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.
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