The binding of the drug quinacrine to its bioreceptor, DNA, was studied by optical methods. Binding of quinacrine to duplex calf thymus DNA produced bathochromic shifts in the absorption spectrum of the drug, indicating the binding of single drug molecules rather than of dimers or aggregates. Results of a spectrophotometric titration of quinacrine with DNA were converted into a nonlinear adsorption isotherm (Scatchard plot) whose curvature suggests that the drug binds to more than one class of binding sites by more than one process. Strong binding with an apparent association constant of 1.2 x 106 M-1 and a stoichiometry of 1 drug molecule/∼4 nucleotides is ascribed to the known intercalation of the drug into DNA. A weaker process, with an apparent association constant of 4.6 x 104 M-1 and a stoichiometry of 1 drug molecule/3 nucleotides, may represent a peripheral electrostatic attraction to phosphates of DNA. Fluorometric titration of quinacrine with DNA at low inorganic ion concentrations showed progressive quenching of fluorescence until a drug to nucleotide ratio (r) of 0.22 was attained. When titration with DNA was continued beyond this point, fluorescence increased and ultimately attained a plateau similar to that to which the fluorescence of the drug decreased in titrations at higher salt concentrations. Poly dA·dT strongly enhanced the fluorescence of quinacrine, while poly dG·dT quenched it. Quinacrine displaced 97.2% of DNA-bound methyl green at rates higher than those caused by aminoacridines which did not possess cationic aliphatic side chains. Depending upon the concentration of free quinacrine, the displacement of methyl green was either firstor second-order with time. Duplex and single-stranded DNAs induced Cotton effects in the optical rotatory dispersion spectrum of the drug, suggesting that the 9-imino group in the molecule is involved in intercalation binding. Quinacrine stabilized native DNA to thermal denaturation and formed a complex with denatured DNA which thermally dissociated in a cooperative manner.
- Copyright ©, 1974, by Academic Press, Inc.