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 alpha-carbon of the tryptophanyl residue. Cleavage of the N-C-alpha 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-alpha-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](.+) 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.