Experimental and theoretical studies on the DNA-binding and spectral properties of water-soluble complex [Ru(MeIm)4(dpq)]2+

A new water-soluble Ru(II) complex [Ru(MeIm)4(dpq)]2+1, has been synthesized and characterized by elemental analysis, 1H NMR, ESI-MS and UV–Vis. The interaction of the complex with CT-DNA has been explored by using electronic absorption titration, competitive binding experiment, circular dichroic (CD) spectra, thermal denaturation and viscosity measurements. The experimental results show that: the title complex can bind to DNA in an intercalative mode and its DNA-binding affinity is weaker (Kb = 1.2 × 104 M−1) than that of the complex [Ru(bpy)2(dpq)]2+2 with bidentate co-ligands (Kb = 4.7 × 104 M−1). Here a very interesting finding is that the hypochromism of the title complex is not linear relation to its DNA-binding affinity. In addition, some significant thermodynamic parameters of the binding of the title complex to DNA, e.g., the changes of free energy at melting temperature, standard enthalpy, and standard entropy (ΔGT0, ΔH0, and ΔS0), were determined and calculated. In order to deeply explain the experimental findings, the DFT/TDDFT computations were carried out. On the basis of the DFT/TDDFT results and the frontier molecular orbital theory, the trend in DNA-binding affinities and the spectral properties as well as the interesting phenomena of larger extent of hypochromism but relatively smaller Kb value for the title complex 1 compared with complex 2 were reasonably explained.