Properties of Benzopyrene-DNA Complexes Investigated by Fluorescence and Triplet Flash Photolysis Techniques

Properties of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP) in physical complexes with native calf thymus DNA dissolved in aqueous solutions at 24 ± 1 °C were studied by means of fluorescence quantum yield, single photon counting fluorescence decay, and triplet flash photolysis techniques. There are three types of binding sites for these polycyclic aromatic hydrocarbons bound to DNA. Between 40 and 60% of the aromatic molecules are located at a type of site where the excited singlet states decay nonradiatively and the other sites are characterized by different fluorescence decay times; for BaP the fluorescence lifetimes are 3.2 and 10 ns, while for BeP the lifetimes are 10 and 33 ns. Addition of silver ions, which in the range of silver ion/nucleotide ratios used (<0.13) bind to GC sites predominantly, results in up to a 70% quenching of the fluorescence of the aromatic hydrocarbons. It is proposed that most of the fluorescence originates from BaP and BeP bound to GC-GC intercalation sites, while the nonfluorescent molecules are bound at AT containing intercalation sites. This conclusion is supported by the relative quenching efficiencies of the fluorescence of BaP by mononucleosides dissolved in aqueous ethanol solutions: 2′-deoxythymidine is more effective (by a factor of >10) as a quencher than cytidine, 2′-deoxyguanosine and -adenosine. The strong quenching of the fluorescence in DNA is partially accompanied by the production of triplet excited states. The triplets are not as susceptible to quenching in DNA as the singlets and the lifetimes of BaP and BeP triplets in DNA are 35 and 155 ms, respectively. The triplet lifetimes are sensitive to the conformation of the DNA in solution, to the ionic strength, and to the oxygen concentration. Employing oxygen quenching techniques, the triplets can be used as probes to determine the accessibility of the polycyclic aromatic molecules in macromolecular complexes. Oxygen quenching constants for triplets of BaP, BeP, benz(a)anthracene, 7,12-dimethylbenz(a)anthracene, and acridine orange are in the range of 1-2 ⨯ 10⁸ 1. mol^-1 s^-1, about 10-20 times smaller than in fluid solution; this decrease is attributed to intercalation.