Collision-Induced dissociations of deprotonated peptides. Dipeptides containing aspartic or glutamic acids

✍ Scribed by Russell J. Waugh; John H. Bowie; Roger N. Hayes

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 473 KB
• Volume: 26
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210260413

✦ Synopsis

Abstract

Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH~2~CH(R^1^) CONHCH(R^2^)CO~2~^−^ → NH~2~^−^C(R^1^)CONHCH(R^2^)CO~2~H → ^−^NHCH(R^2^)CO~2~H + NH~2~C(R^1^)CO, when R^1^ and R^2^ = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α‐side‐chain of Asp or Glu. For example, [Asp‐Leu ‐ H]^−^ and [Leu‐Asp ‐ H]^−^ both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu‐Asp ‐ H]^−^ loses CO~2~ and also forms HO~2~CCHCHCO~2~^−^˙. These fragmentations establish that Asp is the C‐terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu‐Ala ‐ H]^−^ and [Ala‐Glu ‐ H]^−^ undergo similar fragmentation, both competitively losing H~2~O and CO~2~. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium‐labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems.