Salt-dependent binding of iron(II) mixed-ligand complexes containing 1,10-phenanthroline and dipyrido[3,2- a:2′,3′- c]phenazine to calf thymus DNA

Abstract

The salt-dependent binding of racemic iron(II) mixed-ligand complex containing 1,10-phenanthroline (phen) and dipyrido[3,2- a:2′,3′- c]phenazine (dppz), [Fe(phen) 2(dppz)] 2+ to calf thymus DNA (ct-DNA) has been characterized by UV–VIS spectrophotometric titration. The equilibrium binding constant ( K b) of the iron(II) complex to ct-DNA decreases with the salt concentration in the solution. The slope, SK = ( δlog K b / δlog [Na 2+]) has been found to be 0.49, suggesting that, in addition to intercalation, considerable electrostatic interaction is also involved in the ct-DNA binding of [Fe(phen) 2(dppz)] 2+. The calculation of non-electrostatic binding constant ( K t o) based on polyelectrolyte theory has revealed that the non-electrostatic contribution to the total binding constant ( K b) increases significantly with the increase in [Na +] and reaches 36% at 0.1 M NaCl. On the other hand, the contribution of the non-electrostatic binding free energy (Δ G t o) to the total binding free energy change (Δ G o) is considerably large, i.e. 87% at [Na +] = 0.1 M, suggesting that the stabilization of the DNA binding is mostly due to the contribution of non-electrostatic process. Moreover, the effect of specific ligand substitutions on Δ G o has been rigorously evaluated using the quantity ΔΔ G t o, i.e. the difference in Δ G t o relative to that of the parent iron(II) complex, [Fe(phen) 3] 2+, indicating that each substitution of phen by dip and dppz contributes 7.5 and 17.5 kJ mol − 1 , respectively to more favorable ct-DNA binding.