Probing the conformational heterogeneity of the acetylaminofluorene- modified 2'-deoxyguanosine and DNA by 19F NMR spectroscopy

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19F NMR spectroscopy was used to probe the conformation of a DNA adduct derived from the carcinogen 7-fluoro-N-acetyl-2-aminofluorene (FAAF) in three structural contexts: as a monomer and incorporated into single- and double-stranded DNA. The 19F NMR spectrum of dG-C8-FAAF [N-(deoxyguanosin- 8-yl)-N-acetyl-7-fluoro-2-aminofluorene] in methanol at -30 °C exhibited four interconvertible signals in a 11:52:26:11 ratio. Dynamic NMR analysis indicated that the four torsional isomers arise from restricted rotation about the amide (γ) (14.4 kcal/mol) and the guanyl-nitrogen (α) bonds. The conformational heterogeneity persisted in a single strand FAAF-12-mer, d(CTTCTTG[FAAF]ACCTC), whose 19F NMR spectrum at 22 °C and pH 7.0 gave only two signals in a 40:60 ratio, instead of four. The two 19F signals followed a two-site exchange with the rotation barrier of 14.7 kcal/mol about the amide (γ) bond. A similar conformational theme was observed in the FAAF- 12-mer duplex, d(CTTCTTG[FAAF]ACCTC)·d(GAGGTCAAGAAG), which revealed two 19F resonances in a 41:59 ratio at 22 °C and pH 7.0. According to solvent- induced isotope and magnetic anisotropy effects, the two duplex conformers adopt exclusively a base displacement structure, being different only in their relative acetyl group orientations, cis (γ ~ 180°) or trans (γ ~ 0°). Dynamic NMR data indicated that the two conformers do not exchange over a wide range of temperatures. This contrasts with the nonacetylated counterpart, which exhibits an equilibrium between the 'B-type' and 'stacked' conformers [Zhou, L., et al. (1997) J. Am. Chem. Soc. 119, 5384-5389]. The exclusive stacked nature of the AAF adducts may provide insight into why AAF adducts are more mutagenic and prone to repair than the nonacetylated AF adducts.

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