7-Methylguanine in poly(dG-dC).poly(dG-dC) facilitates z-DNA formation

Poly(dG-dC).poly(dG-dC) was methylated at the N-7 position of guanine through the use of dimethyl sulfate. The conversion of poly(dG-dC).poly(dG-dC) from B-DNA to the Z-DNA form was followed by measuring both the circular dichroic spectra and changes in the absorbance ratio A295/A260. Increasing methylation steadily decreases the amount of NaCl or MgCl2 that is required to convert the polymer from B-DNA to Z-DNA. At 100% methylation of the guanine residues, the modified polymer is fully converted to Z-DNA in a physiological salt solution. Kinetic experiments show that methylation markedly accelerates the speed of the conversion from B-DNA to Z-DNA in the presence of added salt. These effects may be partly due to the positive charge on guanine that accompanies N-7 methylation.