Physical binding of pyrene and phenanthrene to native and denatured DNA: Measurements by spectral and coupled‐column liquid chromatography methods

The physical (noncovalent) binding of pyrene and phenanthrene to calf‐thymus DNA in aqueous NaCl solutions was measured by a spectral method (analysis of absorption spectra by Benesi‐Hildebrand plots) and a coupled‐column liquid chromatography method (equilibration of DNA solutions with solid hydrocarbon in a generator column and analysis of dissolved hydrocarbon by liquid chromatography). The measurements yielded values of an affinity constant K ′ = nK , where n is the apparent number of binding sites per nucleotide and K is the apparent binding constant. The affinity of native DNA for pyrene decreases monotonically with increasing NaCl concentration, whereas the affinity of heat‐denatured DNA exhibits a maximum at 0.10 M NaCl. K ′ for the binding of phenanthrene to native DNA is an order of magnitude lower than K ′ for pyrene. The molar enthalpy for the binding of pyrene to native DNA in 10 m M NaCl is (−34.0 ± 1.0) kJ mol −1 . The spectral method data indicate that 50 is an upper limit for the average number of base pairs between intercalation sites for pyrene along the DNA helix and that only a fraction of these sites are bound at the highest binding ratios.