The first example of a nitrile hydratase model complex that reversibly binds nitriles.
The first example of a nitrile hydratase model complex that reversibly binds nitriles.
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DOI：
10.1021/ja012555f
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发表时间：
2002-08
影响因子：
15
通讯作者：
J. Shearer;H. Jackson;D. Schweitzer;D. Rittenberg;Tanya M Leavy;W. Kaminsky;R. Scarrow;J. Kovacs
中科院分区：
文献类型：
--
作者：
J. Shearer;H. Jackson;D. Schweitzer;D. Rittenberg;Tanya M Leavy;W. Kaminsky;R. Scarrow;J. Kovacs
腈水合酶（Nitrile hydratase，简称NHase）是一种将腈类化合物转化为酰胺类化合物的含铁金属酶。这种生化反应发生的机制尚不清楚。已经提出的一种机制涉及水（或氢氧化物）对Fe结合的腈的亲核攻击。本文报道了在类似于NHase的环境中含有Fe（III）的五配位模型化合物（[Fe（III）（S（2）（Me 2）N（3）（Et，Pr））]（+）），其可逆地结合各种腈、醇、胺和硫氰酸盐。X射线吸收光谱表明，五配位的[Fe（III）（S（2）（Me 2）N（3）（Et，Pr））]（+）与甲醇和乙腈反应生成六配位的溶剂键合配合物。竞争结合研究表明，MeCN优先结合ROH，这表明腈能够取代与NHase铁位点配位的H（2）O。测定了乙腈的热力学参数（Δ H = -6.2（+/-0.2）kcal/mol，Δ S = -29.4（+/-0.8）eu），苯甲腈（-4.2（+/-0.6）kcal/mol，Δ S = -18（+/-3）eu）和吡啶（Δ H = -8（+/-1）kcal/mol，Δ S = -41（+/-6）eu）与[Fe（III）（S（2）（Me 2）N（3）（Et，Pr））]（+）的结合。在各种温度下，使用（13）C NMR谱线增宽分析，对乙腈、异丙腈、苯甲腈和4-叔丁基吡啶的配体交换动力学进行了研究。配体交换的活化参数确定为Δ H（+ +）= 7.1（+/-0.8）kcal/mol，Δ S（+ +）= -10（+/-1）eu（乙腈），Δ H（+ +）= 5.4（+/-0.6）kcal/mol，Δ S（+ +）= -17（+/-2）eu（异丙腈），Δ H（+ +）= 4.9（+/-0.8）kcal/mol，Δ S（+ +）= -20（+/-3）eu（苄腈），和Δ H（+ +）= 4.7（+/-1.4）kcal/mol Δ S（+ +）= -18（+/-2）eu（4-叔丁基吡啶）。吡啶与配合物[Fe（III）（S（2）（Me 2）N（3）（Pr，Pr））]（+）结合的热力学参数（Δ H = -5.9（+/-0.8）kcal/mol，Δ S = -24（+/-3）eu），以及4-叔丁基吡啶交换的动力学参数（Δ H（+ +）= 3.1（+/-0.8）kcal/mol，Δ S（+ +）= -25（+/-3）eu）。这些数据首次表明，当它被包含在类似于NHase的配体环境中时，Fe（III）能够与腈形成稳定的络合物。此外，配体交换的速率表明，在这种配体环境中的低自旋Fe（III）比预期的更不稳定。此外，[Fe（III）（S（2）（Me 2）N（3）（Et，Pr））]（+）和[Fe（III）（S（2）（Me 2）N（3）（Pr，Pr））]（+）的比较证明了由配体约束引起的微小畸变如何可以显著地改变金属络合物的反应性。
Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound ([Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+)) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex. Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H(2)O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (DeltaH = -6.2(+/-0.2) kcal/mol, DeltaS = -29.4(+/-0.8) eu), benzonitrile (-4.2(+/-0.6) kcal/mol, DeltaS = -18(+/-3) eu), and pyridine (DeltaH = -8(+/-1) kcal/mol, DeltaS = -41(+/-6) eu) binding to [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, iso-propylnitrile, benzonitrile, and 4-tert-butylpyridine using (13)C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be DeltaH(+ +) = 7.1(+/-0.8) kcal/mol, DeltaS(+ +) = -10(+/-1) eu (acetonitrile), DeltaH(+ +) = 5.4(+/-0.6) kcal/mol, DeltaS(+ +) = -17(+/-2) eu (iso-propionitrile), DeltaH(+ +) = 4.9(+/-0.8) kcal/mol, DeltaS(+ +) = -20(+/-3) eu (benzonitrile), and DeltaH(+ +) = 4.7(+/-1.4) kcal/mol DeltaS(+ +) = -18(+/-2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) (DeltaH = -5.9(+/-0.8) kcal/mol, DeltaS = -24(+/-3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (DeltaH(+ +) = 3.1(+/-0.8) kcal/mol, DeltaS(+ +) = -25(+/-3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) and [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.