Novel rearranged ions observed for protonated peptides via metastable decomposition in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

C-terminal rearrangement ions [b(n-1) + H2O] (where n refers to the total n umber of residues of peptides) are frequently observed for peptides which c ontain basic amino acid(s), especially arginine, at or near their N termini in low- and high-energy collision-induced dissociation or post-source deca y (PSD) spectra. Here we report a novel rearrangement, associated with PSD for serine- or threonine-containing peptides that are susceptible to C-term inal rearrangement. Based on PSD analyses of serine- or threonine-containin g bradykinin and its analogs, which have been ethyl-esterified or O-18 labe led at their C termini, the [b(k) + H2O] (where k denotes the position adja cent to the left of the Ser/Thr residue) ion is generally thought to be for med by the transfer of the hydroxyl moiety of a serine or threonine residue to the carbonyl group of the residue to its left accompanied by the loss o f the remaining C-terminal portion of the peptide. When the Ser/Thr is at o r near the C terminus, the present [b(k) + H2O] ion could be formed via two pathways, i.e., the Ser/Thr-related rearrangement and the conventional C-t erminal rearrangement, which has been clearly verified by O-18 labeling at the C terminus. In addition, the ions which are formally designated as [y(m )b(l) + H2O], where y(m)b(l) denotes a b-type internal ion, are also briefl y described. (J Am Soc Mass Spectrom 2000, 11, 345-351) (C) 2000 American S ociety for Mass Spectrometry.