The interaction of a novel mixed ligand copper (II) complex, [Cu(N-N)(L)(EtOH)](NO3)2 · 2H2O, in which N-N indicates 2,9-dimethyl-1,10-phenanthroline and L indicates N,N-dimethyltrimethylenediamine with calf thymus DNA was investigated by absorption, circular dichroism, voltammetric, and viscosimetric techniques. The absorption spectra of the complex with calf thymus DNA showed a marked hypochromism in the π → π* and metal to ligand charge transfer (MLCT) transitions, with no obvious red shift attributed to a partial intercalation. The intrinsic binding constant (Kb) was determined as 2 × 105 M−1. There was slight to appreciable changes in the relative viscosity of DNA, which is consistent with enhanced hydrophobic interaction of the methyl-substituted phen ring and partial intercalation mode of binding. Electrochemical studies showed a decrease in the peak current, which is ascribed to the strong binding between Cu (II) complex and DNA. The fluorescence spectral characteristics showed that the Cu (II) complex is able to displace the methylene blue bound to DNA, but not as complete as intercalative molecules. It is remarkable that this mixed ligand complex, in contrast to [Cu(2,9-dmp)2]+ (2,9-dmp = 2,9-dimethyl-1,10-phenanthroline), which fails to cleave DNA, has ability to cleave the supercoiled plasmid DNA.
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