Spectroscopic characterizations and comparisons of the structures of the covalent adducts derived from the reactions of 7,8-dihydroxy-7,8,9,10-tetrahydrobenzo [a]pyrene-9,10-oxide, and the 9,10-epoxides of 7,8,9,10-tetrahydrobenzo[a]pyrene and 9,10,11,12-tetrahydrobenzo[e]pyrene with DNA

The conformation of covalent adducts derived from the reactions of racemic 7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaPDE), 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaPE), and 9,10-epoxy-9,10,11,12-tetrahydrobenzo[e]pyrene (BePE) with calf thymus DNA in aqueous buffer solution (25°C, pH 7.0) were investigated and compared by means of absorption, fluorescence and electric linear dichroism techniques. Two types of conformations are recognized. Site I is characterized by a red shift (∼10 nm) in the absorption maximum of the pyrene nucleus, a significantly reduced fluorescence yield, and a negative electric linear dichroism signal (δA); this site is presumed to involve a near-parallel (within 25°) orientation of the planar pyrene residue with the planes of the DNA bases, and a relatively strong interaction between the π electrons of the nucleic acid bases and the pyrene residue. In site II, there is only a small red-shift in the absorption maximum (∼2 nm), a non-zero fluorescence yield, and a positive δA throughout the absorption region of the pyrene residue; in this conformation the pyrene residue is presumed to lie on the outside of the DNA molecule, possibly in one of the grooves. The BaPDE-DNA complex displays predominantly a site II-type conformation while the BaPE- and BePE-DNA complexes display both site I and site II adducts, with site I, conformations predominating. The lack of hydroxyl groups in BaPE and BePE lead to a loss in stereospecificity in covalent adduct formation. The 7 and 8 hydroxyl groups in BaPDE appear to reduce the probability of formation of site I-type of covalent adducts, and appear to be, at least in part, responsible for the enantiomeric stereospecificity in the covalent reaction between BaPDE and DNA.