Monomeric ruthenium(II) bipyridal dihydroxy anthraquinone complexes: Synthesis, characterization and DNA binding

The Ruthenium (II) bipyridal dihydroxy anthraquinone complexes examined are structurally similar, except for the hydrogen-bonding hydroxyl (OH) position. They are considered substituents. This allows examination of the contribution of the role of hydrogen bonding on the DNA binding affinity. Moreover, the anthraquinone moieties are found in many anticancer/antitumor drugs, so examining the binding interactions is useful for both DNA/drug molecular recognition and in the design of new, less toxic drugs. Absorbance and fluorescence spectroscopic techniques were used to study the binding interactions of the ruthenium (II) bipyridyl dihydroxy anthraquinone complexes (1,8-DHAQ, 1,5-DRAQ and 1,4-DHAQ) with calf thymus DNA. The effect of complex concentration on the binding constant was also examined. In order to produce highly pure complexes, modified synthetic procedures were designed and examined. The complexes display a hypochromicity and red shift in the metal-to-ligand charge transfer bands (MLCT) upon DNA addition. The calculated binding constants were 7.5, 5.0 and 3.8 x 104/M for the 1,8-, 1,5- and 1,4-DHAQ respectively. The observed binding constant varied with the [DNA]/[Ru] ratio. The 1,8-DRAQ complex binds stronger at low concentration, whereas the 1,5-DHAQ complex has the reverse case. Fluorescence quenching was observed; the 1,5-DHAQ complex shows the highest quenching ability (I0/I) of 2.60 compared to 1.80 and 1.14 for the 1,8- and 1,4-DHAQ respectively. Based on the spectroscopic and binding results, the complexes may partially intercalate. The introduction of hydrogen bonding functionalities provides no net increases in DNA binding affinity. The modified synthetic procedure produced a highly pure product for the 1,8- and 1,4-DHAQ complexes. The crude product was black indicating the predominance of the final pure complex. The solubility of the complexes varied at different salt concentrations, particularly for the 1,4-DHAQ complex. At buffer concentration 50 mM Tris/50 mM NaCl a fibrous-blue precipitate was formed; decrease in the UV-Visible absorbance with time suggests dimerization and higher order aggregation.