Efficient increase of DNA cleavage activity of a diiron(III) complex by a conjugating acridine group

A new diferric complex, Fe₂L_b, in which a DNA intercalator (acridine) is linked to a precursor diferric complex (Fe ₂L_a), has been designed and synthesised as a hydrolytic cleaving agent of DNA. Compared with Fe₂L_a (without the DNA intercalator) (L_a: 2,6-bis{[(2-hydroxybenzyl)(pyridin-2-yl) methylamino]methyl}-4-methylphenol), Fe₂L_b [L_b: 5-(acridin-9-yl)-N-(3,5-bis{[(2-hydroxybenzyl)(pyridin-2-yl)methylamino]- methyl}-4-hydroxybenzyl)pentanamide] leads to a 14-fold increase in the cleavage efficiency of plasmid DNA due to the binding interaction between DNA and the acridine moiety. The interaction has been demonstrated by UV/Vis absorption, CD spectroscopy, viscidity experiments and thermal denaturation studies. The hydrolytic mechanism is supported by evidence from T4 DNA ligase assay, reactive oxygen species (ROS) quenching and BNPP [bis(4-nitrophenyl) phosphate, a DNA model] cleavage experiments. The pH dependence of the BNPP cleavage by Fe ₂L_a in aqueous buffer media shows a bell-shaped pH-k _obs profile with an optimum point around a pH of 7.0 which is in good agreement with the maximum point of the pH-dependent relative concentration curve of active species from the pH titration experiments. The determination of the initial rates at a pH of 7.36 as a function of substrate concentration reveals saturation kinetics with Michaelis-Menten-like behaviour and Fe ₂L_a shows a rate acceleration increase of 4.7 × 10⁶ times in the hydrolysis of BNPP.