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

[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. (J Am Soc Mass Spectrom 2000, 11, 626-638) (C) 2000 American Society for Mass Spectrometry.