Spectroscopic studies on the interaction of isatin with calf thymus DNA

The interaction of native calf thymus DNA (CT-DNA) with isatin was studied at physiological pH by spectrophotometric, spectrofluorometric, competition experiment, circular dichroism, and viscometric techniques. No red shift and isobestic points are observed in UV absorption band of isatin. A strong fluorescence quenching reaction of DNA to isatin was observed, and the binding constants (Kf) of DNA with isatin and corresponding numbers of binding sites (n) were calculated at different temperatures. Thermodynamic parameters enthalpy change (ΔH) and entropy change (ΔS) were calculated to be +27.42 kJ mol⁻¹ and +201 J mol⁻¹ K⁻¹ according to Van't Hoff's equation, which indicated that the reaction is predominantly entropically driven. DNA viscosity and iodide quenching results suggest that isatin does not intercalate into the DNA base pairs. Circular dichroism spectra of CT-DNA indicated deep conformational changes in the DNA double helix. These results indicate that isatin interacts with CT-DNA via groove binding mode.