Induction of the Z conformation in poly(dG-dC).poly(dG-dC) by binding of N-2-acetylaminofluorene to guanine residues.

Poly(dG-dC).poly(dG-dC) and poly(dG).poly(dC) were modified by treatment with N-acetoxy-N-2-acetylaminofluorene, and their conformations were examined by circular dichroism and susceptibility to nuclease S1 digestion. A sample of poly(dG-dC).poly(dG-dC) modified to an extent of 28% with acetylaminofluorene (AAF) at the C(8) position of the deoxyguanosine residues showed a circular dichroism spectrum that had the characteristics of the Z conformation seen in unmodified poly(dG-dC).poly(dG-dC) at high ethanol or salt concentrations. A sample of poly(dG-dC).poly(dG-dC) modified only 3% by AAF showed a spectrum characteristic of the B form of DNA. However, it was converted to the Z form at ethanol concentrations lower than required to convert unmodified poly(dG-dC).poly(dG-dC) from the B to the Z form. Poly(dG).poly(dC), which does not undergo the B-to-Z transition at high ethanol concentrations, did not show any large conformational changes with high AAF modification. Susceptibility to digestion with nuclease S1 also suggested differences in the conformations of the two modified polynucleotides. Poly(dG-dC).poly(dG-dC) modified by AAF to an extent of 28% was almost completely resistant to nuclease S1 digestion. However, both poly(dG).poly(dC) and DNA modified to similar levels by AAF were highly susceptible to nuclease S1 digestion. Two different conformations for AAF-modified deoxyguanosine are proposed, depending on whether its position is in alternating purine-pyrimidine sequences or in random-sequence DNA.