National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan.
J Mass Spectrom. 2024 Dec;59(12):e5100. doi: 10.1002/jms.5100.
To discriminate amino acid isomers by multiple stage tandem mass spectrometry (MS), the fragmentation of protonated amino acids were investigated by MS with collision-induced dissociation (CID) and density functional theory calculations. The CID of protonated α-amino acids results in a loss of 46 Da, corresponding to HO and CO, and iminium ions appear as resultant fragments. The CID of protonated β-amino acids also produces iminium ions, but the corresponding loss is 60 Da instead of 46 Da. HO loss initiates the fragmentation of protonated β-amino acids, producing protonated β-lactams as an intermediate. Subsequently, protonated β-lactams are easily converted to iminium ions and CHCO. By contrast, HO loss from the protonated forms of γ- and ε-amino acids provides protonated lactams with 5- and 7-membered rings, respectively. Protonated lactams with more than 5-membered rings provide stable fragments and do not undergo further degradation during CID. In addition, protonated forms of γ- and ε-amino acids undergo NH loss as a competitive fragmentation pathway of HO loss, producing protonated lactones. Because the fragmentation of protonated amino acid by CID depends on the position of amino and carboxyl groups, the tandem mass spectrometry with CID can discriminate α-, β-, and other amino acids.
为了通过多级串联质谱（MS）区分氨基酸异构体，通过 MS 与碰撞诱导解离（CID）和密度泛函理论计算研究了质子化氨基酸的碎裂。质子化α-氨基酸的 CID 导致 46 Da 的损失，对应于 HO 和 CO，并且亚氨基离子作为结果碎片出现。质子化β-氨基酸的 CID 也产生亚氨基离子，但相应的损失为 60 Da 而不是 46 Da。HO 的损失引发质子化β-氨基酸的碎裂，产生质子化β-内酰胺作为中间体。随后，质子化β-内酰胺容易转化为亚氨基离子和 CHCO。相比之下，γ-和ε-氨基酸的质子化形式中 HO 的损失分别提供了具有 5 元和 7 元环的质子化内酰胺。具有超过 5 元环的质子化内酰胺提供稳定的片段，并且在 CID 期间不会进一步降解。此外，γ-和ε-氨基酸的质子化形式作为 HO 损失的竞争碎裂途径经历 NH 损失，产生质子化内酯。由于 CID 对质子化氨基酸的碎裂取决于氨基和羧基的位置，因此 CID 的串联质谱可以区分α-、β-和其他氨基酸。
