Title

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

Document Type

Article

Date of Original Version

6-8-1999

Abstract

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.

Publication Title

Biochemistry

Volume

38

Issue

23

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