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

[a ₃ + H] ²⁺ ions generated from Ln ³⁺ /tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [a _n ] ⁺ ions but with protonation at both the terminal NH ₂ and N═CH ₂ 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 ₃ + H] ²⁺ ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b ₂ ] ⁺ or [a ₂ ] ⁺ ions along with internal [a ₁ ] ⁺ ions. For [a ₃ + H] ²⁺ ions containing a tryptophan residue in the central location, in addition to cleavage of the amide bond, losses of neutrals NH ₃ , HN═CHR, (NH ₃ + CO), and HNCO were observed. Dissociations of some unsolvated Ln ³⁺ /tripeptide complexes gave [b ₃ + H] ²⁺ ions in low abundance; formation of these [b ₃ + 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 [b ₃ + H] ²⁺ ions was by the loss of (H ₂ O + CO) and not only CO, indicating that these ions did not have the same type of oxazolone structure as found for [b _n ] ⁺ ions. Density functional theory calculations suggest that the observed [b ₃ + 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 [b ₃ + H] ²⁺ ion would be sufficient to result in further dissociation. Two isomers of the [b ₃ + H] ²⁺ ion of ProHisGly have been created, one from the [Ce(ProHisGly)] ³⁺ complex that characteristically dissociates by the combined loss of (H ₂ O + CO) and the other by the loss of glycine from [ProHisGlyGly + 2H] ²⁺ . The [b ₃ + H] ²⁺ ion derived from [ProHisGlyGly + 2H] ²⁺ dissociated by the loss of only CO.