In-Source Decay Characterization of Isoaspartate and β-Peptides

Deamidation and the subsequent formation of isoaspartic acid (isoAsp) are common modifications of asparagine (Asn) residues in proteins. Differentiation of isoAsp and Asp residues is a challenging task owing to their similar chemical properties and identical molecular mass. Recent studies showed that they can be differentiated using electron capture dissociation (ECD) which generates diagnostic fragments c'+57 and z^•-57 specific to the isoAsp residue. However, the ECD approach is only applicable towards multiply charged precursor ions and generally does not work for β-amino acids other than isoAsp. In this study, the potential of in-source decay (ISD) in characterization of isoAsp and other β-amino acids was explored. For isoAsp-containing peptides, ISD with a conventional hydrogen-donating matrix produced ECD-like, c'+57 and z^•-57 diagnostic ions, even for singly charged precursor ions. For other β-amino acids, a hydrogen-accepting matrix was used to induce formation of site-specific a-14 ions from a synthetic β-analogue of substance P. These results indicated that ISD can be broadly applied for β-peptide characterization.