T-jump fluorescence relaxation study of the binding of ethidium cation to natural DNA

T-jump chemical relaxation experiments with fluorescence detection were carried out on the binding of ethidium cation to calf thymus DNA over the temperature range from 11 to 35 degrees C for ethidium concentrations from 500 nM to 2 microM, DNA concentrations from 7.5 to 60 microM (base pairs), and KCl concentrations from 20 to 500 mM. In contrast to the findings of earlier investigations, no salt or DNA concentration or temperature dependences were seen that would support a mechanism of intercalation directly from solution or from an electrostatically bound exterior site in rapid equilibrium with solution. It is suggested that the rate-determining process of intercalation in vivo involves large scale dynamics of the DNA itself.