Fragmentation of Lanthanide (III) Cationized Small Peptides: Generation of Peptide Radical Cations and Dipositive a and b ions.

This research work examines the dissociation chemistry of tripositive complexes formed by trivalent lanthanide ions and small peptides with tandem mass spectrometry under low-energy collision-induced dissociation (CID). By fragmentation of the tripositive lanthanide(III) cationized small peptide, a new route to generate peptide radical cations has been discovered. The dipositive b ions are also observed and the mechanisms by which they fragment are investigated by MSn. Tripositive complexes of lanthanide(III)/peptide have similar fragmentation chemistries in the gas phase when lanthanide = yttrium, lanthanum, cerium, samarium, gadolinium and terbium; [a3+H]2+ ions are formed and there are no peptide radical cations observed. When the lanthanide is europium(III), radical cations of tryptophan-, tyrosine-, phenylalanine-, methionine-containing peptides and of aliphatic peptides have been generated. Fragmentations of tripositive Ce(III)/peptide and Eu(III)/peptide complexes show very different behaviours. Abundant CO loss is only observed for dissociation of Ce(III)/peptide complexes, whereas CO2 loss is the predominant channel for Eu(III)/peptide complexes. Similarly, CO loss and CO2 loss are the predominant channels for the dissociations of [Ce(peptide-H)]2+ and [Eu(peptide-H)]2+, respectively. Peptide radical cations are only generated by the fragmentation of Eu(III)/peptide complexes, while protonated a and b ions are only observed when Ce(III)/peptide complexes dissociate. The dissociations of aliphatic [peptide]+ions generate [b3-H]+/ [b2-H]+ions for most peptides. In the dissociation of [a3+H]+ions, [b2-H]+ions are formed from most peptides. [a3+H]2+ ions usually cleave at the C-terminal amide bonds, creating two singly charged ions, a [b2]+ ion and an iminium ion derived from the C-terminal residue. Some [a3+H]2+ ions also lose small neutral molecules. The composition of the peptides dictates the preferred mode of the fragmentation of [b3+H]2+ ions, either loss of CO to form [a3+H]2+, or loss of CO plus H2O. Fragmentations of [Ce(peptide-H)]2+ ions show CO loss, and CO2 losses are observed for peptides with aromatic side chains or a methionine residue at C-terminus. For [Ce(peptide-H)(peptide)]2+ complexes, neutral losses are also observed but formation of two singly charged ions is dominant. The dissociation behaviour of [Ce(peptide-H)(CH3CN)]2+ and [Eu(peptide-H)(CH3CN)]2+ complexes are quite different. The former loses only CH3CN whereas the latter loses only CO2.