Photoemission probes of hydrocarbon-DNA interactions: A comparison of DNA influences on the reactivities of (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene, benzo[a]pyrene 4,5-oxide, and benz[a]anthracene 5,6-oxide

Article abstract of DOI:10.1021/ja00010a033

Time-resolved fluorescence and UV photoelectron measurements have been employed to examine the influence of calf thymus DNA on the reactivities of epoxides derived from benzo[a]pyrene (BP) and benz[a]anthracene (BA). By monitoring the increase in fluorescence intensity, which accompanies reaction at 23°C, overall, pseudo-first-order rate constants have been measured for reactions of the highly carcinogenic bay region epoxide (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and of two less carcinogenic K region epoxides benzo[a]pyrene 4,5-oxide (BPO) and benz[a]anthracene 5,6-oxide (BAO). Overall rate constants for hydrolysis and rearrangement reactions have been measured for BPDE, BPO, and BAO in buffer alone (1.0 mM sodium cacodylate, pH 7.1). The rate constants increase in the order BPO ((3.8 ± 0.1) × 10^-6 s^-1) < BAO ((5.7 ± 2.6) × 10^-5 s^-1) < BPDE ((7.2 ± 1.0) × 10^-4 s^-1). These results have been compared with overall rate constants for reactions, carried out in calf thymus DNA, which result in catalyzed hydrolysis and rearrangement, as well as DNA adduct formation. In DNA, the ordering of the rate constants for BPO and BAO changes from that observed in buffer alone. The rate constants increase in the order BAO ((2.8 ± 0.1) × 10^-3 s^-1) < BPO ((1.2 ± 0.2) × 10^-2 s^-1) < BPDE(～1 × 10^-1 s^-1). This ordering is the same as the ordering of association constants for the reversible binding to DNA of the fluorescent diols trans-7,8-dihydroxy-7,8-dihydro-BP (BP78D), trans-4,5-dihydroxy-4,5-dihydro-BP (BP45D) and cis-5,6-dihydroxy-5,6-dihydro-BA (BAD), which are model compounds of BPDE, BPO, and BAO, respectively. For the model compounds, the association constants for intercalation increase in the order BAD ((3.6 ± 0.9(× 10² M^-1) < BP45D ((9.6 ± 0.5) × 10³ M^-1) < BP78D ((3.4 ± 0.1) × 10⁴ M^-1). This ordering is consistent with the ordering of the association constants of BPDE ((2.5 ± 0.3) × 10⁴ M^-1) and of BPO ((6.0 ± 1.0) × 10³ M^-1). The temperature dependence of the association constants of the model compounds demonstrates that, for the intercalation of the BP diols into DNA, differences in the enthalpy of binding contribute significantly to differences in the free energy of binding. UV photoelectron data and results from ab initio molecular orbital calculations on BPDE, BPO, and BAO indicate that, for these three epoxides, the association constants increase as the ionization potentials decrease and the polarizabilities increase. The percentage of epoxide reaction that yields DNA adducts has been compared under varying conditions. For long reaction times (>1 h) in systems containing native, calf thymus DNA at low salt concentrations, the ordering of adduct yields is BPO (14.9 ± 1.1%) > BPDE (10.1 ± 3.0%) > BAO (3.6 ± 0.4%). For short reaction times (10 min) in systems containing native DNA stabilized with 0.10 mM Mg²⁺, the ordering of adduct yields is BPDE (7.3 ± 1.9%) > BPO (1.3 ± 0.1%) > BAO (0.1 ± 0.1%). These results suggest that the ability of an epoxide to form adducts with exposed DNA during long reaction times is less indicative of the genotoxic potency of the epoxide than its ability to form adducts with stabilized DNA during short reaction times.