Effect of Cysteic Acid Position on the Negative Ion Fragmentation of Proteolytic Derived Peptides

A study on the effect of cysteic acid position on the types of fragment ions formed by collision-induced dissociation (CID) of M – H]⁻ ions is presented. Of particular note is the observation of d-type fragment ions for peptides that contain an N-terminal cysteic acid (fixed negative charge) and cleavable amino acid side chains possessing a β-γ carbon–carbon bond. For example, the CID mass spectrum of oxidized cys-kemptide (C_oxLRRASLG) M – H + O₃]⁻ ions contains abundant series of d-type fragment ions, and similar results are observed for oxidized cysteine-containing ribonuclease A proteolytic peptides. The d _i fragment ions are assumed to arise by a charge-remote and/or charge-assisted fragmentation mechanism, which both occur at high collision energies and involve consecutive reactions (i.e., the formation of a _i ions followed by the elimination of the side chain to form d _i ions).