The (1S,2R,3S,4R)-N-6-[1-(1,2,3,4-tetrahydro-2,3,4-trihydroxybenz[a]anthracenyl)]-2'-deoxy-adenosyl adduct at X-6 Of 5'-d(CGGACXAGAAG)-3'.5'-d(CTTCTTGTCCG)-3', incorporating codons 60, 61 (underlined), and 62 of the human N-ras protooncogene, results from trans opening of (1R,2S,3S,4R)-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthracenyl 3,4-diol by the exocyclic N-6 Of adenine. Two conformations of this adduct exist, in slow exchange on the NMR time scale. A structure for the major conformation, which represents approximately 80% of the population, is presented. In this conformation, an anti glycosidic torsion angle is observed for all nucleotides, including (S,R,S,R)A(6). The refined structure is a right-handed duplex, with the benz[a]anthracene moiety intercalated on the 3'-face of the modified base pair, from the major groove. It is located between (S.R,S,R)A(6). The refined A(7).T-16. Intercalation is on the opposite face of the modified (S,R,S,R)A(6). The base pair as compared to the (1R,2S,3R,4S)-N-6-[1-(1,2,3,4-tetrahydro-2,3,4-trihydroxybenz[a]anthracenyl)] 2'-deoxyadenosyl adduct, which intercalated 5' to the modified (R,S,R,S)A(6).T-17 base pair [Li, Z., Mao, H., Kim, H.-Y., Tamura, P. J., Harris, C. M., Harris, T. M., and Stone, M. P. (1999) Biochemistry 38, 2969-2981]. The spectroscopic data do not allow refinement of the minor conformation, but suggest that the adenyl moiety in the modified nucleotide (S,R,S,R)A(6) adopts a syn glycosidic torsion angle. Thus, the minor conformation may create greater distortion of the DNA duplex. The results are discussed in the context of site-specific mutagenesis studies which reveal that the (S,R,S,R)A(6) lesion is less mutagenic than the (R.S,R,S)A(6) lesion.
