Anaerobic DNA cleavage in red light by dicopper(II) complexes on disulphide bond activation

Abstract

Binuclear complexes [Cu(µ-RSSR)]2 (1) and [M2(µ-PDS)(H2O)]2 (M = Cu(II), 2; Fe(II), 3), where H2RSSR is a reduced Schiff base derived from 2-(thioethyl)salicylaldimine having a disulphide moiety and H2PDS is derived from dimerization of D-penicillamine, have been prepared, structurally characterized, and their photo-induced DNA cleavage activity studied. The crystal structure of 1 shows the complex as a discrete binuclear species with each metal in a CuN2O2 square-planar geometry (Cu…Cu, 6·420 Å). The tetradentate RSSR2− acts as a bridging ligand. The sulphur atoms in the disulphide unit do not interact with the metal ions. Complexes 1–3 do not show any DNA cleavage activity in darkness. The copper(II) complexes exhibit chemical nuclease activity in the presence of 3-mercaptopropionic acid. Cleavage of supercoiled DNA has been observed in UV-A light of 365 nm for 1 and red light of 647·1 nm for both 1 and 2 in air. Mechanistic data reveal the involvement of the disulphide unit as photosensitizer generating hydroxyl radicals (•OH) as the reactive species. Photo-induced DNA cleavage in red light seems to involve sulphide radicals in a type-I process and hydroxyl radicals. The dicopper(II) complexes show significant anaerobic photo-induced DNA cleavage activity in red light under argon following type-I pathway without involving any reactive oxygen species.