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

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Abstract
Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH2CH(R1) CONHCH(R2)CO2− → NH2−C(R1)CONHCH(R2)CO2H → −NHCH(R2)CO2H + NH2C(R1)CO, when R1 and R2 = 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 CO2 and also forms HO2CCHCHCO2−˙. 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 H2O and CO2. 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.

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