Photocytotoxicity and near-IR light DNA cleavage activity of oxovanadium(IV) Schiff base complexes having phenanthroline bases

Oxovanadium(IV) complexes VO(L)(B)] (1-3), where H₂L is a Schiff base ligand 2-(2-hydroxybenzylideneamino)phenol and B is 1,10-phenanthroline (phen for 1), dipyrido3,2-d:2′,3′-f]quinoxaline (dpq for 2) or dipyrido3,2-a:2′,3′-c]phenazine (dppz for 3), have been prepared, characterized and their DNA binding property and photo-induced DNA cleavage activity studied. Complex 3 which is structurally characterized by X-ray crystallography shows the presence of an oxovanadium(IV) moiety in a six coordinate VO₃N₃ coordination geometry. The complexes show a d-d band within 800-850 nm in DMF. The complexes display an oxidative response near 0.7 V versus SCE for V(V)-V(IV) and a reductive response within −1.1 to −1.3 V due to V(IV)-V(III) couple in DMF-0.1 M TBAP. The complexes are avid binders to calf thymus DNA giving binding constant values of 4.2 × 10⁴ to 1.2 × 10⁵ M⁻¹. The complexes do not show any “chemical nuclease” activity in dark. The dpq and dppz complexes are photocleavers of plasmid DNA in UV-A light of 365 nm via ¹O₂ pathway and in near-IR light (752.5 to 799.3 nm IR optics) by HO^• pathway. Complex 3 exhibits significant photocytotoxicity in visible light in HeLa cells giving IC₅₀ value of 13 μM, while it is less toxic in dark (IC₅₀ = 97 μM).