Department of Chemistry, M.V. Lomonosov Moscow State University, Moscow, Russian Federation.
J Am Soc Mass Spectrom. 2013 Jul;24(7):1037-44. doi: 10.1007/s13361-013-0632-y. Epub 2013 Apr 30.
Collision-induced dissociation (CID) spectra of long non-tryptic peptides are usually quite complicated and rather difficult to interpret. Disulfide bond formed by two cysteine residues at C-terminus of frog skin peptides precludes one to determine sequence inside the forming loop. Thereby, chemical modification of S-S bonds is often used in "bottom up" sequencing approach. However, low-energy CID spectra of natural non-tryptic peptides with C-terminal disulfide cycle demonstrate an unusual fragmentation route, which may be used to elucidate the "hidden" C-terminal sequence. Low charge state protonated molecules experience peptide bond cleavage at the N-terminus of C-terminal cysteine. The forming isomeric acyclic ions serve as precursors for a series of b-type ions revealing sequence inside former disulfide cycle. The reaction is preferable for peptides with basic lysine residues inside the cycle. It may also be activated by acidic protons of Asp and Glu residues neighboring the loop. The observed cleavages may be quite competitive, revealing the sequence inside disulfide cycle, although S-S bond rupture does not occur in this case.
长非胰蛋白酶肽的碰撞诱导解离（CID）谱通常非常复杂，难以解释。蛙皮肽 C 末端两个半胱氨酸残基形成的二硫键阻止了人们确定形成环内的序列。因此，在“自上而下”的测序方法中，经常使用 S-S 键的化学修饰。然而，具有 C 末端二硫键环的天然非胰蛋白酶肽的低能量 CID 谱表现出一种不寻常的碎裂途径，可用于阐明“隐藏”的 C 末端序列。低电荷状态质子化分子在 C 末端半胱氨酸的 N 末端经历肽键断裂。形成的非环异构离子作为前二硫键环内一系列 b 型离子的前体，揭示了序列。该反应有利于环内具有碱性赖氨酸残基的肽。该反应也可以被邻近环的天冬氨酸和谷氨酸残基的酸性质子激活。尽管在此情况下不会发生 S-S 键断裂，但观察到的裂解可能具有很强的竞争性，揭示了二硫键环内的序列。
