Effect of copper-sulphur bond on the DNA photo-cleavage activity of 2-(methylthio)ethylpyridine-2-carbaldimine copper(II) complexes

The binding and photo-induced DNA cleavage activity of a binary complex [CuL2](ClO4)2 (1) and the in situ generated ternary complexes [CuLB](ClO4)2 from 1 (B: 1,10-phenanthroline, phen, 2; dipyrido[3,2-d: 2′,3′-f]quinoxaline, dpq, 3) are studied, where L is a N2S-donor tridentate Schiff base 2- (methylthio)ethylpyridine-2-carbaldimine. Complex 1, structurally characterized by X-ray diffraction study, has six-coordinate meridional geometry showing CuN4S2 coordination. The Cu-N bond lengths are in the range of 1·968(3) to 2·158(4) Å. The Cu-S bond lengths of 2·599(2) and 2·705(2) Å are significantly long indicating weak covalent interaction between copper and sulphur atoms. The thiomethyl groups are cis to each other giving S-Cu-S angle of 75·82(5)°. The Cu-N(pyridyl) bond distances are longer than the Cu-N(imine) bonds. The complexes are redox active and display a quasi-reversible cyclic voltammetric response assignable to the Cu(II)/Cu(I) couple near 0·0 V vs SCE in DMF-Tris buffer (1: 4 v/v) using 0·1 M KCl as supporting electrolyte. Electronic spectra of the complexes show a d-d band in the range 630 to 700 nm in DMF along with higher energy charge transfer bands. While complex 1 is a poor binder to DNA, the ternary complexes show good DNA binding propensity. The photo-nuclease activity of 1–3 is studied using UV and visible wavelengths. The DNA cleavage activity at 365 nm follows the order: 3 > 2 > 1. The cleavage reaction involves the formation of singlet oxygen as the reactive species in a type-II process.