Spectroscopic characteristics and site I/site II classification of cis and trans benzo[a]pyrene diolepoxide enantiomer-guanosine adducts in oligonucleotides and plynucleotides

Abstract

The highly tumorigenic isomer (+)-7,8-dihydroxy-anti-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE] and its non-tumorigenic enantiomer (-)-anti-BPDE are known to react predominantly with the exocyclic amino group (N2) of deoxyguanine in DNA and to form adducts of different conformations. The spectroscopic characteristics (UV absorbance, fluorescence and circular dichroism) of stereochemically defined (+)-trans, (-)-trans, (+)-cis and (-)-cis d(5'-CACATGBPDETACAC) adducts in the single-stranded form, or complexed with the complementary strand d(5'-GTGTACATGTG) in aqueous solution, were investigated. The spectroscopic characteristics of the double-stranded d(5'-CACATGBPDETACAC).d(5'-GTGTACATGTG) adducts can be interpreted in terms of two types of conformations. In site I-type conformations, there is an approximately 10 nm red shift in the absorption maxima, which is attributed to significant pyrenyl residue-base interactions; in site II-type adducts, the red shift is only approximately 2-3 nm, and the pyrene ring system is located at external, solvent-exposed binding sites. The spectroscopic characteristics of the BPDE-modified duplexes are of the site II type for the (+)- and (-)-trans, and of the site I type for the (+)- and (-)-cis adducts. In adducts derived from the binding of (+)-anti-BPDE to poly(dG-dC).(dG-dC) and poly(dG).(dC), the trans/cis BPDE-N2-dG adduct ratio is 6 +/- 1; in the case of (-)-anti-BPDE this ratio is only 0.4 +/- 0.1 and 0.6 +/- 0.15 in poly(dG-dC).(dG-dC) and poly(dG).(dC) respectively. The spectroscopic properties of these BPDE-modified polynucleotide adducts are consistent with those of the BPDE-modified oligonucleotide complexes; the cis adducts are correlated with site I adduct conformations, while the trans adducts are of the site II type. The correlations between adduct characteristics and biological activities of the two BPDE enantiomers are discussed.