Identification of conformationally different binding sites in benzo[a]pyrene diol epoxide--DNA adducts by low-temperature fluorescence spectroscopy.

It is known that the covalent binding of the two enantiomers of trans-7,8-diol-anti-9,10-epoxy-benzo[a]pyrene (BPDE) to native double-stranded DNA gives rise to two distinct classes of adducts. Type I adducts involve significant interactions of the pyrenyl residues with the DNA bases and are similar but not identical to intercalation complexes. Type II adducts involve external solvent-exposed binding sites and their predominance in adducts derived from the covalent reaction of (+)-BPDE with DNA has been associated with the higher tumorigenicity and mutagenic activity of (+)-BPDE in mammalian cells. These two distinct binding sites in covalent BPDE-DNA adducts can be readily resolved by synchronous scanning fluorescence methods at low temperatures (77 K) using commercially available fluorescence spectrophotometers. The site I adducts are particularly unstable in the presence of UV light, and this method can be used to follow their selective photodegradation.