Alternative conformations of DNA modified by N-2-acetylaminofluorene.

Modification of DNA by the carcinogen N-acetoxy-N-2-acetylaminofluorene gives two adducts, a major one at the C-8 position of guanine and a minor one at the N-2 position with differing conformations. Binding at the C-8 position results in a large distortion of the DNA helix referred to as the "base displacement model" with the carcinogen inserted into the DNA helix and the guanosine displaced to the outside. The result is increased susceptibility to nuclease S1 digestion due to the presence of large, single-stranded regions in the modified DNA. In contrast, the N-2 adduct results in much less distortion of the helix and is less susceptible to nuclease S1 digestion. A third and predominant adduct is formed in vivo, the deacetylated C-8 guanine adduct. The conformation of this adduct has been investigated using the dimer dApdG as a model for DNA. The attachment of aminofluorene (AF) residues introduced smaller changes in the circular dichroism (CD) spectra of dApdG than binding of acetylaminofluorene (AAF) residues. Similarly, binding of AF residues caused lower upfield shifts for the H-2 and H-8 protons of adenine than the AAF residues. These results suggest that AF residues are less stacked with neighboring bases than AAF and induce less distortion in conformation of the modified regions than AAF. An alternative conformation of AAF-modified deoxyguanosine has been suggested based on studies of poly(dG-dC).(dG-dC). Modification of his copolymer with AAF to an extent of 28% showed a CD spectrum that had the characteristics of the left-handed Z conformation seen in unmodified poly(dG-dC).poly(dG-dC) at high ethanol or salt concentrations. Poly(dG-dC).poly(dG-dC) which does not undergo the B to Z transition at high ethanol concentrations, did not show this type of conformational change with high AAF modifications. Differences in conformation were suggested by single-strand specific nuclease S1 digestion and reactivity with anticytidine antibodies. Highly modified poly(GS-dC).poly(DG-dC) was almost completely resistant to nuclease S1 hydrolysis, while, modified DNa and poly(dG).poly(dC) are highly susceptible to digestion. Two possible conformations for deoxyguanosine modified at the C-8 position by AAF are compared depending on whether its position is in alternating purine-pyrimidine sequences or random sequence DNA.