Dipositively Charged Protonated a3and a2Ions: Generation by Fragmentation of [La(GGG)(CH3CN)2]3+

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 [a_n]⁺ 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 [b_n]⁺ 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.

Electron capture dissociation of trivalent metal ion-peptide complexes

With electrospray ionization from aqueous solutions, trivalent metal ions readily adduct to small peptides resulting in formation of predominantly (peptide + M(T) - H)(2+), where M(T) = La, Tm, Lu, Sm, Ho, Yb, Pm, Tb, or Eu, for peptides with molecular weights below ~1000 Da, and predominantly (peptide + M(T))(3+) for larger peptides. ECD of (peptide + M(T) - H)(2+) results in extensive fragmentation from which nearly complete sequence information can be obtained, even for peptides for which only singly protonated ions are formed in the absence of the metal ions. ECD of these doubly charged complexes containing M(T) results in significantly higher electron capture efficiency and sequence coverage than peptide-divalent metal ion complexes that have the same net charge. Formation of salt-bridge structures in which the metal ion coordinates to a carboxylate group are favored even for (peptide + M(T))(3+). ECD of these latter complexes for large peptides results in electron capture by the protonation site located remotely from the metal ion and predominantly c/z fragments for all metals, except Eu(3+), which undergoes a one electron reduction and only loss of small neutral molecules and b/y fragments are formed. These results indicate that solvation of the metal ion in these complexes is extensive, which results in the electrochemical properties of these metal ions being similar in both the peptide environment and in bulk water.

Infrared multiple-photon dissociation spectroscopy of tripositive ions: lanthanum-tryptophan complexes

Collision-induced charge disproportionation limits the stability of triply charged metal ion complexes and has thus far prevented successful acquisition of their gas-phase IR spectra. This has curtailed our understanding of the structures of triply charged metal complexes in the gas phase and in biological environments. Herein we report the first gas-phase IR spectra of triply charged La(III) complexes with a derivative of tryptophan (N-acetyl tryptophan methyl ester), and an unusual dissociation product, a lanthanum amidate. These spectra are compared with those predicted using density functional theory. The best structures are those of the lowest energies that differ by details in the π-interaction between La(3+) and the indole rings. Other binding sites on the tryptophan derivative are the carbonyl oxygens. In the lanthanum amidate, La(3+) replaces an H(+) in the amide bond of the tryptophan derivative.

Doubly charged protonated a ions derived from small peptides

Protonated a(2) and a(3) (therefore doubly charged) ions in which both charges lie on the peptide backbone are formed in collision-induced dissociations of [La(III)(peptide)(CH(3)CN)(m)](3+) complexes. Abundant (a(3)+H)(2+) ions are formed from triproline (PPP) and peptides with a proline residue at the N-terminus; these peptides are the most effective in producing ions of the type (a(2)+H)(2+) and (a(3)+H)(2+). A systematic study of the effect of the location of the proline residue and other residues of aliphatic amino acids on the generation of protonated a ions is reported. Density functional theory calculations at B3LYP/6-311++G(d,p) gave the proton affinity of the a(3) ion derived from PPP to be 167.6 kcal mol(-1), 2.6 kcal mol(-1) higher than that of water. The protonated a(2) ions of diglycine and diproline and a(3) ions of triglycine have lower proton affinities and are only observed in lower abundances, possibly due to proton transfer to water in ion-molecule reactions.

Fragmentation of doubly-protonated Pro-His-Xaa tripeptides: formation of b(2)(2+) ions

When ionized by electrospray from acidic solutions, the tripeptides Pro-His-Xaa (Xaa = Gly, Ala, Leu) form abundant doubly-protonated ions, [M + 2H]2+. Collision-induced dissociation (CID) of these doubly-protonated species results, in part, in formation of b(2)(2+) ions, which fragment further by loss of CO to form a(2)(2+) ions; the latter fragment by loss of CO to form the Pro and His iminium [immonium is commonly used in peptide MS work] ions. Although larger doubly-charged b ions are known, this represents the first detailed study of b(2)(2+) ions in CID of small doubly protonated peptides. The most abundant CID products of the studied doubly-protonated peptides arise mainly in charge separation involving two primary fragmentation channels, formation of the b2/y1 pair and formation of the a1/y2 pair. Combined molecular dynamics and density functional theory calculations are used to gain insight into the structures and fragmentation pathways of doubly-protonated Pro-His-Gly including the energetics of potential protonation sites, backbone cleavages, post-cleavage charge-separation reactions and the isomeric structures of b(2)(2+) ions. Three possible structures are considered for the b(2)(2+) ions: the oxazolone, diketopiperazine, and fused ring isomers. The last is formed by cleavage of the His-Gly amide bond on a pathway that is initiated by nucleophilic attack of one of the His side-chain imidazole nitrogens. Our calculations indicate the b(2)(2+) ion population is dominated by the oxazolone and/or fused ring isomers.