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 + O ₃] ^- ions contains abundant series of d-type fragment ions, and similar results are observed for oxidized cysteinecontaining ribonuclease A proteolytic peptides. The di 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 ai ions followed by the elimination of the side chain to form di ions).