Differentiation of α- or β-aspartic isomers in the heptapeptides by the fragments of [M + Na]+ using ion trap tandem mass spectrometry.

Electrospray ionization coupled with low energy collision induced dissociation (CID) in an ion trap mass spectrometer was used to examine the fragmentation patterns of the [M + Na](+) of eight pairs of heptapeptides containing α- or β-Asp residues in second and sixth amino acid positions, respectively. Selective cleavages at the peptide backbone C-terminal to two Asp residues were observed, which generated a series of C-terminal y(5) ions and N-terminal b(6) ions. Two typical ions: [y5 + Na - H]+ and [b6 + Na + OH]+, produced by α-Asp containing peptides were noted to be much more abundant than those of the peptides with β-Asp, which could be used for distinction of the isomers in Asp2 and Asp6, respectively.