PREVIOUSLY UNCHARACTERIZED IONS OBSERVED IN THE HIGH-ENERGY COLLISION-INDUCED DISSOCIATION MASS-SPECTRA OF PEPTIDES CONTAINING S-ALKYL CYSTEINE

The high-energy collision-induced dissociation (CID) spectra of a numb er of peptides containing S-beta-(4-pyridylethyl)-, S-carbamidomethyl- and S-carboxymethylcysteine have been generated and several hitherto undescribed fragmentation pathways identified. For peptides containing pyridylethylcysteine at the nth position, these include x(n) - 105, a (n+1) + 1 - 106, a(n+1) + 1 - 138, z(n+1) + 1 - 106, z(n+1) + 1 - 138, and combined side chain loss ions. The analogous fragment ions were a lso observed in the spectra of peptides containing other S-alkylcystei nes. Also, two fragmentations unique to pyridylethylcysteine-containin g peptides, resulting in the formation of b(n+greater than or equal to 1) - R-C + H and CO + peC ions, were studied. Precursor scanning, exa ct mass measurement and CID of the appropriate source-generated fragme nt ions have been employed to identify the origin of these ions and to facilitate the proposal of mechanisms for their formation. (C) 1997 E lsevier Science B.V.