Interaction of macrocyclic copper(II) complexes with calf thymus DNA: effects of the side chains of the ligands on the DNAbinding behaviors

Three macrocyclic copper(II) complexes of formula CuL1(ClO4)2 (L1 = 1,4,8,11-tetraazacyclotetradecane), CuL2Cl2 (L2 = 3,10-bis-phenethyl1,3,5,8,10,12-hexaazacyclotetradecane) and CuL3Cl2 (L3 = 3,10-bis-n-propyl-1,3,5,8,10,12-hexaazacyclotetradecane) have been synthesized and characterized by elemental analysis, infrared spectroscopy and mass spectrometry. X-Ray structural analysis of CuL2(ClO4)2 showed each copper atom to be coordinated to four nitrogen atoms in the macrocycle, which has a square-planar coordination geometry. Absorption and fluorescence spectral studies, circular dichroic spectral and viscometric studies have been carried out to assess the interaction of the three complexes with calf thymus DNA. The results suggest that three complexes can bind to DNA by different binding modes. [CuL1]2+ and [CuL3] 2+ can bind to DNA by electrostatic interaction whereas [CuL2]2+ may bind to DNA by intercalation of the aromatic ring into the base pairs of DNA. The functional groups on the side chain of the macrocycle play a key role in deciding the mode and extent of binding of the copper complexes to DNA. The copper complexes exhibit nuclease activities, in which circular plasmid pUC18 DNA is initially converted to nicked DNA.