Impact of metal on the DNA photocleavage activity and cytotoxicity of ferrocenyl terpyridine 3d metal complexes

Abstract

Ferrocenyl terpyridine 3d metal complexes and their analogues, viz. [M(Fc-tpy)₂](ClO₄)₂ (1–4), [Zn(Ph-tpy)₂](ClO₄)₂ (5) and [Zn(Fc-dpa)₂]X₂ (X = ClO₄, 6; PF₆, 6a), where M = Fe(II) in 1, Co(II) in 2, Cu(II) in 3 and Zn(II) in 4, Fc-tpy is 4′-ferrocenyl-2,2′:6′,2′′-terpyridine, Ph-tpy is 4′-phenyl-2,2′:6′,2′′-terpyridine and Fc-dpa is ferrocenyl-N,N-dipicolylmethanamine, are prepared and their DNA binding and photocleavage activity in visible light studied. Complexes 2, 4, 5 and 6a that are structurally characterized by X-ray crystallography show distorted octahedral geometry with the terpyridyl ligands binding to the metal in a meridional fashion, with Fc-dpa in 6a showing a facial binding mode. The Fc-tpy complexes display a charge transfer band in the visible region. The ferrocenyl (Fc) complexes show a quasi-reversible Fc⁺-Fc redox couple within 0.48 to 0.66 V vs. SCE in DMF-0.1 M TBAP. The DNA binding constants of the complexes are ∼10⁴ M⁻¹. Thermal denaturation and viscometric data suggest DNA surface binding through electrostatic interaction by the positively charged complexes. Barring the Cu(II) complex 3, the complexes do not show any chemical nuclease activity in the presence of glutathione. Complexes 1–4 exhibit significant plasmid DNA photocleavage activity in visible light via a photoredox pathway. Complex 5, without the Fc moiety, does not show any DNA photocleavage activity. The Zn(II) complex 4 shows a significant PDT effect in HeLa cancer cells giving an IC₅₀ value of 7.5 μM in visible light, while being less toxic in the dark (IC₅₀ = 49 μM).