Solution structure of the N-(deoxyguanosin-8-yl)-1-aminopyrene ([AP]dG) adduct opposite dA in a DNA duplex

Solution structural studies have been undertaken on the aminopyrene-C-8-dG ([AP]dG) adduct in the d(CS-[AP]GG-C7).d(G16-A17-G18) sequence context in a n 11-mer duplex with dA opposite [AP]dG, using proton-proton distance and i ntensity restraints derived from NMR data in combination with distance-rest rained molecular mechanics and intensity-restrained relaxation matrix refin ement calculations. The exchangeable and nonexchangeable protons of the ami nopyrene and the nucleic acid were assigned following analysis of two-dimen sional NMR data sets on the [AP]dG.dA 11-mer duplex in H2O and D2O solution . The broadening of several resonances within the d(G16-A17-G18) segment po sitioned opposite the [AP]dG6 lesion site resulted in weaker NOEs, involvin g these protons in the adduct duplex. Both proton and carbon NMR data are c onsistent with a syn glycosidic torsion angle for the [AP]dG6 residue in th e adduct duplex. The aminopyrene ring of [AP]dG6 is intercalated into the D NA helix between intact Watson-Crick dC5.dG18 and dC7.dG16 base pairs and i s in contact with dC5, dC7, dG16, dA17, and dG18 residues that form a hydro phobic pocket around it. The intercalated AP ring of [AP]dG6 stacks over th e purine ring of dG16 and, to a lesser extent dG18, while the looped out de oxyguanosine ring of [AP]dG6 stacks over dC5 in the solution structure of t he adduct duplex. The dA17 base opposite the adduct site is not looped out of the helix but rather participates in an in-plane platform with adjacent dG18 in some of the refined structures of the adduct duplex. The solution s tructures are quite different for the [AP]dG.dA 11-mer duplex containing th e larger aminopyrene ring (reported in this study) relative to the previous ly published [AF]dG.dA 11-mer duplex containing the smaller aminofluorene r ing (Norman et al., Biochemistry 28, 7462-7476, 1989) in the same sequence context. Both the modified syn guanine and the dA positioned opposite it ar e stacked into the helix with the aminofluorene chromophore displaced into the minor groove in the latter adduct duplex. By contrast, the aminopyrenyl ring participates in an intercalated base-displaced structure in the prese nt study of the [AP]dG.dA 11-mer duplex and in a previously published study of the [AP]dG.dC 11-mer duplex (Mao et al.., Biochemistry 35, 12659-12670, 1996). Such intercalated base-displaced structures without hydrogen bondin g between the [AP]dG adduct and dC or mismatched dA residues positioned opp osite it, if present at a replication fork, may cause polymerase stalling a nd formation of a slipped intermediate that could produce frameshift mutati ons, the most dominant mutagenic consequence of the [AP]dG lesion.