Noncovalent Intercalative Complex Formation and Kinetic Flow Linear Dichroism of Racemic syn- and anti-Benzo[a]pyrenediol Epoxide-DNA Solutions

In order to gain a better understanding of the molecular basis underlying the differences in biological activities of the diastereomeric syn and anti diol epoxides of benzo [a] pyrene (BPDE) (trans-7,8-dihydroxy-syn-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and trans-7,8-dihydroxy-anti-9,10-ep-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene, respectively), their interactions with DNA in aqueous solutions were studied and compared. Kinetic flow linear dichroism experiments indicate that both diastereomers (racemic mixtures) form intercalation complexes immediately after mixing; the association constant (23 °C, ionic strength ~0.005) is signficantly smaller (5200 M^-1) in the case of the syn than in the case of the anti diastereomer (12 200 M^-1). This difference is attributed to the greater bulkiness of the 7,8,9,10 ring of the syn stereoisomer, which is in the quasi-diaxial conformation as compared to the less bulky quasi-diequatorial conformation of the anti diastereomer. In contrast, the intercalative association constants of the tetraols derived from the hydrolysis of the two diol epoxides are similar in value. Upon formation of noncovalent syn-BPDE-DNA intercalation complexes, the reaction rate constant for the formation of tetraols (~98%) and covalent adducts (~2%) increases from 0.009 to 0.05 s^-1at pH 9.5 in 5 mM tris(hydroxy-methyl)aminomethane buffer. The conformations of the aromatic chromophores of BPDE were followed by the kinetic flow dichroism technique as a function of reaction time; while the anti diastereomer changes conformation from an intercalative to an apparently external binding site, the syn diol epoxide molecules do not appear to undergo any measurable reorientation during or after the covalent binding reaction. These significant differences in the characteristics of the interactions of these two BPDE diasteromers with DNA, and the differences in the conformations of the adducts formed, are discussed in relation to the known differences in the mutagenic and tumorigenic activities of these compounds.