Shaffer Christopher J, Marek Aleš, Nguyen Huong T H, Tureček František
Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA.
J Am Soc Mass Spectrom. 2015 Aug;26(8):1367-81. doi: 10.1007/s13361-015-1139-5. Epub 2015 Apr 23.
Electron transfer dissociation of peptide ions with the diazirine-containing residue photomethionine (M*) results in side-chain dissociations by loss of C3H7N2 radicals in addition to standard backbone cleavages. The side-chain dissociations are particularly prominent upon activation of long-lived, charge-reduced, cation radicals (GMGGR + 2H)(+•). Investigation of these cation radicals by near-UV photodissociation and collisional activation revealed different fragmentation products and mechanisms resulting from these ion activation modes. The dissociations observed for photomethionine were dramatically different from those previously reported for the lower homologue photoleucine; here, a difference by a single methylene group in the side chain had a large effect on the chemistries of the cation radicals upon ETD and further activation. ETD intermediates and products were probed by tandem 355-nm UV photodissociation-collision induced dissociation and found to contain chromophores that resulted from electron attachment to the diazirine ring. The nature of the newly formed chromophores and ion energetics and kinetics were investigated by electron structure calculations combining ab initio and density functional theory methods and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The dramatic difference between the dissociations of L and M* containing peptide cation radicals is explained by electronic effects that play a role in stabilizing critical reaction intermediates and steer the dissociations into kinetically favored reaction channels. In addition, a new alternating UVPD-ETD-UVPD MS(4) experiment is introduced and utilized for ion structure elucidation.
含重氮丙啶残基光甲硫氨酸（M*）的肽离子的电子转移解离，除了标准的主链裂解外，还会因C3H7N2自由基的损失导致侧链解离。在长寿命、电荷减少的阳离子自由基（GMGGR + 2H）（+•）活化时，侧链解离尤为显著。通过近紫外光解离和碰撞活化对这些阳离子自由基进行研究，揭示了由这些离子活化模式产生的不同碎片产物和机制。观察到的光甲硫氨酸的解离与先前报道的较低同系物光亮氨酸的解离有显著不同；在此，侧链中单个亚甲基的差异对ETD及进一步活化时阳离子自由基的化学性质有很大影响。通过串联355 nm紫外光解离-碰撞诱导解离对ETD中间体和产物进行探测，发现其含有因电子附着到重氮丙啶环而产生的发色团。通过结合从头算和密度泛函理论方法以及莱斯-拉姆齐-卡斯尔-马库斯（RRKM）理论的电子结构计算，研究了新形成发色团的性质以及离子的能量学和动力学。含L和M*的肽阳离子自由基解离之间的显著差异，是由电子效应解释的，这些电子效应在稳定关键反应中间体以及将解离导向动力学有利的反应通道中发挥作用。此外，还引入了一种新的交替紫外光解离-ETD-紫外光解离MS(4)实验，并将其用于离子结构解析。
