DNA binding of benzophenanthridine compounds sanguinarine versus ethidium: Comparative binding and thermodynamic profile of intercalation

There is compelling evidence that cellular DNA is the target of many small molecule anticancer agents. Consequently, elucidation of the molecular nature governing the interaction of small molecules to DNA is paramount to the progression of the rational drug design strategies. In this study, we have compared the binding and thermodynamic aspects of two known DNA binding agents, ethidium and sanguinarine with calf thymus DNA. The study revealed non-cooperative binding phenomena for both the drugs to DNA with an affinity similar for ethidium and sanguinarine as observed from different techniques. The binding phenomena analyzed from isothermal titration calorimetry showed exothermic binding for both compounds that was favoured by negative enthalpy and positive entropy changes typical of intercalative binding. The binding of both the drugs was further characterized by strong stabilization of DNA against thermal strand separation in optical melting as well as differential scanning calorimetry studies. The data of the salt dependence of binding of sanguinarine and ethidium from the plot of log K versus log [Na+] revealed a slope of −0.711 and −0.875, respectively, consistent with the values predicted by the theories for the binding of monovalent cations and the binding free energy has been analyzed for contributions from polyelectrolytic and non-polyelectrolytic forces. The salt dependence of the binding was also evident from the conformational changes in the circular dichroism where both extrinsic and induced changes were lowered on increasing the salt concentration. The heat capacity changes obtained from temperature dependence of enthalpy change gave values of (−590 and −670) J · mol−1 · K−1, respectively for the binding of sanguinarine and ethidium to DNA. Overall the DNA binding of ethidium was slightly more favoured over sanguinarine.