Fragmentation of α-Radical Cations of Arginine-Containing Peptides

Fragmentation pathways of peptide radical cations, M +·, with well-defined initial location of the radical site were explored using collision-induced dissociation (CID) experiments. Peptide radical cations were produced by gas-phase fragmentation of Co III(salen)–peptide complexes [salen = N,N'-ethylenebis (salicylideneiminato)]. Subsequent hydrogen abstraction from the β-carbon of the side-chain followed by C α–C β bond cleavage results in the loss of a neutral side chain and formation of an α-radical cation with the radical site localized on the α-carbon of the backbone. Similar CID spectra dominated by radical-driven dissociation products were obtained for a number of arginine-containing α-radicals, suggesting that for these systems radical migration precedes fragmentation. In contrast, proton-driven fragmentation dominates CID spectra of α-radicals produced via the loss of the arginine side chain. Radical-driven fragmentation of large M +· peptide radical cations is dominated by side-chain losses, formation of even-electron a-ions and odd-electron x-ions resulting from C α–C bond cleavages, formation of odd-electron z-ions, and loss of the N-terminal residue. In contrast, charge-driven fragmentation produces even-electron y-ions and odd-electron b-ions. The competition between proton-driven and radical-driven fragmentation of peptide radical cations is determined by the presence of a basic residue in the sequence.