Copper-mediated peptide radical ions in the gas phase

Abstract

Molecular radical cations, M ̇+, of amino acids and oligopeptides are produced by collision-induced dissociation of mixed complex ions, [Cu II(dien)M] ̇2+, that contain Cu II, an amine, typically diethylenetriamine (dien), and the oligopeptide, M. With dien as the amine ligand, abundant M ̇+ formation is observed only for the amino acids tryptophan and tyrosine, and oligopeptides that contain either the tryptophanyl or tyrosyl residue. Dissociation of the M ̇+ ion is rich and differs considerably from that of protonated amino acids and peptides. Facile fragmentation occurs around the α-carbon of the tryptophanyl residue. Cleavage of the N-C α bond and proton transfer from the exocyclic methylene group in the side chain to the N-terminal residue results in formation of the [z n -H] ̇+ ion and elimination of the N-terminal fragment as ammonia or an amide, depending on the position of the tryptophanyl residue. Cleavage of the C α - C bond of an oligopeptide containing a C-terminal tryptophan residue and proton transfer from the carboxylic group to the N-terminal fragment (a carbonyl oxygen atom) results in formation of the [a n + H] dot;+ ion and elimination of carbon dioxide. Both types of fragmentation have no analogous reactions in protonated peptides. For the M ̇+ of tryptophanylgly-cylglycine, WGG, elimination of the tryptophanyl side chain results in GGG ̇+. This radical cation fragments by eliminating its C-terminal glycine to give the [b 2 - H] ̇+ ion, which is an oxazolone and shares much of the structure and reactivity of the b 2 + ion from protonated triglycine. Density functional theory shows the mechanism of forming the [b 2 - H] ̇+ ion is similar to that of the b 2 + ion, although the free-energy barrier at 29.4 kcal/mol is lower. The [b 2 - H] ̇+ ion eliminates CO readily to give the [a 2 - H] ̇+ ion, which is an iminium radical ion.