Synthesis, Crystal Structures and DNA-Cleaving Activities of [Cemp]2[MCl4] (Cemp=N-Carbethoxymethyl-1,10-phenanthrolinium, M=CuII, ZnII, CoII, NiII and MnII)

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N-Carbethoxymethyl-1,10-phenanthrolinium bromide (CempBr) and its five ionic metal complexes, [Cemp]2[MCl4] where M=CuII (1), ZnII (2), CoII (3), NiII (4) and MnII (5) were synthesized and fully characterized. Complexes 1–5 have similar structures, and consist of isolated [Cemp]+ cations and [MCl4]2− anions in which there are no obvious interactions between the oxygen or nitrogen donor atoms in [Cemp]+ and the metal center in [MCl4]2−. Agarose gel electrophoresis studies on the cleavage of plasmid pBR322 DNA by complexes 1–5 indicated that complex 1 was capable of efficiently cleaving DNA under physiological conditions, most probably via an oxidative mechanism. Kinetic assay of complex 1 afforded the maximal catalytic rate constant kmax of 0.55 h−1 and Michaelis constant KM of 47.6 µ<span style="font-variant: small-caps;">M</span>, respectively, which gives about 1.5×107-fold rate acceleration over uncatalyzed cleavage of supercoiled DNA. Ethidium bromide displacement experiments indicated that complex 1 had a binding affinity of (1.58±1.12)×106 <span style="font-variant: small-caps;">M</span>−1 toward calf-thymus DNA, 20–100-fold higher than those shown by CempBr and complexes 2–5. The high cleaving efficacy of complex 1 is thought to be due to the efficient catalysis of the copper(II)-coordinated center and the efficient binding of the quaternized 1,10-phenanthroline cation to DNA.