Photocleavage of DNA by copper(II) complexes

The chemistry of ternary and binary copper(II) complexes showing efficient visible lightinduced DNA cleavage activity is summarized in this article. The role of the metal in photo-induced DNA cleavage reactions is explored by designing complex molecules having a variety of ligands. Ternary copper(II) complexes with amino acid like L-methionone or L-lysine and phenanthroline base are efficient photocleavers of DNA. Complexes of formulation $[Cu(L^n)(phen)](ClO_4)$ with NSO-donor Schiff base $(HL^n)$ and NN-donor heterocyclic base 1,10-phenanthroline (phen) show significant cleavage of supercoiled (SC) DNA on exposure to red light at \approx 700 nm. The d–d and CT electronic bands of the copper(II) complexes play important roles in DNA cleavage reactions. The mechanistic pathways are found to be dependent on the types of ligands present in the copper(II) complexes and the photo-excitation energy. While UV exposure generally proceeds via a type-II process forming singlet oxygen as the reactive species, red-light exposure leads to DNA cleavage following different mechanistic pathways, viz. type-I, type-II and photo-redox pathways. Ternary copper(II) complexes with phen as DNA binder and Schiff base with a thiomethyl group as photosensitizer, cleave SC DNA to its nicked circular (NC) form in a type-II process in red-light. The binary complex $[Cu(dpq)_2(H_2O)](ClO_4)_2$ (dpq, dipyridoquinoxaline) cleaves DNA by photo-redox pathway at 694 nm. The binuclear complex $[Cu^{II} _2(RSSR)_2]$, where $H_2RSSR$ is a Schiff base derived from 2-(thioethyl)salicylaldimine, cleaves SC DNA at 632.8nm (CW He–Ne laser) and 694 nm (ruby laser) involving sulphide (type-I process) and hydroxyl radicals (photo-redox pathway) as the reactive species