Previous studies have established that 4-hydroxy-2- nonenal is a lipid hydroperoxide-derived aldehydic bifunctional electrophile that reacts with DNA and proteins. However, it has now been recognized that 4-oxo- 2-nonenal is also a major product of lipid hydroperoxide decomposition. Furthermore, 4-oxo-2-nonenal is more reactive than 4-hydroxy-2-nonenal toward the DNAbases 2'-deoxyguanosine, 2'-deoxyadenosine, and 2'-deoxycytidine and proteins. The formation of 4-oxo-2-nonenal can be induced through vitamin C-mediated or transition metal ion-mediated homolytic decomposition of polyunsaturated ω-3 lipid hydroperoxides such as 13(S)-hydroperoxyoctadecadienoic acid. We have discovered that synthetic 4-oxo-nonenal or 4-oxo-2-nonenal- generated from 13(S)-hydroperoxyoctadecadienoic acid recognizes the specific amino acid motifs of His75, Ala76, and Lys77 in bovine histone H4. Reaction of the histidine and lysine residues with 4-oxo-2-nonenal results in the formation of a novel cyclic structure within the protein. The cyclic structure incorporates the histidine imidazole ring and a newly formed pyrrole derived from the lysine. The cyclic imidazole-pyrrole derivative that is formed from the small Nα-acetyl-His-Ala-Lys peptide exists as a mixture of two atropisomers that interconvert upon heating. Such lipid hydroperoxidederived modifications could potentially modulate transcriptional activation in vivo. Furthermore, the ability to synthesize cyclic peptides using 4-oxo-2-nonenal will facilitate the preparation of novel structural analogs with potential biological activity.