Generation of Peptide Radical Dications via Low-Energy Collision-Induced Dissociation of [CuII(terpy)(M + H)]·3+

The first example of the formation of hydrogen-deficient radical cations of the type [M + H]·2+ is demonstrated to occur through a one-electron-transfer mechanism upon low-energy collision-induced dissociation (CID) of gas-phase triply charged [CuII(terpy)(M + H)]·3+ complex ions (where M is an angiotensin III or enkephalin derivative; terpy = 2,2′:6′,2″-terpyridine). The collision-induced dissociation of doubly charged [M + H]·2+ radical cations generates similar product ions to those prepared through hot electron capture dissociation (HECD). Isomeric isoleucine and leucine residues were distinguished by observing the mass differences between [zn + H]·+ and wn+ ions (having the same residue number, n) of the Xle residues. The product ion spectrum of [zn + H]·+ reveals that the wn+ ions are formed possibly from consecutive fragmentations of [zn + H]·+ ions. Although only the first few [M + H]·2+ species have been observed using this approach, these hydrogen-deficient radical cations produce fragment ions that have more structure-informative patterns and are very different from those formed during the low-energy tandem mass spectrometry of protonated peptides.