Ferrocene-Conjugated l-Tryptophan Copper(II) Complexes of Phenanthroline Bases Showing DNA Photocleavage Activity and Cytotoxicity

Ferrocene-conjugated l-tryptophan (l-Trp) reduced Schiff base (Fc-TrpH) copper(II) complexes [Cu(Fc-Trp)(L)](ClO4) of phenanthroline bases (L), viz. 2,2′-bipyridine (bpy in 1), 1,10-phenanthroline (phen in 2), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 3), and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 4), were prepared and characterized and their photocytotoxicity studied. Cationic reduced Schiff base (Ph-TrpH) complexes [Cu(Ph-Trp)(L)(H2O)](ClO4) (L = phen in 5; dppz in 6) having the ferrocenyl moiety replaced by a phenyl group and the Zn(II) analogue (7) of complex 4 were prepared and used as control species. The crystal structures of 1 and 5 with respective square-planar CuN3O and square-pyramidal CuN3O2 coordination geometry show significantly different core structures. Complexes 1–4 exhibit a Cu(II)–Cu(I) redox couple near −0.1 V and the Fc+–Fc couple at ∼0.5 V vs SCE in DMF–0.1 M [Bun4N](ClO4) (Fc = ferrocenyl moiety). The complexes display a copper(II)-based d–d band near 600 nm and a Fc-centered band at ∼450 nm in DMF–Tris-HCl buffer. The complexes are efficient binders to calf thymus DNA. They are synthetic chemical nucleases in the presence of thiol or H2O2, forming hydroxyl radicals. The photoactive complexes are cleavers of pUC19 DNA in visible light, forming hydroxyl radicals. Complexes 2–6 show photocytotoxicity in HeLa cancer cells, giving IC50 values of 4.7, 10.2, 1.3, 4.8, and 4.3 μM, respectively, in visible light with the appearance of apoptotic bodies. The complexes also show photocytotoxicity in MCF-7 cancer cells. Nuclear chromatin cleavage has been observed with acridine orange/ethidium bromide (AO/EB) dual staining with complex 4 in visible light. The complexes induce caspase-independent apoptosis in the HeLa cells.