Doubly Charged Small Organic Fragments Derived from [Ce(tripeptide)(CH3CN)m]3+ Complexes: Observation of the Elusive [bn + H]2+ Ions.

[a3 + H]2+ ions generated from Ln3+/tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [an]+ ions but with protonation at both the terminal NH2 and N═CH2 groups. Ion stability is favored by having the basic secondary amine of the proline residue at the N-terminus and by an amino acid residue accommodating one of the protons on the side chain. Dissociation of [a3 + H]2+ ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b2]+ or [a2]+ ions along with internal [a1]+ ions. For [a3 + H]2+ ions containing a tryptophan residue in the central location, in addition to cleavage of the amide bond, losses of neutrals NH3, HN═CHR, (NH3 + CO), and HNCO were observed. Dissociations of some unsolvated Ln3+/tripeptide complexes gave [b3 + H]2+ ions in low abundance; formation of these [b3 + H]2+ ions was favored by the presence of a proline residue at the N-terminus and by either a histidine or tryptophan residue in the central position. Dissociation of these [b3 + H]2+ ions was by the loss of (H2O + CO) and not only CO, indicating that these ions did not have the same type of oxazolone structure as found for [bn]+ ions. Density functional theory calculations suggest that the observed [b3 + H]2+ ions of ProGlyGly were formed from [Ce(ProGlyGly)]3+ complexes in which the peptide was bound to the metal ion as an enolate. Dissociation of the slightly lower-energy complex, where the peptide is bound in the keto form, would produce an oxazolone but the high barrier required to create this isomer of the [b3 + H]2+ ion would be sufficient to result in further dissociation. Two isomers of the [b3 + H]2+ ion of ProHisGly have been created, one from the [Ce(ProHisGly)]3+ complex that characteristically dissociates by the combined loss of (H2O + CO) and the other by the loss of glycine from [ProHisGlyGly + 2H]2+. The [b3 + H]2+ ion derived from [ProHisGlyGly + 2H]2+ dissociated by the loss of only CO.