Collision-activated cleavage of a peptide/antibiotic disulfide linkage: possible evidence for intramolecular disulfide bond rearrangement upon collisional activation

Ceftiofur is an important veterinary l-lactam antibiotic whose bioactive metabolite, desfuroylceftiofur, has a free thiol group. Desfuroylceftiofur (DFC) was reacted with two peptides, [Arg(8)]-vasopressin and reduced glutathione, both of which have cysteine residues to form disulfide-linked peptide/antibiotic complexes. The products of the reaction, [vasopressin + (DFC-H) + (DFC-H) + H](+), [(vasopressin+H) + (DFC-H) + H](+) and [(glutathione-H) + (DFC-H) + H](+), were analyzed using collision-activated dissociation (CAD) with a quadrupole ion trap tandem mass spectrometer. MS/MS of [vasopressin + (DFC-H) + (DFC-H) + H](+) resulted in facile dissociative loss of one and two covalently bound DFC moieties. Loss of one DFC resulted from either homolytic or heterolytic dissociation of the peptide/antibiotic disulfide bond with equal or unequal partitioning of the two sulfur atoms between the fragment ion and neutral loss. Hydrogen migration preceded heterolytic dissociation. Loss of two DFC moieties from [vasopressin + (DFC-H) + (DFC-H) + H](+) appears to result from collision-activated intramolecular disulfide bond rearrangement (IDBR) to produce cyclic [vasopressin + H](+) (at m/z 1084) as well as other cyclic fragment ions at m/z 1084+/-32 and +64. The cyclic structure of these ions could only be inferred as MS/MS may result in rearrangement to non-cyclic structures prior to dissociative loss. IDBR was also detected from MS3 experiments of [vasopressin + (DFC-H) + (DFC-H) + H](+) fragment ions. MS/MS of [(glutathione-H) + (DFC-H) + H](+) resulted in cleavage of the peptide backbone with retention of the DFC moiety as well as heterolytic cleavage of the peptide/antibiotic disulfide bond to produce the fragment ion: [(DFC-2H) + H](+). These results demonstrate the facile dissociative loss by CAD of DFC moieties covalently attached to peptides through disulfide bonds. Published in 2004 by John Wiley Sons, Ltd.