School of Chemistry, The University of Melbourne, Victoria 3010, Australia.
Rapid Commun Mass Spectrom. 2011 Jan 30;25(2):251-61. doi: 10.1002/rcm.4830.
The fragmentation reactions of the radical cations, M(·+), of histidine, 2-oxo-histidine and 2-thioxo-histidine were examined using a combination of experiments performed on a linear ion trap and density functional theory (DFT) calculations at the UB3-LYP/6-311++G(d,p) level of theory. Low-energy collision-induced dissociation (CID) on Cu(II)(terpy)(M) complexes, formed via electrospray ionisation, produced the radical cations in sufficient yield to examine their unimolecular chemistry via an additional stage of CID. The CID spectrum of the radical cation of histidine is dominated by loss of water with the next most abundant ion arising from the combined loss of H(2)O and CO. In contrast, the CID spectra of the radical cations of 2-oxo-histidine and 2-thioxo-histidine are dominated by the combined loss of CO(2) and NH=CH(2). The observed differences are rationalised via DFT calculations which reveal that the barrier associated with loss of CO(2) from the histidine radical cation is higher than that for loss of H(2)O. In contrast, the introduction of an oxygen or sulfur atom into the side chain of histidine results in a reversal of the order of these barrier heights, thus making CO(2) loss the preferred pathway.
使用线性离子阱和密度泛函理论（DFT）计算相结合的方法，在 UB3-LYP/6-311++G(d,p)理论水平上，研究了组氨酸、2-氧代组氨酸和 2-硫代组氨酸自由基阳离子的碎裂反应。通过电喷雾电离形成的 [Cu(II)(terpy)(M)]（2+）配合物的低能碰撞诱导解离（CID）以足够的产率产生自由基阳离子，通过 CID 的附加阶段来研究它们的单一分子化学。组氨酸自由基阳离子的 CID 谱主要由失去水主导，其次丰富的离子来自 H（2）O 和 CO 的联合损失。相比之下，2-氧代组氨酸和 2-硫代组氨酸自由基阳离子的 CID 谱主要由 CO（2）和 NH=CH（2）的联合损失主导。通过 DFT 计算可以合理地解释观察到的差异，该计算表明，从组氨酸自由基阳离子中失去 CO（2）的相关势垒高于失去 H（2）O 的势垒。相反，在组氨酸侧链中引入一个氧或硫原子会导致这些势垒高度的顺序反转，从而使 CO（2）损失成为首选途径。
