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

F-19 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 F-19 NMR spectrum of dG-C8-FAAF [N-(deoxyguanosin-8-yl)-N-acetyl-7-fluoro-2-aminoflourene] in methanol at -30 degrees 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 (gamma) (14.4 kcal/mol) and the guanyl-nitrogen (alpha) bonds. The conformational heterogeneity persisted in a single strand FAAF-120-mer, d(CTTCTTG[FAAF]ACCTC); whose F-19 NMR spectrum at 22 degrees 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 (gamma') bond. A similar conformational theme was observed in the. FAAF-12-mer duplex, d(CTTCTTG[FAAF]ACCTC).d(GAGGTCAAGAAG), which revealed two F-19 resonances in a 41:59 ratio at 22 degrees 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 (gamma' similar to 180 degrees) or trans (gamma' similar to 0 degrees). 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, 384-5389]. 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.