Properties of covalent benzo[a] pyrene diol eporide-dna adducts investigated by fluorescence techniques

The spectroscopic absorption and fluorescence properties of adducts derived from the covalent binding of (±)trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,l0-tetrahydrobenzo[a]pymne (BPDE) to DNA are re-examined in view of conflicting interpretations regarding the Coaformations of these adducts which currently exist in the literature. The fluorescence decay profiles were accurately determined utilizing synchrotron-pulsed light source excitation and the time-correlated single photon counting technique. The coaformational properties of the adducts were probed by determining their accessibilities to acrylamide, a known fluorescence quencher, and by comparing the accessibilities of the BPDE-DNA adducts with those of known model systems with intercalative, partially intercalative and minor groove binding conformations. In contrast toanyofthesemodelsystems,tkfhmesmmofthearomahic pyrenyl residues in the covalent BPDE - DNA ad- exhibit significant sensitivity to acrylamide, suggesting that these residues are located at binding sites with significant solvent exposure. A quantitative analysis of the acrylamide fluorescence quenching according to a dynamic Stem-Volmer quenching model suggests the following characteristics: the major (65%) component (1.4 ns lifetime) is characterized by significant exposure to the sotvent environment; the second component (6-7 ns lifetime) can be subdivided into a solvent-accessible and a solvent-inaccessible component, the inaccessible fraction being attributed to minor adducts, possibly with a quasi-intercalative conformation. The amplitude of the third, long-lived (200-ns) component is variable; it arises from the photochemical decomposition of the adducts which gives rise to tetraols (7,8,9,1O-tetrahydro-tetrahydroxybenzo[a]pyrene). The variable content of these degradation products accounts for most discrepancies in the fluorescence properties of the covalent BPDE-DNA adducts previously reported.