Conformations and selective photodissociation of heterogeneous benzo( a)pyrene diol epoxide enantiomer-DNA adducts

Abstract

The covalent binding of the tumorigenic (+) enantiomer and the nontumorigenic (−) enantiomer of trans-7,8-dihydroxy- anti-9, 10-epoxy-7,8,9,19-tetrahydrobenzo( a)pyrene (BPDE) to double-stranded native DNA gives rise to heterogeneous adducts, especially in the case of (−)-BPDE. The covalent (+)-BPDE-DNA adducts are predominantly of the external site II type, while the (−)-BPDE-DNA adducts are predominantly of the quasi-intercalative, site I type (65%), with 35% of site II adducts. The site I adducts can be selectively photodissociated with near-ultraviolet light (quantum yields in the range 0.0003–0.005); the external site II adducts (photodissociation quantum yield 3 × 10 −5) are 10–100-times more stable. The photolability of covalent (−)-BPDE-DNA adducts accounts for the discrepancies in the linear dichroism properties of these complexes reported previously. Fluorescence quenching data, previously utilized to assess the degree of solvent exposure of the pyrenyl residues in covalent adducts, were in some cases significantly influenced by the presence of highly fluorescent tetraol dissociation products. After correcting for this effect, it is shown that the fluorescence of the external site II (+)-BPDE-DNA adducts is sensitive to acrylamide, while the fluorescence of the dominant site I (−)-BPDE-DNA adducts is not affected by this fluorescence quencher, as expected for adducts with considerable carcinogen-base stacking interactions.