Electrochemical, phosphate hydrolysis, DNA binding and DNA cleavage properties of new polyaza macrobicyclic dinickel(ii) complexes

Abstract

A new class of macrobicyclic dinickel(II) complexes [Ni₂L^1,2B](ClO₄)₄ (1–6), where L^1,2 are polyaza macrobicyclic binucleating ligands, and B is a N,N-donor heterocyclic base (viz. 2,2′–bipyridine (bipy) and 1,10–phenanthroline (phen)) are synthesized and characterized. The redox, catalytic, DNA binding and DNA cleavage properties were studied. They exhibit two irreversible waves in the cathodic region around E_pc = −0.95 V and E_pa = −0.85 V vs. Ag/Ag⁺ in CH₃CN–0.1 M TBAP, respectively. The first order rate constants for the hydrolysis of 4-nitrophenylphosphate to 4-nitrophenolate by the dinickel(II) complexes 1–6 are in the range from 3.36 × 10⁻⁵ to 10.83 × 10⁻⁵ Ms⁻¹. The complexes 3 and 6 show good binding propensity to calf thymus DNA giving binding constant values (K_b) in the range from 3.08 × 10⁵ to 5.37 × 10⁵ M⁻¹. The binding site sizes and viscosity data suggest the DNA intercalative and/or groove binding nature of the complexes. The complexes display significant hydrolytic cleavage of supercoiled pBR322DNA at pH 7.2 and 37 °C. The hydrolytic cleavage of DNA by the complexes is supported by the evidence from free radical quenching and T4 ligase ligation. The pseudo Michaelis–Menten kinetic parameters k_cat = 5.44 × 10⁻² h⁻¹ and K_M = 6.23 × 10⁻³ M for complex 3 were obtained. Complex 3 also shows an enormous enhancement of the cleavage rate, of 1.5 × 10⁶, in comparison to the uncatalysed hydrolysis rate (k = 3.6 × 10⁻⁸ h⁻¹) of ds-DNA.