Peptide Toxin Identification by Liquid Chromatography/Mass Spectrometry Using an External Electrospray Ionization Source Combined with Ion Trap Mass Spectrometry

Three mutated synthetic peptides, analogs of the channel toxin BGK, were studied by liquid chromatography/mass spectrometry ⁿ (LC/MS ⁿ ) using an external electrospray ionization (ESI) source. The reduction of disulfide bridges, combined with alkylation of the free cysteine residues, is shown to be a useful tool to determine their number as well as their location in conjunction with proteolysis of the peptides. The modified toxins were identified by LC/MS, while LC/MS/MS experiments were conducted to determine and locate modifications. Low-energy collision-induced dissociation spectra of S-alkyl cysteine derivative display ions (MH − 91)⁺ attributed to the loss of HS–CH₂–CONH₂ as confirmed by sequential MS³ experiments. Identification and localization of cysteine residues can be achieved providing definitive identification for unknown proteins containing S-carbamidomethylcysteine.