Synthesis, characterization, DNA-binding and spectral properties of complexes [Ru(L)4(dppz)]2+ (L = Im and MeIm)

Two new Ru(II) complexes [Ru(L)4(dppz)]2+ (L = imidazole (Im), 1-methylimidazole (MeIm); dppz = dipyrido[3,2-a:2′,3′-c]phenazine), have been synthesized and characterized in detail by elemental analysis, 1H NMR, Electrospray ionization mass spectrometry (ESI-MS) and UV–visible (UV–Vis) spectroscopic techniques. The interaction of these complexes with calf thymus DNA (CT-DNA) has been explored by using electronic absorption titration, competitive binding experiment, circular dichroism (CD), thermal denaturation and viscosity measurements. The experimental results show that: both the two complexes can bind to DNA in an intercalation mode; the DNA-binding affinity of complex [Ru(Im)4(dppz)]2+1 (Kb = 2.5 × 106 M−1) is greater than that of complex [Ru(MeIm)4(dppz)]2+2 (Kb = 1.1 × 106 M−1). Moreover, it is very interesting to find that the circular dichroic spectrum of DNA–complex 1 adduct, in which both bands centered at 277 nm and 236 nm are all negative, is very different from those of DNA–complex 2 adduct and other Ru(II) complexes binding to DNA in general intercalation mode. It may be due to the hydrogen-bonding effect or the contribution of induced CD signals of complex 1. Another interesting finding is that the hypochromism of the complexes is not linear relation to their DNA-binding affinities. In order to deeply study these experimental phenomena and trends, the density functional theory (DFT) and time-dependent DFT (TDDFT) computations were carried out, and on the basis of the DFT/TDDFT results and the frontier molecular orbital theory, the trend in DNA-binding affinities, the spectral properties as well as the interesting phenomena of larger extent of hypochromism but relatively smaller Kb values for the title complexes have been reasonably explained.