Photocytotoxicity and DNA cleavage activity of l-arg and l-lys Schiff baseoxovanadium(iv) complexes having phenanthroline bases

Abstract

Oxovanadium(IV) complexes [VO(sal-argH)(B)]Cl (1–3) and [VO(sal-lysH)(B)]Cl (4–6), where sal-argH₂ and sal-lysH₂ are N-salicylidene-L-arginine and N-salicylidene-L-lysine Schiff bases and B is a phenanthroline base, viz. 1,10-phenanthroline (phen in 1 and 4); dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 2 and 5) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 3 and 6), have been prepared, characterized and their DNA photocleavage activity studied. Complex 1, characterized by X-ray crystallography, shows the presence of a vanadyl group in V^IVO₃N₃ coordination geometry with a tridentate Schiff base having a pendant guanidinium moiety and bidentate phen ligand. The complexes exhibit a d–d band at ∼715 nm in 20% DMF-Tris-HCl buffer. The complexes are redox active showing cathodic and anodic responses near −1.0 V and 0.85 V (vs. SCE) for the V(IV)–V(III) and V(V)–V(IV) couples, respectively, in DMF-Tris-HCl buffer. The complexes bind to calf thymus DNA giving K_b values in the range of 3.8 × 10⁴ to 1.6 × 10⁵ M⁻¹. Thermal denaturation and viscosity data suggest DNA groove binding nature of the complexes. The complexes do not show any “chemical nuclease” activity in dark in the presence of 3-mercaptopropionic acid or H₂O₂. The dpq and dppz complexes are efficient photocleavers of plasmid DNA in UV-A (365 nm) and red light (676 nm) via singlet oxygen pathway. The dppz complexes exhibit photocytotoxicity in HeLa cancer cells giving IC₅₀ values of 15.4 μM for 3 and 17.5 μM for 6 in visible light while being non-toxic in dark giving IC₅₀ values of >100 μM.