Conformation base-displaced stacked

P212121 Z — 8 Dx= 1.57 R = 0.085 for 1,743 intensities. The two independent molecules have similar conformations. The glycosyl dispositions are anti (90.1°, 91.2°), and the D-ribosyl groups are 3T4 (24.0°, 34.1° 15.6°, 35.5°). The exocyclic, C-4 -C-5 bond orientations are gauche+ (63.1°, 53.8°). The orientation of the methyl groups in both molecules is such that it is directed away from the imidazole moiety of the base, that is, the 0-6-C-7 bond is trans to the C-5-C-6 bond this arrangement constitutes an obstacle to formation of Watson-Crick hydrogen-bonds to the complementary base cytosine. In molecule A, 0-6 and C-7 are displaced from the purine plane by 79 and 87 pm, and, in molecule B, by 49 and 16 pm. The bases are stacked. 

While all three B[a]P-N2-dG adducts destabilize double-stranded DNA, the extent of destabilization is significantly smaller in the case of the (+)-ds-anti adduct (ATm = -4°C) than in the two trans-anti-B[a P-N2-dG adducts (ATm = -8 to —10°C). Nevertheless, the NER efficiency of the (+)-cis-anti-B[a]P-N2-dG adduct is 5-7 times greater than that of either trans-anti adduct. The smaller extent of destabilization caused by the (+)-cis-anti-B[a]P-N2-dG adduct is attributed to carcinogen base n-n stacking interactions that counteract the loss of Watson-Crick hydrogen-bonding and base-base stacking interactions in this base-displaced/intercalated conformation [38]. 

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