The binding parameters of ethidium bromide (EtBr) with DNA of various GC/AT ratios were determined using absorption and fluorescence spectroscopy. Our experimental data clearly demonstrate the co-existence of fluorescing and non-fluorescing types of "strong" binding at low concentration of EtBr. The fluorescent complex corresponds to the ordinary intercalative model. The non-fluorescent complex is referred to semi-intercalative binding of EtBr. The binding constant (K) and the number of base pairs corresponding to a binding site (n) of the fluorescent (Kf and n f) and non-fluorescent (Knf, n nf) types of interactions were determined. The average size of binding site (n) is equal to 1.5 bp (absorption spectroscopy), and n f ca. 2 bp (fluorescence spectroscopy). It was shown that n nf is dependent on GC-content and total n is independent of it.
DNA; ethidium bromide; adsorption isotherms; spectrophotometry; spectrofluorometry
