The interaction of native calf thymus DNA with FeIII-dipyrido[3,2-a:2′,3′-c]phenazine

The mono and bis dipyrido[3,2-a:2′,3′-c]phenazine (dppz) adducts of iron(III) chloride, i.e. [Fe(dppz)]Cl3 and [Fe(dppz)2]Cl3, have been synthesized and characterized. The interaction of the FeIIIdppz hydrolyzed aquo complex with native calf thymus DNA has been monitored as a function of the metal complex–DNA molar ratio, by variable temperature UV absorption spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. The results obtained in solution at various ionic strength values give support for a tight intercalative binding of the FeIIIdppz cation with DNA. In particular, the appearance of induced CD bands, caused by the addition of FeIIIdppz, indicate the existence of a rigid metal complex–DNA-binding leading to dominating chiral organization of FeIIIdppz species within the DNA double helix. The trend of selected CD bands with the molar concentration of FeIIIdppz emphasizes that the presence of high amounts of metal complex induces also the formation of DNA–FeIIIdppz supramolecular aggregates in solution. The analysis of fluorescence measurements allowed us to calculate a value of the intercalative binding constant comparable to that obtained by UV spectrophotometric titration. Finally, the temperature dependence of the absorbance at 258 nm shows that the metal complex strongly increases the DNA melting temperature already at metal complex–DNA molar ratio equal to 0.25 suggesting that metal complex intercalation effectively hinders DNA denaturation. Overall, the results of the present study point out that the FeIIIdppz aquo complex has DNA-binding properties analogous to those previously reported for the tris-chelate FeII(phen)2dppz complex (phen = 1,10-phenantroline).