SOLUTION CONFORMATION OF [AF]DG OPPOSITE A -1-DELETION SITE IN A DNA DUPLEX - INTERCALATION OF THE COVALENTLY ATTACHED AMINOFLUORENE RING INTO THE HELIX WITH BASE DISPLACEMENT OF THE C-8-MODIFIED SYN GUANINE INTO THE MAJOR GROOVE

This paper reports on the solution structure of the [AF]dG adduct positioned opposite a deletion site in a DNA oligomer duplex that defines the alignment of the covalent aminofluorene-C-8-guanine adduct relative to the deletion site. The combined NMR molecular mechanics computational studies were undertaken on the [AF]dG adduct embedded in the d(C5-[AF]G6-C7). d(G16-G17) sequence context in a duplex containing 11 residues on the modified strand and 10 on the partner, with no base opposite the modification. The exchangeable and nonexchangeable protons of the aminofluorene moiety and the nucleic acid were assigned following analysis of two-dimensional NMR data sets in H2O and D2O solution. The solution conformation of the [AF]dG . del 11-mer duplex has been determined by incorporating intramolecular and intermolecular proton-proton distances defined by lower and upper bounds deduced from NOESY spectra as restraints in molecular mechanics computations in torsion angle space. The aminofluorene ring of [AF]dG6 is intercalated between intact Watson-Crick dC5 . dG17 and dC7 . dG16 base pairs with the guanine base of [AF]dG6 in a syn alignment displaced into the major groove. The syn glycosidic torsion angle at [AF]dG6 is supported by both carbon and proton chemical shift data for the sugar resonances of the modified guanine residue. The long axis of the aminofluorene ring is parallel to the long axis of the flanking dG . dC base pairs with the AF ring undergoing rapid 180 degrees flips on the NMR time scale. The intercalation site is wedge shaped with a pronounced propeller-twisting and buckling of the dC5 . dG17 base pair. The guanine base of [AF]dG6, which is positioned in the major groove, is inclined relative to the helix axis and stacks over the 5'-flanking dC5 residue in the solution structure. The intercalative-base displacement structure of the [AF]dG . del 11-mer duplex exhibits several unusually shifted proton resonances that can be readily accounted for by the ring current contributions of the guanine purine and carcinogen fluorene aromatic rings of the [AF]dG6 adduct. There are similarities between this structure of the AF-C-8-dG covalent adduct positioned opposite a deletion site and the (+)-trans-antiBP-N-2-dG covalent adduct positioned opposite a deletion site in the same sequence context reported previously from this laboratory [Cosman et al. (1994) Biochemistry 33, 11507-11517]. The chromophores are intercalated into the helix opposite the deletion site with displacement of the modified guanine into the major groove in both cases. The important difference is that the modified guanine of the N-2-linked benzo[a]pyrenyl-dG adduct adopts an anti alignment while it adopts a syn alignment for the C-8-linked aminofluorene adduct, both of which have their guanine bases tilted relative to the helix axis and stacked over the major groove edge of the 5'-flanking dC5 residue.