Microcalorimetry and binding studies of DNA upon interaction with [pyridine diamine]2[Co(phenanthroline dicarboxylate)2]

The interaction of calf thymus DNA with [pyridine diamine]2[Co(phenanthroline dicarboxylate)2], [pyda]2[Co(phendc)2], complex has been studied at 10 mM phosphate buffer at 27 °C, pH 7.0 using various techniques including isothermal titration calorimetry (ITC), UV-vis spectrophotometry and potentiometric titration. Sigmoidal absorption profile implies the existence of cooperative interactions between DNA and [pyda]2[Co(phendc)2] that induces a conformational change in DNA structure. Scatchard plots confirm this cooperative interaction. The calorimetric enthalpy curve of [pyda]2[Co(phendc)2] upon interaction with DNA is measured by ITC. The enthalpy of binding (ΔHbin) of [pyda]2[Co(phendc)2] with DNA is also obtained spectrophotometrically by determining of equilibrium binding constants at 27 and 37 °C based on Wyman binding potential and van't Hoff relation. The enthalpy of ionization (ΔHion) is also obtained by acid-base titration at 27 and 37 °C in 100 mM KCl solution using modified differential van't Hoff relation for estimation of (ΔHion). The binding data was analyzed in a two-set binding sites model based on the Hill equation. The first phase of the calorimetric enthalpy curve is consistent with a first set of binding sites and further phases are consistent with a second set of binding sites.