ^-单元，从而允许进行N–N键形成的关键步骤（通过自由基型偶联机理）。

更新日期：2018-04-02

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