Binding of new Fe ( II ) - containing porphyrins to DNA . Effect of ionic strength

We employed Circular Dichroism (CD) and UV/Vis-spectrophotometric methods to study the interaction of two Fe-containg cationic porphyrins: meso-tetra (4N-butylpyridyl) porphyrin [FeTButPyP4] and meso-tetra (4N-oxyethylpyridyl) porphyrin [FeTOEtPyP4] with DNA at various ionic strengths. The temperature-dependent absorption titration data were used to determine the binding constants and stoichiometry for each ligand-DNA complexes. It was shown that the decrease in ionic strength of solution leads to decrease in the number of binding sites of porphyrins to DNA and increase in the efficiency of their binding. The CD spectra analysis shows that the binding mode of both porphyrins to DNA depends on the ionic strength of buffer. It changes from outside random binding at lower ionic strength (μ = 0.02M) to semi-intercalation at higher strength (μ = 0.2M). The magnitude and sign of enthalpy and entropy, calculated using Van’t Hoff equation, confirm these conclusions.