Rearrangements of doubly charged acylium ions from lysyl and ornithyl peptides.

The study of large-scale rearrangements of [b']2+ ions produced by electrospray ionization of Substance P (Tang et al., Anal. Chem. Vol. 65, p. 2824 (1993)) has been extended to 18 other peptides containing either a lysine or ornithine residue remote from the C-terminus. Evidence for wholesale transfer of one or more residues, from the C-terminus of the [b']2+ precursor to the omega-amino group of the Lys (or Orn) residue, was observed for 12 of the 18 peptides studied. Unfortunately, no rigorous predictive rules, relating features of the peptide sequence to the propensity to undergo such rearrangements, could be discerned although a significant correlation with presence of a proline residue close to the lysine or ornithine on the C-terminal side was apparent. The resulting mass-shifts can complicate derivation of peptide sequences from fragment-ion spectra of [M + 2H]2+ peptide ions, for example, since the cyclized [b']2+ ions responsible for the rearrangements are readily formed as intermediate species in the fragmentation mechanisms.