Lam K H Brian, Lau Justin Kai-Chi, Lai Cheuk-Kuen, Hopkinson Alan C, Siu K W Michael
Department of Chemistry and Centre for Research in Mass Spectrometry, York University , 4700 Keele Street, Toronto, ON M3J 1P3, Canada.
Department of Chemistry and Biochemistry, University of Windsor , 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada.
J Phys Chem B. 2017 Oct 19;121(41):9541-9547. doi: 10.1021/acs.jpcb.7b07586. Epub 2017 Oct 6.
Collision-induced dissociations of isotopically labeled protonated tetraglycines establish that the [b] ion formed by loss of water from the second amide bond (structure II) rearranges to form N-protonated 3,5-dihydro-4H-imidazol-4-one (structure I), the product of water loss from the first amide bond. Structure II is slightly higher in energy than I (ΔH at 0 K is 5.1 kJ mol, as calculated at M06-2X/6-311++G-(d,p)), and the barrier to interconversion is 139.8 kJ mol above I. The dominant dissociation pathway is the loss of methanimine (HN=CH) from ion I with a barrier of 167.1 kJ mol, giving [GlyGlyGlyGly + H - HO - HN=CH], ion III; a minor channel, loss of NH, has a slightly higher barrier (181.5 kJ mol). Using labeled glycine (C) it was determined that loss of the imine is from the same residue as that from which water was initially lost. The collision-induced dissociation spectra of ion III derived from both I and II were identical, and their energy-resolved curves were also very similar. Ion III fragments by losses of a glycine molecule (the dominant channel), a water molecule, and a glycine residue (57 Da), giving ions IV, V, and VII, respectively. Isotopic labeling established the origins of each of the neutral molecules that are lost. Using glycine (2,2 D), rapid deuterium exchange was observed for both ions I and II for the α-hydrogens that are from the same residue as that from which the water had been eliminated.
同位素标记的质子化四甘氨酸的碰撞诱导解离表明，由第二个酰胺键失去水形成的[b]离子（结构II）重排形成N-质子化的3,5-二氢-4H-咪唑-4-酮（结构I），即第一个酰胺键失去水的产物。结构II的能量略高于I（在0 K时ΔH为5.1 kJ mol，在M06-2X/6-311++G-(d,p)水平计算），相互转化的能垒比I高139.8 kJ mol。主要的解离途径是离子I失去甲亚胺（HN=CH），能垒为167.1 kJ mol，生成[甘氨酰甘氨酰甘氨酰甘氨酸 + H - HO - HN=CH]，离子III；一个次要通道，失去NH，能垒略高（181.5 kJ mol）。使用标记的甘氨酸（C）确定，亚胺的失去与最初失去水的残基相同。由I和II衍生的离子III的碰撞诱导解离光谱相同，它们的能量分辨曲线也非常相似。离子III通过失去一个甘氨酸分子（主要通道）、一个水分子和一个甘氨酸残基（57 Da）进行碎片化，分别生成离子IV、V和VII。同位素标记确定了每个失去的中性分子的来源。使用甘氨酸（2,2-D），观察到离子I和II中与已消除水的残基相同的α-氢的快速氘交换。
