DNA photonuclease activity of four new copper(ii) complexes under UV and red light: theoretical/experimental correlations with active species generation

Abstract

In this study, four new mononuclear copper(II) complexes, [Cu(L₁)(dpq)]ClO₄ (2), [Cu(L₁)(phen)]ClO₄ (3), [Cu(L₂)(dpq)]ClO₄ (4), [Cu(L₁)(OAc)] (5), and the known complex [Cu(dpq)₂(H₂O)]ClO₄ (1), were synthesized and characterized by various methods (dpq = dipyrido[3,2-d:2′,3′-h]quinoxaline, L₁ = (3,5-di-tert-butyl-2-hydroxybenzyl)(2-pyridylmethyl)imine, L₂ = (2-hydroxybenzyl)(2-pyridylmethyl)imine). The structure of [Cu(L₁)(dpq)]ClO₄ (2) was determined by X-ray crystallography and reveals that the Cu^II center is coordinated by dpq and by the N₂O donor ligand L₁ in a distorted square pyramidal arrangement. The interaction of 1–5 with calf thymus DNA was measured and the binding constant K_b was obtained for the complexes, with values strongly dependent on the intercalating and/or binding properties of the ligands present in the structure of each complex. Also, the nuclease and photonuclease activity in terms of plasmid DNA cleavage with addition of reactants was monitored, in order to verify the mechanism through which the complexes cleave DNA. The complexes displayed good activity under both UV (365 nm) and red light (635 nm) irradiation, with the Type II mechanism prevailing in the case of the former and Type I mechanism for the latter. Finally, two new correlations between the experimental and theoretical parameters are discussed and some hypotheses regarding the active species generation are proposed.