The guanidine group present in the amino acid arginine was found to react with the lipid hydroperoxide-derived bifunctional electrophile, 4-oxo-2-nonenal. The reaction between Nα-tert-butoxycarbony-L-arginine and 4-oxo-2-nonenal resulted in the formation of an adduct (adduct A) that subsequently dehydrated on heating to adduct B. Liquid chromatography/mass spectrometry and nuclear magnetic resonance spectroscopy were used to assign the structure of adduct B as (Nδ,Nω′ -etheno-2′-heptanon-2″-one)-Nα-t-Boc-arginine. The reaction proceeded from initial reaction of the primary Nω -amino group at the C-1 aldehyde of 4-oxo-2-nonenal. Subsequently, an intramolecular Michael addition of a secondary Nδ-amino group occurring at C-3 resulted in formation of the cyclic carbinolamine adduct A. Dehydration and rearrangement of the exocyclic imine resulted in the formation of adduct B, which contained a stable imidazole ring. The tetra peptide LRDE reacted with 4-oxo-2-nonenal primarily at arginine rather than at the amino terminus. This suggests that arginine-containing proteins can react with lipid hydroperoxide-derived 4-oxo-2-nonenal to form a novel imidazole modification.