Physical Binding of Tetraols Derived from 7,8-Dihydroxy-9,10-epoxybenzo[a]pyrene to DNA

The major reactive metabolite of the carcinogen trans- 7,8-dihydtoxy-anti-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene (ANTI-BPDE) either reacts covalently with DNA or is hydrolyzed to the tetraol 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT). In this work it is shown that BPT binds noncovalently to DNA in aqueous buffer solution (5 mM sodium cacodylate buffer, pH 7.1) at 25 °C. This binding, at values of the binding ratio r=10~3, defined as the ratio of bound BPT molecules per DNA base, is characterized by two types of binding sites. Site I is characterized by a 10-nm red shift in the absorption spectrum (a shift from 343 to 353 nm for the most intense absorption band of BPT), a complete quenching of the fluorescence of BPT at this site, and a negative linear dichroism spectrum. These properties are characteristic of an intercalation-type complex, in which the BPT molecule is sandwiched between adjacent base pairs of DNA. Equilibrium dialysis and absorption and fluorescence spectroscopy are the techniques utilized to demonstrate that there is a second type of binding site (II). This binding site is characterized by the following: (1) no shift in the absorption spectrum with respect to that of free BPT molecules in the buffer solution; (2) unchanged fluorescence yield, decay time, and susceptibility to oxygen quenching. It is proposed that site II corresponds to an external type of binding site of BPT on the DNA molecule, and the similarity between this type of binding and that of the covalent adduct formed between anti-BPDE and DNA is noted. At the low’ values of r studied here (1/950-1/4700) the ratio of BPT molecules at sites I and II lies in the range of 2-4, while 16-38% of the total BPT molecules initially added remain free in solution.