Abstract The gas phase dissociation of B-subunits of Shiga toxin 2 (Stx2) subtypes expressed by pathogenic Escherichia coli have been analyzed using antibiotic induction, MALDI-TOF-TOF tandem mass spectrometry and post-source decay (PSD). Two Stx2 subtypes (a and c) are distinguished by two amino acid substitutions involving aspartic acid (D), asparagine (N) and alanine (A) residues. In addition to polypeptide backbone cleavage (PBC) on the C-terminal side of D and glutamic acid (E) residues by the aspartic acid effect (AAE) fragmentation mechanism, PBC is also observed on the C-terminal side of N-residues. It is estimated that N-residues are at least an order-of-magnitude less efficient than D-residues at PBC. As the side-chains of D and N residues are of equal length, the difference in PBC efficiency is likely due to differences in the gas phase acidities (ΔΔHacid ≈ 0.4 eV) of their respective side-chain functional groups: [−(C=O)OH] for D versus [−(C=O)NH2] for N. It is proposed that the AAE mechanism for N-residues results in the formation of a b-type fragment with a 2,5-pyrrolidinedione (or succinimide) end-group at its C-terminus similar to previous proposals that predict formation of a b-type fragment with a 2,5-furanedione (or succinic anhydride) end-group for D-residues. As a D ↔ N substitution differs in mass by only 1 Da, the b-fragment would also differ by 1 Da (if there are no other residue substitutions) and the complementary y-type fragment would have exactly the same mass. In consequence, the relative abundance of fragment ions caused by a D versus N residue might be a better indicator as to which residue was responsible for PBC rather than relying solely upon fragment ion m/z for this determination.

中文翻译：

---

通过 MALDI-TOF-TOF-MS/MS 和源后衰变分析亚稳态蛋白质离子天冬酰胺残基 C 端的多肽骨架裂解

摘要 使用抗生素诱导、MALDI-TOF-TOF 串联质谱和源后衰减 (PSD) 分析了致病性大肠杆菌表达的志贺毒素 2 (Stx2) 亚型 B 亚基的气相解离。两个 Stx2 亚型（a 和 c）的区别在于涉及天冬氨酸 (D)、天冬酰胺 (N) 和丙氨酸 (A) 残基的两个氨基酸取代。除了通过天冬氨酸效应 (AAE) 断裂机制在 D 和谷氨酸 (E) 残基的 C 端侧发生多肽骨架裂解 (PBC) 外，还在 N 残基的 C 端侧观察到 PBC。据估计，在 PBC 中，N 残基的效率至少比 D 残基低一个数量级。由于 D 和 N 残基的侧链长度相等，PBC 效率的差异可能是由于其各自侧链官能团的气相酸度 (ΔΔHacid ≈ 0.4 eV) 的差异所致：D 的 [-(C=O)OH] 与 [-(C=O)NH2] ] 对于 N。建议 N 残基的 AAE 机制导致 b 型片段的形成，在其 C 端具有 2,5-吡咯烷二酮（或琥珀酰亚胺）端基，类似于先前预测的提议形成带有 2,5-呋喃二酮（或琥珀酸酐）端基的 b 型片段，用于 D 残基。由于 D ↔ N 置换的质量仅相差 1 Da，因此 b 片段也将相差 1 Da（如果没有其他残基置换），并且互补的 y 型片段将具有完全相同的质量。结果，