The fragmentation reactions of the protonated a-monomethyl and b-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 from protonated H-Asp-OMe, there is a methoxy group migration which is followed by loss of H 2 O CO from the a-carboxyl position. This migration also is observed for the ion derived from
The fragmentation reactions of a variety of protonated a-and b-dipeptides, H-Asp-Xxx-OR and H-Asp(Xxx)-OH, also have been elucidated. For both types of linkage facile formation of the ion is observed, y 1 / 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 b-linkage is elimination of H 2 O + CO from the a-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, Protonated dipeptides with the H 2 N'xCHwCHxCxO. a-linkage show loss of from the aspartic acid b-carboxyl group. This water loss is followed by elimination of H 2 O ketene or ammonia. These fragment ions, which are most abundant at low collision energies, serve to identify dipeptides with the a-linkage.