Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL 60115, USA.
J Am Soc Mass Spectrom. 2013 Apr;24(4):513-23. doi: 10.1007/s13361-013-0594-0. Epub 2013 Mar 20.
In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations-a distonic indolyl N-radical (H3N(+) - TrpN(•)) and a canonical aromatic π (Trp(•+)) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary Cu(II)(terpy)(Trp) complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm(-1) region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.
在这项工作中，我们特异地生成并比较了两种色氨酸自由基阳离子的异构体在气相中的性质——一个是离域吲哚 N-自由基（H3N(+) - TrpN(•)），另一个是经典的芳香族π（Trp(•+)）自由基阳离子。离域自由基阳离子是通过在溶液中硝酰化色氨酸的吲哚氮，然后对得到的质子化 N-亚硝基色氨酸进行碰撞诱导解离（CID）生成的。π-自由基阳离子是通过三元Cu(II)(terpy)(Trp)配合物的 CID 产生的。发现这两种异构体的 CID 谱非常不同，表明它们之间没有互变异构。在气相离子-分子反应中，离域氮自由基阳离子与正丙硫醚不反应，而π自由基阳离子则通过氢原子提取反应。DFT 计算表明，离域吲哚基自由基阳离子的能量比色氨酸的π自由基阳离子高约 82 kJ/mol。离域氮自由基阳离子的低反应性可以通过自由基在芳香环上的自旋离域和远程、局域电荷（在氨基氮上）来解释。在捕获和 CID 条件下，异构体之间没有互变异构，这可以用大约 137 kJ/mol 的高重排势垒来解释。最后，通过在~1000-1800 cm(-1) 区域的红外多光子解离（IRMPD）光谱对两种异构体进行了表征。发现两个物种之间的一些主要实验红外特征重叠，使得通过该区域的 IRMPD 光谱区分它们变得困难。此外，DFT 理论计算表明，IR 光谱强烈依赖于构象。
