Fragmentation of Peptide Radical Cations Containing a Tyrosine or Tryptophan Residue: Structural Features That Favor Formation of [<i>x</i>_{(<i>n</i>–1)} + H]^•+ and [<i>z</i>_{(<i>n</i>–1)} + H]^•+ Ions

Abstract

Peptide radical cations A_<i>n</i>Y^•+ (where <i>n</i> = 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 [<i>c</i> + 2H]⁺ or [<i>z</i> – H]^•+ ions and radical-driven fragmentation at the C_α–C bond to give <i>a</i>⁺ ions, unusual product ions [<i>x</i> + H]^•+ and [<i>z</i> + 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 <i>both</i> the charge and radical be located on the peptide backbone. Energy-resolved spectra established that the [<i>z</i> + H]^•+ ion can be produced either directly from the peptide radical cation or via the fragment ion [<i>x</i> + H]^•+. Additionally, backbone dissociation by loss of the C-terminal amino acid giving [<i>b</i>_{(<i>n</i>–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^•+