Spectral, viscometric and electrochemical studies on mixed ligand cobalt(III) complexes of certain diimine ligands bound to calf thymus DNA

The complex [Co(5,6-dmp)3]Cl3 (1) and the mixed ligand complexes [Co(bipy)2(5,6-dmp)]Cl3 (2), [Co(bipy)2(phen)]Cl3 (3), [Co(phen)2(bipy)]Cl3 (4), [Co(bipy)2(imp)]Cl3 (5) and [Co(phen)2(imp)]Cl3 (6) where bipy=2,2′-bipyridine, phen=1,10-phenanthroline, 5,6-dmp=5,6-dimethyl-1,10-phenanthroline and imp=imidazo[4,5-f][1,10]-phenanthroline, have been isolated and characterised by elemental analysis. The interaction of these complexes with calf thymus DNA has been explored by using absorption, emission and circular dichroic spectral and electrochemical studies and viscosity measurements. Absorption spectral studies and viscosity measurements reveal that the complexes 1 and 2 interact with DNA primarily by groove binding while 3 and 4 may possibly bind by two modes. The complexes 5 and 6 associate with DNA involving the non-classical intercalation of the imp ligand with DNA base pairs. Circular dichroic spectral studies reveal that calf thymus B-DNA becomes more A-like in structure on interaction with all the complexes except 5. The complex 5, in contrast, exhibit a spectral band shift of about 10 nm to longer wavelength, which is characteristic of A conformation of DNA. Electrochemical studies reveal that the phen complexes facilitate the electrostatic interaction of cobalt(III) complexes, preferentially in the higher oxidation state of cobalt. On the other hand, the presence of 5,6-dimethyl groups on phen favour DNA groove binding of complexes. The presence of hydrophobic imp ligand facilitates its intercalative interaction with the hydrophobic interior of the nucleic acid bases, interestingly, in the lower oxidation state of cobalt.