Characterization of Dipeptide Isomers by Tandem Mass Spectrometry of Their Mono- versus Dilithiated Complexes

Abstract

The Li+ complexes of the isomeric dipeptide pairs PheGly/GlyPhe, PheAla/AlaPhe, and TrpAla/AlaTrp, namely, [Pep + Li]+, and of the corresponding lithium carboxylates, namely, [Pep - H + 2Li]+, are produced in the gas phase by desorption ionization, and their unimolecular chemistry is probed by tandem mass spectrometry experiments at various activation conditions. At low internal energies, monolithiated isomers dissociate to the same products, formed through a mixed anhydride intermediate in which the sequence information is lost. Isomerization to the mixed anhydride is less competitive at higher internal energies, which start promoting sequence-specific fragmentations. On the other hand, dilithiated isomers (they contain a permanent COO-Li+ salt bridge) do not rearrange to an anhydride and give rise to substantially different fragmentation patterns; structurally diagnostic c1- and y1-type fragments are observed at all internal energies, allowing for unequivocal sequence assignment. The mono- and dilithiated peptides undergo loss of their aromatic side chain to form distonic radical ions carrying Li+ charge(s) and one unpaired electron at an alpha-C atom of the peptide backbone. The yield of such metal-bound peptide radicals is particularly high from the dilithiated complexes, [Pep - H + 2Li]+. Upon activation, the Li+ ions become mobile and can be shuttled to the various basic sites of the dipeptides, where they may initiate backbone fragmentation or the elimination of small neutral molecules.

Unimolecular chemistry of metal ion-coordinated α-dipeptide radicals

“…Mass spectrometry offers a convenient means for the isolation and the study of transient intermediates, if these are charged. Kenttämaa and coworkers [11][12][13][14][15][16] and, more recently, our group [17][18][19][20][21] have demonstrated that organic radicals containing inert charges, including alkali metal ion or quaternary pyridinium sites, primarily react at their radical centers. Such systems can therefore serve as templates for the investigation of the gas-phase chemistry of the corresponding neutral radicals.…”

“…The corresponding dilithiated complexes, in which the -COOH termini are derivatized to COO Li + , are also investigated in order to determine the influence of salt bridges on radical reactivity. We recently showed that such radical ions can be generated by collisionally activated dissociation (CAD) of mono-or dilithiated dipeptides containing an aromatic amino acid residue at the position where the Gly • radical is to be introduced [20]. This method complements the synthetic procedure recently introduced by Siu et al [22], in which radical ions of complete peptides (they have one more H atom than the ␣-peptide radicals studied here) are prepared via gas-phase redox reactions from ternary complexes of a multiply charged transition metal ion, the peptide of interest and an auxiliary multidentate ligand [22][23][24][25].…”

Fragmentation ofα-radical cations of arginine-containing peptides

“…In contrast, charge-driven fragmentation produces even-electron y-ions and odd-electron b-ions. (n-1)ϩ· ions produced by capture of low-energy electrons by multiply protonated molecules or by electron-transfer processes [1][2][3][4][5]9], hydrogen deficient radical anions [M Ϫ nH] (n-1)-· generated by electron detachment and photodetachment from multiply deprotonated molecules [10,11], peptides cationized on lithium [12], or transition metals [13][14][15][16], and M ϩ· peptide radical cations. [In this study, we used standard notation for molecular radical cations, M ϩ· , which implies that the molecule has a charge and one unpaired electron, and specified the initial location of the radical site, when possible].…”

An Isotope (18O, 15N, and 2H) Technique to Investigate the Metal Ion Interactions Between the Phosphoryl Group and Amino Acid Side Chains by Electrospray Ionization Mass Spectrometry

“…These data are consistent with the rearrangement mechanism proposed in Schemes 6 and 7. Consequently, these results show that ESI-MS/MS is a useful tool for isomeric differentiation through collisionally induced dissociation of the metal ion complexes [59,60]. …”