Abstract
Three Cu(II) complexes of bis-pyrazole based ligands have been synthesized and structurally characterized by X-ray crystallography. One of the ligand (L2) contains a methionine ester conjugated to a bis-pyrazole carboxylate through an amide linkage. The binding constant for complexes 1-3 with CT DNA are of the order of 10(4) M(-1). The crystal structure suggests that the axial Cu-O bonds (ca. 2.31(4) Å) are relatively labile and hence during the redox cycle with ascorbic acid and oxygen one or both the axial Cu-O bonds might open to promote copper oxygen reaction and generate ROS. The chemical nuclease activity of complexes 1-3 in dark, show complete relaxation of supercoiled DNA at 100 μM concentration in presence of ascorbic acid (H2A). The mechanistic investigation suggests that the complexes 1 and 2 show involvement of peroxo species whereas 3 shows involvement of both singlet oxygen and peroxo species in DNA cleavage. The singlet oxygen formation in dark is otherwise unfavourable but the presence of methionine as pendant arm in L2 might activate the generation of singlet oxygen from the metal generated peroxo species. The results of DNA cleavage studies suggest that methionine based copper(II) complexes can promote dual pathway for DNA cleavage. Probing the cytotoxic activity of these complexes on MCF-7, human breast cancer cell line shows that 3 is the most effective one with an IC50 of 70(2) μM.
Highlights
Ascorbic acid (H2A) and glutathione (GSH) are two main extracellular and intracellular physiological reductants which may promote the redox process of metal ions
In our quest to study the effect of methionine when conjugated in a CuII complex we present the syntheses, structure, DNA cleavage and cytotoxicity studies of three CuII complexes of bispyrazolyl ligands (Scheme 1) to show that presence of methionine in copper complexes may lead to activation of a dual pathway promoting peroxide and singlet oxygen based DNA cleavage in dark leading to better cytotoxicity
The DNA cleavage studies emphasize that in CuII complexes the presence of methionine as a pendant moiety in the ligand can help to generate additional reactive oxygen species (ROS) and induce DNA damage through multiple pathways
Summary
Ascorbic acid (H2A) and glutathione (GSH) are two main extracellular and intracellular physiological reductants which may promote the redox process of metal ions. The time dependent DNA cleavage studies for 1–3 (100 μM) were done same as above except that the samples were incubated for variable time (0–60 min) in presence of 5 molar equivalent of H2A and reaction mixture was quenched by addition of loading dye followed by keeping at −30 °C. Cisplatin was dissolved in DMSO just before the experiment and a calculated amount of drug solution was added to the growth medium containing cells such that the final DMSO concentration in the wells were no more than 0.2%. After 48 h the media containing compound was removed and fresh media was added to each well and successively treated with 20 μL of a 2 mg mL−1 MTT in saline solution, followed by 3 h of incubation at 37 °C in 5% carbon dioxide atmosphere. IC50 values represent the drug concentration that reduces the mean absorbance at 515 nm to 50% as compared to the untreated control wells
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