C-H activation by a mononuclear manganese(III) hydroxide complex: Synthesis and characterization of a manganese-lipoxygenase mimic?
C-H activation by a mononuclear manganese(III) hydroxide complex: Synthesis and characterization of a manganese-lipoxygenase mimic?
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DOI：
10.1021/ja039283w
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发表时间：
2005-07-13
影响因子：
15
通讯作者：
Stack, TDP
中科院分区：
文献类型：
--
作者：
Goldsmith, CR;Cole, AP;Stack, TDP
脂氧合酶是单核非血红素金属酶，可区域和立体特异性地将含有 1,4-戊二烯亚基的脂肪酸转化为烷基过氧化物。通常认为，速率决定步骤是通过活性金属(III)-氢氧化物从底物的戊二烯亚基中夺取氢原子，得到金属(II)-水物质和有机自由基。所有已知的植物和动物脂氧合酶均含有铁作为活性金属；然而，最近发现锰是真菌脂氧合酶中的活性金属。本文报道了单核 Mn(III) 配合物 [Mn-III(PY5)(OH)](CF3SO3)(2) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymacet)-pyridine) 的合成和表征，该配合物以与氢原子提取最一致的方式与碳氢化合物底物反应，并为拟议的 反应机理。 PY5 的中性五吡啶基连接赋予金属中心强路易斯酸性特征，允许 Mn(III) 化合物进行这种氧化化学反应。 [Mn-III(PY5)(H2O)](2+)和还原产物[Mn-II(PY5)(H2O)](2+)的热力学分析估计Mn(II)络合物中金属结合水中的O-H键强度为82(+/-2) kcal mol(-1)，略小于相关还原铁络合物中的O-H键强度， [Fe-II(PY5)(MeOH)](2+)。 [Mn-III(PY5)(OH)](2+) 在 323 K 下与烃底物反应的速率与类似的 [Fe-III(PY5)(OMe)](2+) 相当。[Mn-III(PY5)(OH)](2+) 的晶体结构显示出 Jahn-Teller 畸变，而 [Mn-II(PY5)(H2O)](2+)，特别是沿 Mn(III)-OH 轴的压缩。因此，预计氢原子转移会发生大的内部结构重组，这可能与与其他金属配合物相比，底物氧化速率对底物键解离能的依赖性降低有关。这里提出的结果表明，锰是脂氧合酶活性的可行金属，并且具有相似的配位层，铁和锰可以通过相似的机制氧化底物。
Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active metal (III)-hydroxide species to give a metal(II)-water species and an organic radical. All known plant and animal lipoxygenases contain iron as the active metal; recently, however, manganese was found to be the active metal in a fungal lipoxygenase. Reported here are the synthesis and characterization of a mononuclear Mn(III) complex, [Mn-III(PY5)(OH)](CF3SO3)(2) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)-pyridine), that reacts with hydrocarbon substrates in a manner most consistent with hydrogen atom abstraction and provides chemical precedence for the proposed reaction mechanism. The neutral pentapyridyl ligation of PY5 endows a strong Lewis acidic character to the metal center allowing the Mn(Ill) compound to perform this oxidation chemistry. Thermodynamic analysis of [Mn-III(PY5)(H2O)](2+) and the reduced product, [Mn-II(PY5)(H2O)](2+), estimates the strength of the O-H bond in the metal-bound water in the Mn(II) complex to be 82 (+/- 2) kcal mol(-1), slightly less than that of the O-H bond in the related reduced iron complex, [Fe-II(PY5)(MeOH)](2+). [Mn-III(PY5)(OH)](2+) reacts with hydrocarbon substrates at rates comparable to those of the analogous [Fe-III(PY5)(OMe)](2+) at 323 K. The crystal structure of [Mn-III(PY5)(OH)](2+) displays Jahn-Teller distortions that are absent in [Mn-II(PY5)(H2O)](2+), notably a compression along the Mn(III)-OH axis. Consequently, a large internal structural reorganization is anticipated for hydrogen atom transfer, which may be correlated to the lessened dependence of the rate of substrate oxidation on the substrate bond dissociation energy as compared to other metal complexes. The results presented here suggest that manganese is a viable metal for lipoxygenase activity and that, with similar coordination spheres, iron and manganese can oxidize substrates through a similar mechanism.