Fragmentation reactions of deprotonated peptides containing aspartic acid

The fragmentation reactions of deprotonated peptides containing aspartic acid have been elucidated using MS2 and MS3 experiments and accurate mass measurements where necessary. The disposition of labile (N and O bonded) hydrogens in the fragmentation products has been studied by exchanging the labile hydrogens for deuterium whereby the [MD]− ion is formed on electrospray ionization. α-Aspartyl and β-aspartyl dipeptides give very similar fragment ion spectra on collisional activation, involving for both species primarily formation of the y1 ion and loss of H2O from [MH]− followed by further fragmentation, thus precluding the distinction of the isomeric species by negative ion tandem mass spectrometry. Dipeptides of sequence HXxxAspOH give characteristic spectra different from the α- and β-isomers. For larger peptides containing aspartic acid a common fragmentation reaction involves nominal cleavage of the NC bond N-terminal to the aspartic acid residue to form a c ion (deprotonated amino acid amide (c1) or peptide amide (cn)) and the complimentary product involving elimination of a neutral amino acid amide or peptide amide. When aspartic acid is in the C-terminal position this fragmentation reaction occurs from the [MH]− ion while when the aspartic acid is not in the C-terminal position the fragmentation reaction occurs mainly from the [MHH2O]− ion. The products of this NC bond cleavage reaction serve to identify the position of the aspartic acid residue in the peptide.