Reactivity and selectivity of charged phenyl radicals toward amino acids in a Fourier transform ion cyclotron resonance mass spectrometer.
Reactivity and selectivity of charged phenyl radicals toward amino acids in a Fourier transform ion cyclotron resonance mass spectrometer.
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在傅里叶变换离子回旋共振质谱仪中带电苯基对氨基酸的反应性和选择性。
DOI：
10.1021/ja111280t
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发表时间：
2011
影响因子：
15
通讯作者：
Kenttämaa,HilkkaI
中科院分区：
文献类型：
--
作者：
Pates,GeorgeO;Guler,Leonard;Nash,JohnJ;Kenttämaa,HilkkaI
在双池傅里叶变换离子回旋共振质谱仪中，在气相中检查了 10 个带电苯基对几种氨基酸的反应性。正如预期的那样，所有自由基都会从氨基酸中提取一个氢原子。最亲电的自由基（在自由基位点具有最大的计算垂直电子亲和力 (EA)）也通过 NH2 抽象（非自由基亲核加成-消除反应）与这些氨基酸发生反应。自由基（氢原子抽象）和非自由基（NH2 抽象）反应效率都随着自由基的亲电性（EA）而增加。然而，NH2 提取受 EA 的影响更为强烈。与早期的报告相反，氨基酸的电离能似乎没有起到一般反应性控制作用。使用几种部分氘标记的氨基酸进行的研究表明，只有甘氨酸优选从 α-碳中提取氢原子；对于其他氨基酸，氢原子优先从侧链上被夺走。自由基的亲电性似乎对氢原子被夺取的位点没有重大影响。因此，氢原子夺取的区域选择性似乎与自由基的结构无关，但取决于氨基酸的结构。令人惊讶的是，观察到N-(3-硝基-5-脱氢苯基)吡啶鎓自由基被夺去两个氢原子，这表明自由基上的取代基不仅影响自由基的EA，而且还可以参与反应。与之前的报告不同的是，脯氨酸被发现显示出几种前所未有的反应途径，这些途径可能不是通过激进机制进行的，而是通过亲核加成-消除机制进行的。 NH2 和 15NH2 基团都是从侧链上用 15N 标记的赖氨酸中提取出来的，表明 NH2 提取既发生在氨基末端，也发生在侧链上。采用量子化学计算来深入了解一些反应机制。
The reactivity of 10 charged phenyl radicals toward several amino acids was examined in the gas phase in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. All radicals abstract a hydrogen atom from the amino acids, as expected. The most electrophilic radicals (with the greatest calculated vertical electron affinities (EA) at the radical site) also react with these amino acids via NH2abstraction (a nonradical nucleophilic addition–elimination reaction). Both the radical (hydrogen atom abstraction) and nonradical (NH2abstraction) reaction efficiencies were found to increase with the electrophilicity (EA) of the radical. However, NH2abstraction is more strongly influenced by EA. In contrast to an earlier report, the ionization energies of the amino acids do not appear to play a general reactivity-controlling role. Studies using several partially deuterium-labeled amino acids revealed that abstraction of a hydrogen atom from the α-carbon is only preferred for glycine; for the other amino acids, a hydrogen atom is preferentially abstracted from the side chain. The electrophilicity of the radicals does not appear to have a major influence on the site from which the hydrogen atom is abstracted. Hence, the regioselectivity of hydrogen atom abstraction appears to be independent of the structure of the radical but dependent on the structure of the amino acid. Surprisingly, abstraction of two hydrogen atoms was observed for theN-(3-nitro-5-dehydrophenyl)pyridinium radical, indicating that substituents on the radical not only influence the EA of the radical but also can be involved in the reaction. In disagreement with an earlier report, proline was found to display several unprecedented reaction pathways that likely do not proceed via a radical mechanism but rather by a nucleophilic addition–elimination mechanism. Both NH2and15NH2groups were abstracted from lysine labeled with15N on the side chain, indicating that NH2abstraction occurs both from the amino terminus and from the side chain. Quantum chemical calculations were employed to obtain insights into some of the reaction mechanisms.