Intramolecular addition of cysteine thiyl radicals to phenylalanine in peptides: formation of cyclohexadienyl type radicals

The intra-molecular addition of peptide cysteine thiyl radicals to phenylalanine yields alkylthio-substituted cyclohexadienyl radicals for the peptides Phe–Cys and Phe–Gly–Cys–Gly, i.e. even when Phe and Cys are separated by a Gly residue, and presents a possible free radical pathway to thioether-containing peptide and protein cross-links.