Fragmentation reactions of deprotonated peptides containing aspartic acid

Abstract

The fragmentation reactions of deprotonated peptides containing aspartic acid have been elucidated using MS 2 and MS 3 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 [M D] − 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 y 1 ion and loss of H 2O from [M H] − followed by further fragmentation, thus precluding the distinction of the isomeric species by negative ion tandem mass spectrometry. Dipeptides of sequence H Xxx Asp OH 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 N C bond N-terminal to the aspartic acid residue to form a c ion (deprotonated amino acid amide (c 1) or peptide amide (c n )) 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 [M H] − ion while when the aspartic acid is not in the C-terminal position the fragmentation reaction occurs mainly from the [M H H 2O] − ion. The products of this N C bond cleavage reaction serve to identify the position of the aspartic acid residue in the peptide.