Ion chemistry of protonated aspartic acid derivatives

The fragmentation reactions of the protonated α-monomethyl and β-monomethyl esters of aspartic acid have been elucidated using low-energy collision-induced dissociation (CID) and metastable ion studies. It is shown that, following loss of H2O from protonated H–Asp–OMe, there is a methoxy group migration which is followed by loss of CO from the α-carboxyl position. This migration also is observed for the [MH+-NH3]+ ion derived from H–Asn–OMe. The fragmentation reactions of a variety of protonated α- and β-dipeptides, H–Asp–Xxx–OR and H–Asp(Xxx)–OH, also have been elucidated. For both types of linkage facile formation of the y1″ ion is observed, in line with earlier observations that protonated peptides undergo preferential cleavage at the C-terminus side of acidic residues. A characteristic reaction of protonated dipeptides with the β-linkage is elimination of H2O+CO from the α-carboxyl group of the aspartic acid residue. This is followed by elimination of the C-terminus amino acid to give a characteristic fragment ion at m/z 70, H2N+=CH−CH=C=O. Protonated dipeptides with the α-linkage show loss of H2O from the aspartic acid β-carboxyl group. This water loss is followed by elimination of ketene or ammonia. These fragment ions, which are most abundant at low collision energies, serve to identify dipeptides with the α-linkage.