Significant photocytotoxic effect of an iron(iii) complex of a Schiff base ligand derived from vitamin B6 and thiosemicarbazide in visible light

Abstract

Iron(III)–Schiff base complexes, namely, [Fe(tsc-py)₂](NO₃) (1), [Fe(tsc-acpy)₂](NO₃) (2) and [Fe(tsc-VB₆)₂](NO₃) (3), where tsc-py, tsc-acpy and tsc-VB₆ are the respective Schiff bases derived from thiosemicarbazide (tsc) and pyridine-2-aldehyde (tsc-py), 2-acetyl pyridine (tsc-acpy) and vitamin B₆ (pyridoxal, tsc-VB₆), have been prepared, structurally characterized and their photocytotoxicity studied in cancer HeLa cells. The single crystal X-ray structures of the complexes 1 and 2 show a distorted octahedral geometry formed by the FeN₄S₂ core. The low-spin and 1 : 1 electrolytic complexes display a broad absorption band in the visible region. Complexes 1 and 2, without any VB₆ moiety are not cytotoxic under light or dark conditions. Complex 3 is significantly photocytotoxic under visible light of 400–700 nm giving an IC₅₀ value of 22.5 μM in HeLa cells with no dark toxicity (IC₅₀ > 100 μM). The photo-induced cell death is attributable to apoptotic pathways involving photo-assisted generation of intracellular ROS. The observed photocytotoxicity of complex 3 could be the result of its better photosensitizing property combined with its enhanced uptake into cancer cells via a VB₆ transporting membrane carrier (VTC) mediated diffusion pathway due to the presence of the VB₆ moiety compared to the two non-vitamin B₆ analogues, complexes 1 and 2.