Fragmentation chemistry of [M + Cu]⁺ peptide ions containing an N-terminal arginine

Abstract

[M + Cu]⁺ peptide ions formed by matrix-assisted laser desorption/ionization from direct desorption off a copper sample stage have sufficient internal energy to undergo metastable ion dissociation in a time-of-flight mass spectrometer. On the basis of fragmentation chemistry of peptides containing an N-terminal arginine, we propose the primary Cu⁺ ion binding site is the N-terminal arginine with Cu⁺ binding to the guanidine group of arginine and the N-terminal amine. The principal decay products of [M + Cu]⁺ peptide ions containing an N-terminal arginine are [a _n + Cu − H]⁺ and [b _n + Cu − H]⁺ fragments. We show evidence to suggest that [a _n + Cu − H]⁺ fragment ions are formed by elimination of CO from [b _n + Cu − H]⁺ ions and by direct backbone cleavage. We conclude that Cu⁺ ionizes the peptide by attaching to the N-terminal arginine residue; however, fragmentation occurs remote from the Cu⁺ ion attachment site involving metal ion promoted deprotonation to generate a new site of protonation. That is, the fragmentation reactions of [M + Cu]⁺ ions can be described in terms of a “mobile proton” model. Furthermore, proline residues that are adjacent to the N-terminal arginine do not inhibit formation of [b _n + Cu − H]⁺ ion, whereas proline residues that are distant to the charge carrying arginine inhibit formation of [b _n + Cu − H]⁺ ions. An unusual fragment ion, [c _n + Cu + H]⁺, is also observed for peptides containing lysine, glutamine, or asparagine in close proximity to the Cu⁺ carrying N-terminal arginine. Mechanisms for formation of this fragment ion are also proposed.