Photo-activated cytotoxicity of a pyrenyl-terpyridine copper(II) complex in HeLa cells

Terpyridine copper(II) complexes [Cu(L)₂](NO₃)₂, where L is (4′-phenyl)-2,2′:6′,2′′-terpyridine (ph-tpy in 1) and [4′-(1-pyrenyl)]-2,2′:6′,2′′-terpyridine (py-tpy in 2), are prepared, characterized and their photocytotoxic activity studied. The crystal structure of complex 1 shows distorted octahedral CuN₆ coordination geometry. The 1:2 electrolytic and one-electron paramagnetic complexes show a visible band near 650 nm in DMF–H₂O. The complexes show emission band at 352 nm for 1 and 425 nm for 2 when excited at 283 and 346 nm, respectively. The Cu(II)–Cu(I) redox couple is observed near −0.2 V versus SCE in DMF–0.1 M TBAP. The complexes are avid partial-intercalative binders to calf thymus DNA giving binding constant (K_b) values of ∼10⁶ M⁻¹. Complex 2 with its photoactive pyrenyl moiety exhibits significant photocleavage of pUC19 DNA in red light via singlet oxygen pathway. Complex 2 also exhibits significant photo-activated cytotoxicity in HeLa cancer cells in visible light giving IC₅₀ value of 11.9 μM, while being non-toxic in dark with an IC₅₀ value of 130.5 μM.