trans-4-Hydroxynonenal (HNE) is a peroxidation product of omega-6 polyunsaturated fatty acids. The Michael addition of deoxyguanosine to HNE yields four diastereomeric exocyclic 1,N-2-dG adducts. The corresponding acrolein- and crotonaldehyde-derived exocyclic 1,N-2-dG adducts undergo ring-opening to N-2-dG aldehydes, placing the aldehyde functionalities into the minor groove of DNA. The acrolein- and the 6R-crotonaldehyde-derived exocyclic 1,N-2-dG adducts form interstrand N-2-dG:N-2-dG cross-links in the 5'-CpG-3' sequence context. Only the HNE-derived exocyclic 1,N-2-dG adduct of (6S,8R,11S) stereochemistry forms interstrand N-2-dG:M-dG cross-links in the 5'-CpG-3' sequence context. Moreover, as compared to the exocyclic 1,N-2-dG adducts of acrolein and crotonaldehyde, the cross-linking reaction is slow (Wang, H.; Kozekov, I. D.; Harris, T. M.; Rizzo, C. J. J. Am. Chem. Soc. 2003, 125, 5687-5700). Accordingly, the chemistry of the HNE-derived exocyclic 1,N-2-dG adduct of (6S,8R,11S) stereochemistry has been compared with that of the (6R,8S,11R) adduct, when incorporated into 5'-d(GCTAGCXAGTCC)-3'-5'-d(G-GACTCGCTAGC)-3', containing the 5'-CpG-3' sequence (X = HNE-dG). When placed complementary to dC in this duplex, both adducts open to the corresponding N-2-dG aldehydic rearrangement products, suggesting that the formation of the interstrand cross-link by the exocyclic 1,N-2-dG adduct of (6S,8R,11 S) stereochemistry, and the lack of cross-link formation by the exocyclic 1,N-2-dG adduct of (6R,8S,l 1 R) stereochemistry, is not attributable to inability to undergo ring-opening to the aldehydes in duplex DNA. Instead, these aldehydic rearrangement products exist in equilibrium with stereoisomeric cyclic hemiacetals. The latter are the predominant species present at equilibrium. The trans configuration of the HNE H6 and H8 protons is preferred. The presence of these cyclic hemiacetals in duplex DNA is significant as they mask the aldehyde species necessary for interstrand cross-link formation.