8,9-Dihydro-8-(N7-guanyl-[d(ATCGAT)])-9-hydroxyaflatoxin B1.d(ATCGAT) and 8,9-dihydro-8-(N7-guanyl-[d(ATGCAT)])-9-hydroxyaflatoxin B1.8,9-dihydro-8-(N7-guanyl-[d(ATGCAT)])-9-hydroxyaflatoxin B1 were prepared by direct addition of afltoxin B1 8,9-epoxide to d(ATCGAT)2 and d(ATGCAT)2, respectively. In contrast to reaction of aflatoxin B1 8,9-epoxide with d(ATCGAT)2 which exhibits a limiting stoichiometry of 1:1 aflatoxin B1:d(ATCGAT)2 [Gopalakrishnan, S., Stone, M. P., & Harris, T. M. (1989) J. Am. Chem. Soc. 111, 7232-7239], reaction of aflatoxin B1 8,9-epoxide with d(ATGCAT)2 exhibits a limiting stoichiometry of 2:1 aflatoxin B1:d(ATGCAT)2. 1H NOE experiments, nonselective 1H T1 relaxation measurements, and 1H chemical shift perturbations demonstrate that in both modified oligodeoxynucleotides the aflatoxin moiety is intercalated above the 5'-face of the modified guanine. The oligodeoxynucleotides remain right-handed, and perturbation of the B-DNA structure is localized adjacent to the adducted guanine. Aflatoxin-oligodeoxynucleotide 1H NOEs are observed between aflatoxin and the 5'-neighbor base pair and include both the major groove and the minor groove. The aflatoxin methoxy and cyclopentenone ring protons face into the minor groove; the furofuran ring protons face into the major groove. No NOE is observed between the imino proton of the modified base pair and the imino proton of the 5'-neighbor base pair; sequential NOEs between nucleotide base and deoxyribose protons are interrupted in both oligodeoxynucleotide strands on the 5'-side of the modified guanine. The protons at C8 and C9 of the aflatoxin terminal furan ring exhibit slower spin-lattice relaxation as compared to other oligodeoxynucleotide protons, which supports the conclusion that they face into the major groove. Increased shielding is observed for aflatoxin protons; chemical shift perturbations of the oligodeoxynucleotide protons are confined to the immediate vicinity of the adducted base pair. The imidazole proton of the modified guanine exchanges with water and is observed at 9.75 ppm. The difference in reaction stoichiometry is consistent with an intercalated transition-state complex between aflatoxin B1 8,9-epoxide and B-DNA. Insertion of aflatoxin B1-8,9 epoxide above the 5'-face of guanine in d(ATCGAT)2 would prevent the binding of a second molecule of aflatoxin B1 8,9-epoxide. In contrast, two intercalation sites would be available with d(ATGCAT)2.(ABSTRACT TRUNCATED AT 400 WORDS)