Minor Groove Orientation for the (1S,2R,3S,4R)-N²- [1-(1,2,3,4-Tetrahydro-2,3,4-trihydroxybenz[a]anthracenyl)]-2‘-deoxyguanosyl Adduct in the N-ras Codon 12 Sequence^†

The conformation of the trans-anti-(1S,2R,3S,4R)-N²-[1-(1,2,3,4-tetrahydro-2,3,4-trihydroxybenz[a]anthracenyl)]-2‘-deoxyguanosyl adduct in d(G¹G²C³A⁴G⁵X⁶T⁷G⁸G⁹T¹⁰G¹¹)·d(C¹²A¹³C¹⁴C¹⁵A¹⁶C¹⁷C¹⁸T¹⁹G²⁰C²¹C²²), bearing codon 12 of the human N-ras protooncogene (underlined), was determined. This adduct had S stereochemistry at the benzylic carbon. Its occurrence in DNA is a consequence of trans opening by the deoxyguanosine amino group of (1R,2S,3S,4R)-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthracenyl-3,4-diol. The resonance frequencies, relative to the unmodified DNA, of the X⁶ H1‘ and H6 protons were shifted downfield, whereas those of the C¹⁸ and T¹⁹ H1‘, H2‘, H2‘ ‘, and H3‘ deoxyribose protons were shifted upfield. The imino and amino resonances exhibited the expected sequential connectivities, suggesting no interruption of Watson−Crick pairing. A total of 426 interproton distances, including nine uniquely assigned BA−DNA distances, were used in the restrained molecular dynamics calculations. The refined structure showed that the benz[a]anthracene moiety bound in the minor groove, in the 5‘-direction from the modified site. This was similar to the (+)-trans-anti-benzo[a]pyrene−N²-dG adduct having S stereochemistry at the benzylic carbon [Cosman, M., De Los Santos, C., Fiala, R., Hingerty, B. E., Singh, S. B., Ibanez, V., Margulis, L. A., Live, D., Geacintov, N. E., Broyde, S., and Patel, D. J. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1914−1918]. It differed from the (−)-trans-anti-benzo[c]phenanthrene−N²-dG adduct having S stereochemistry at the benzylic carbon, which intercalated in the 5‘-direction [Lin, C. H., Huang, X., Kolbanovskii, A., Hingerty, B. E., Amin, S., Broyde, S., Geacintov, N. E., and Patel, D. J. (2001) J. Mol. Biol. 306, 1059−1080]. The results provided insight into how PAH molecular topology modulates adduct structure in duplex DNA.