Induced Circular Dichroism of Benzo(a)pyrene-7,8-dihydrodiol 9,10-Epoxide Stereoisomers Covalently Bound to Deoxyribooligonucleotides Used to Probe Equilibrium Distribution between Groove Binding and Intercalative Adduct Conformations

Binding conformations of single anti-BPDE-N-2-dG adducts in oligonucleotides of varying base composition have been studied by induced circular dichroism (ICD). The sign of the ICD around 350 nm of single-stranded oligonucleotide adducts and the sign of an exciton type of CD component at 260 nm in both single strand and duplex farms of adducts correlate with the absolute configuration of the cyclohexyl moiety of the adduct. Changes in magnitude and sign of the ICD around 350 nm were observed upon duplex formation. The results show that adducts displaying external (minor groove) binding characteristics are associated with a significant positive ICD. Conversely, adducts displaying intercalation binding characteristics were found to have a positive or negative ICD. The magnitude of the ICD is dependent on the sequence context and the particular adduct isomer studied. Duplexes with (+)-trans-anti-BPDE-N-2-dG in 5\u27-d(CCTATCGCTATCC) or 5\u27-d(CCTATAGATATCC) exhibit a relatively strong positive ICD. In contrast, the duplexes with (+)-trans-anti-BPDE-N-2-dG in 5\u27-d(CCTATTGCTATCC) and 5\u27-d(CCTATTGTTATCC) display a small positive and negative ICD, respectively, in both cases suggesting conformational heterogeneity. Partially complementary duplexes (dA, dT, or do) localized opposite the (+)-trans-anti-BPDE-N-2-dG adduct in 5\u27-d(CCTATCGCTATCC) or 5\u27-d(CCTATAGATATCC) also demonstrated negative ICD. These results together with light absorption characteristics suggest a preferred conformation of intercalation for the mismatched duplexes. Evidence of an equilibrium between the external and intercalative adduct conformation is provided by the results from the temperature dependence of the near-UV absorption and ICD characteristics of (+)-trans-anti-BPDE-N-2-dG complex in a 5\u27-d(CCTATAGATATCC) duplex

Studies on the Adduct Heterogeneity of Benzo(a)pyrene 7,8-Dihydrodiol 9,10-Epoxide Stereoisomers Covalently Bound to Deoxyribooligonucleotides by Induced Circular Dichroism and Light Absorption Spectroscopy

The binding conformations of single anti- and syn-BPDE-N-2-dG adducts in oligonucleotides of varying base composition have been studied by induced circular dichroism (ICD) and light absorption spectroscopy. The sign of the ICD in single-stranded oligonucleotide adducts correlates with the absolute configuration of the cyclohexyl moiety of the BPDE. Adducts in oligonucleotide duplexes with UV lambda(max) 350 nm exhibiting either positive or negative contributions to the ICD should have intercalated binding as the predominant conformation. The magnitude of the ICD is dependent on the sequence context of the adducted strand and the particular BPDE-adduct isomer under study. In some cases, the results suggest structural heterogeneity. For instance, the (+)- and the (-)-trans-anti-BPDE-N-2-dG adducts in duplexes where a dT flanks the lesion site exhibit weak positive ICD or negative ICD. These results reflect a bimodal conformational adduct distribution with contributions from both externally and internally located adducts. A key observation for the (+)-cis-syn-BPDE-N-2-dG complexes in 5\u27-d(TGC) and 5\u27-d(CGC) sequence contexts is that the near-UV absorption spectra showed distinct bands corresponding to minor groove binding (lambda(max) congruent to 346 nm) as well as intercalative binding (lambda(max) congruent to 354 nm). Evidence for an equilibrium between the different modes of localization is provided by the results from the temperature dependence of the near-UV absorption and ICD characteristics of(+)-cis-synBPDE-N-2-dG complexes in 5\u27-d(TGC) and 5\u27-d(CGC) sequence contexts, respectively