Thermodynamics of the interaction of aristololactam-beta-D-glucoside with DNA. Ionic strength dependence of enthalpy and entropy.

The interaction of aristololactam-beta-D-glucoside with calf thymus DNA has been studied by measuring the changes in the absorbance of the alkaloid over a wide range of temperatures and sodium chloride concentrations. The binding parameters obtained are best fit by the neighbour exclusion model. The salt and temperature dependence of the binding constants are used to estimate the thermodynamic parameters involved in the interaction of the alkaloid with DNA. It is observed that aristololactam-beta-D-glucoside binding to DNA is an exothermic process over the entire range of salt and temperature, and the estimated values of enthalpy and entropy change are strongly dependent on the ionic strength of the solution. The enthalpy and entropy changes compensate one another to produce a relatively small Gibbs' free energy change. The possibility that aristololactam-beta-D-glucoside exists as a monovalent cation at neutral pH and the possible molecular contribution to the enthalpy and entropy changes of the aristololactam-beta-D-glucoside-DNA complex are discussed.