Reactivity and binding of benzo(a)pyrene diol epoxide to poly(dG-dC)·(dG-dC) and poly(dG-m⁵dC)·(dg-m⁵dC) in the B and Z forms

The physical and covalent binding of the carcinogen benzo (a)pyrene-7,8-diol-9,10-oxide (BaPDE) to poly(dG-dC)·(dG-dC) and poly(dG-m⁵dC)·s(dG-m⁵dC) in the B and Z forms were studied utilizing absorbance, fluorescence and linear dichroism techniques. In the case of poly(dG-dC)·(dG-dC) the decrease in the covalent binding of BaPDE with increasing NaCl concentration (0.1-4 M) as the B form is transformed to the Z form is attributed to the effects of high ionic strengths on the reactivity and physical binding of BaPDE to the polynucleotides; these effects tend to obscure differences in reactivities with the B and Z forms of the nucleic acids. In the case of poly(dG-m⁵dC)·(dG-m⁵dC) the B-to-Z transition is induced at low ionic strength (2 mM NaCl + 10 μM Co(NH₃)₆Cl ₃) and the covalent binding is found to be 2-3-times lower to the Z form than to the B form. Physical binding of BaPDE by intercalation, which precedes the covalent binding reaction, is significantly lower in the Z form than in the B form, thus accounting, in part, for the lower covalent binding. The linear dichroism characteristics of BaPDE covalently bound to the Z and B forms of poly(dG-m⁵dC)·(dG-m⁵dC) are consistent with nonintercalative, probably external conformations of the aromatic pyrenyl residues.