The Negative Ion Mass Spectra of [M-H]− Ions Derived From Caeridin and Dynastin Peptides. Internal Backbone Cleavages Directed Through Asp and Asn Residues

The collision-induced spectra of [M–H]− ions of peptides containing Asp and Asn residues exhibit characteristic backbone cleavage ions produced via the enolate anions of the Asp or Asn side chains. Such reactions do not occur from the analogous Glu of Gln residues. Asp and Asn residues may be distinguished because the backbone cleavage ion formed from Asp undergoes pronounced loss of water: the corresponding loss of ammonia from the Asn backbone cleavage ion is less pronounced. The presence of Asp or Asn in a peptide usually results in the normal α and β backbone cleavage ions being minor in comparison to backbone cleavage ions formed via the enolate anions of Asp or Asn.