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 oligonucle otides of varying base composition have been studied by induced circul ar dichroism (ICD). The sign of the ICD around 350 nm of single-strand ed oligonucleotide adducts and the sign of an exciton type of CD compo nent at 260 nm in both single strand and duplex farms of adducts corre late with the absolute configuration of the cyclohexyl moiety of the a dduct. Changes in magnitude and sign of the ICD around 350 nm were obs erved 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 th e particular adduct isomer studied. Duplexes with (+)-trans-anti-BPDE- N-2-dG in 5'-d(CCTATCGCTATCC) or 5'-d(CCTATAGATATCC) exhibit a relativ ely strong positive ICD. In contrast, the duplexes with (+)-trans-anti -BPDE-N-2-dG in 5'-d(CCTATTGCTATCC) and 5'-d(CCTATTGTTATCC) display a small positive and negative ICD, respectively, in both cases suggestin g conformational heterogeneity. Partially complementary duplexes (dA, dT, or do) localized opposite the (+)-trans-anti-BPDE-N-2-dG adduct in 5'-d(CCTATCGCTATCC) or 5'-d(CCTATAGATATCC) also demonstrated negative ICD. These results together with light absorption characteristics sug gest a preferred conformation of intercalation for the mismatched dupl exes. Evidence of an equilibrium between the external and intercalativ e adduct conformation is provided by the results from the temperature dependence of the near-UV absorption and ICD characteristics of (+)-tr ans-anti-BPDE-N-2-dG complex in a 5'-d(CCTATAGATATCC) duplex.