A pair of dicationic metalloporphyrins with only two meso-substituents, i.e. copper(II) and zinc(II) complexes of 5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrin (H2-1), have been synthesized by insertion reaction of the metal ions. Interactions of [5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrinato]copper(II) (Cu-I) and [5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrinato]zinc(II) (Zn-1) with calf thymus DNA (CT-DNA) and double-helical synthetic polynucleotides ([poly(dA-dT)]2 and [poly(dG-dC)]2) have been studied by melting temperature, viscometric, visible absorption, CD and MCD spectroscopic measurements. The copper(II) complex, Cu-1, intercalates into the base pairs of DNA such as CT-DNA, [poly(dA-dT)]2 and [poly(dG-dC)]2 to stabilize the DNA duplex due to its high affinity for them. In contrast, the zinc(II) complex, Zn-1, binds edge-on to CT-DNA, [poly(dA-dT)]2 and [poly(dG-dC)]2 as an outside binder. The binding constant of Cu-1 to DNA is in the order of 106 M-1 in a phosphate buffer (pH 6.8) at 25 °C, while that of Zn-1 ranges from 104 to 10 5 M-1 under the same experimental conditions. The binding of Cu-1 and Zn-1 to DNA is entropically driven, although that of H 2-1 is enthalpically driven. As a result, it has been revealed that the kind of central metal ions in dicationic metalloporphyrins influences the binding properties of the dicationic porphyrin to DNA.
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