Doubly Charged Small Organic Fragments Derived from [Ce(tripeptide)(CHCN)] Complexes: Observation of the Elusive [b + H] Ions.

[a + H] ions generated from Ln/tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [a] ions but with protonation at both the terminal NH and N═CH 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 [a + H] ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b] or [a] ions along with internal [a] ions. For [a + H] ions containing a tryptophan residue in the central location, in addition to cleavage of the amide bond, losses of neutrals NH, HN═CHR, (NH + CO), and HNCO were observed. Dissociations of some unsolvated Ln/tripeptide complexes gave [b + H] ions in low abundance; formation of these [b + H] 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 [b + H] ions was by the loss of (HO + CO) and not only CO, indicating that these ions did not have the same type of oxazolone structure as found for [b] ions. Density functional theory calculations suggest that the observed [b + H] ions of ProGlyGly were formed from [Ce(ProGlyGly)] 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 [b + H] ion would be sufficient to result in further dissociation. Two isomers of the [b + H] ion of ProHisGly have been created, one from the [Ce(ProHisGly)] complex that characteristically dissociates by the combined loss of (HO + CO) and the other by the loss of glycine from [ProHisGlyGly + 2H]. The [b + H] ion derived from [ProHisGlyGly + 2H] dissociated by the loss of only CO.