Abstract
Seven-coordinate Fe(III) complexes [Fe(dapsox)(H2O)(2)](+), where [dapsox = 2,6-diacetylpyridine-bis(semioxamazide)] is an equatorial pentadentate ligand with five donor atoms (2O and 3N), were studied with regard to their acid-base properties and complex formation equilibria. Stability constants of the complexes and the pK (a) values of the ligands were measured by potentiometric titration. The interaction of [Fe(dapsox)(H2O)(2)](+) with the DNA constituents, imidazole and methylamine center dot HCl were investigated at 25 A degrees C and ionic strength 0.1 mol center dot dm(-3) NaNO3. The hydrolysis constants of the [Fe(dapsox)(H2O)(2)](+) cation (pK (a1) = 5.94 and pK (a2) = 9.04), the induced ionization of the amide bond and the formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with DNA constituents supporting the hepta-coordination mode of Fe(III). The concentration distributions of the various complex species were evaluated as a function of pH. The thermodynamic parameters Delta HA degrees and Delta SA degrees calculated from the temperature dependence of the equilibrium constants were investigated for interaction of [Fe(dapsox)(H2O)(2)] with uridine.