Continuous mixing experiments allow to determine the size of binding sites for anthracyclines complexed to DNA

The size of binding sites of a ligand forming a complex with a polymer can be determined by continuous mixing experiments and measuring spectrophotometrically the various mixtures in the absorption band of the ligand or polymer. Only the total concentrations of both stock solutions and the molar extinction coefficient of the ligand or polymer have to be known. Here, this method is applied to DNA complexes with differently strongly interacting anthracyclines (adriamycin, daunomycin, beta-rhodomycin-I, iremycin). For all the anthracyclines studied a binding site of about three base pairs per ligand was found. Consideration of the self-association of the anthracyclines practically does not influence the results obtained.