Synthesis and characterization of water‐soluble copper(II), cobalt(II) and zinc(II) complexes derived from 8‐hydroxyquinoline‐5‐sulphonic acid: DNA binding and cleavage studies

Abstract

Three water‐soluble complexes, [Cu2L2Cl2] (1), [CoL2(im)2] (2) and [ZnLClH2O] (3) (HL = 8‐hydroxyquinoline‐5‐sulphonic acid; im = N‐methylimidazole), were prepared and characterized using various spectral techniques. The DNA binding behaviour of complexes 1–3 was studied using UV–visible and circular dichroism (CD) spectra and cyclic voltammetry. All three complexes exhibit hypochromism but complexes 1 and 3 alone give a red shift of 4 nm with a significant binding constant of Kb = 2.1 × 104 and 1.0 × 104 M−1, respectively, but complex 2 shows no red shift with lower Kb of 4.1 × 103 M−1. The voltammetric E1/2 of complex 1 on interaction with herring sperm DNA shifts to a more positive potential, as expected, than complex 2 due to higher DNA affinity. Additionally, analysis of electrochemical data yields a value of K+/K2+ greater than one suggesting that complex 1 binds to DNA through intercalation in the M(I) state. Evidently in CD spectral analysis, complex 1 exhibits a decrease in molar ellipticity with a red shift of 10 nm and a significant decrease in intensity compared to complexes 2 and 3. This clearly indicates that complex 1 induces the B → A transition to a greater extent than 2 and 3. Oxidative cleavage using circular plasmid pUC18 DNA with complex 1 was investigated using gel electrophoresis. Interestingly, complex 1 displays a strong DNA binding affinity and is efficient in cleaving DNA in the presence of H2O2 at pH = 8.0 at 37 °C.