Fragmentation of Peptide Radical Cations Containing a Tyrosine or Tryptophan Residue: Structural Features That Favor Formation of [x_(n–1) + H]^•+ and [z_(n–1) + H]^•+ Ions

Peptide radical cations A_nY^•+ (where n = 3, 4, or 5) and A₅W^•+ have been generated by collision-induced dissociation (CID) of [Cu^II(tpy)­(peptide)]^•2+ complexes. Apart from the charge-driven fragmentation at the N–C_α bond of the hetero residue producing either [c + 2H]⁺ or [z – H]^•+ ions and radical-driven fragmentation at the C_α–C bond to give a⁺ ions, unusual product ions [x + H]^•+ and [z + H]^•+ are abundant in the CID spectra of the peptides with the hetero residue in the second or third position of the chain. The formation of these ions requires that both the charge and radical be located on the peptide backbone. Energy-resolved spectra established that the [z + H]^•+ ion can be produced either directly from the peptide radical cation or via the fragment ion [x + H]^•+. Additionally, backbone dissociation by loss of the C-terminal amino acid giving [b_(n–1) – H]^•+ increases in abundance with the length of the peptides. Mechanisms by which peptide radical cations dissociate have been modeled using density functional theory (B3LYP/6-31++G** level) on tetrapeptides AYAG^•+, AAYG^•+, and AWAG^•+.