Fluorescence and absorption spectroscopy and linear dichroism have been used to study the covalent adducts of calf thymus DNA with the two stereoisomers of benzo(a)pyrene 7,8-dihydrodiol-9,10-oxide, (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (anti-BPDE; a strong carcinogen) and (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (syn-BPDE; a weak carcinogen). Each stereoisomer gives rise to two types of complexes, I and II, with characteristic spectral properties: type I with light absorption and fluorescence excitation maxima at 322, 337, and 354 nm; and type II with the corresponding maxima at 316, 330, and 345 nm. In anti-BPDE-DNA, the type II component dominates and the type I component amounts to <10%. In syn-BPDE-DNA, approximately 35%, of the adduct is of type II and approximately 65% of type I. From linear dichroism, it is concluded that the type II component of anti-BPDE-DNA has the plane of the chromophore molecule nearly parallel to the helix axis. The type I component of syn-BPDE-DNA has a very different geometry with the chromophore probably intercalated between the DNA bases. This is also in accord with the large wavelength shift of the light absorption and the weak quenching of the fluorescence by O2 for the type I complex. The properties of complex type II of anti-BPDE-DNA are in agreement with a wedge-type geometry at the binding site. The Stern-Volmer quenching curves are bent, and the fluorescence decays are not monoexponential, which demonstrates that there is heterogeneity in the microenvironment of the chromophore. From the dynamic quenching constants with O2, it is found that different subcategories of the chromophore are differently exposed to the medium. Addition of Ag+ to anti-BPDE-DNA (type II complexes) leads to increased fluorescence and longer decay times. The Ag+-induced effects are probably due to a conformational change of DNA when Ag+ is bound, causing the anti-BPDE adducts to interact less strongly with the DNA. Component type I of syn-BPDE-DNA is not affected by Ag+ in a similar way. Instead, a weak quenching is observed.
Upon denaturation, both anti- and syn-BPDE-DNA give a type of single-stranded complex with light absorption and fluorescence excitation maxima at 316, 332, and 351 nm. The chromophores are probably sandwiched between the DNA bases.
This work was supported by grants from the Swedish Cancer Society, the Swedish Council for Coordination and Planning of Research, the Swedish Natural Science Research Council, NIH (Grant 1RC1CA26261-01), and the Magn. Bergvalls Stiftelse.