Different modes of DNA cleavage activity of dihydroxo-bridged dicopper(II) complexes having phenanthroline bases

Abstract

Dihydroxo-bridged dicopper(II) complexes [{Cu(phen)}₂(μ-OH)₂](ClO₄)₂ (1), [{Cu(dpq)}₂(μ-OH)₂](ClO₄)₂ (2) and [{Cu(dppz)(DMF)}₂(μ-OH)₂](PF₆)₂ (3), where phen, dpq and dppz are 1,10-phenanthroline, dipyridoquinoxaline and dipyridophenazine, respectively, are prepared and their DNA binding and cleavage properties studied. Complex 3 has been structurally characterized by X-ray crystallography. The complexes have a {Cu₂(μ-OH)₂}₂₊ core with an essentially planar arrangement of two CuN₂O₂ basal planes. The complexes are avid binder to calf thymus DNA (K_app value of 4.8 × 10⁶ and 5.9 × 10⁶ M^-1 for 2 and 3, respectively, from ethidium displacement assay) and exhibits significant cleavage of supercoiled (SC) pUC19 DNA in dark in presence of mercaptopropionic acid. Besides, the dpq and dppz complexes display photo-induced DNA cleavage on UV (312 nm) and red light (632.8 nm) irradiations in absence of any additives. Mechanistic investigations reveal minor groove binding for the phen and dpq complexes, and major groove preference for the dppz species. The oxidative DNA cleavage reactions in presence of mercaptopropionic acid as a reducing agent involve hydroxyl radicals. The photo-cleavage reactions at UV light involve singlet oxygen as the reactive species, while similar reactions on red light irradiation (632.8 nm) proceed through the formation of hydroxyl radical. The complexes show significant DNA hydrolase activity in absence of any additives under dark reaction conditions.