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/MSn) using an external electrospray ionization (EST) 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.