A Series of Oxyimine-Based Macrocyclic Dinuclear Zinc(II) Complexes Enhances Phosphate Ester Hydrolysis, DNA Binding, DNA Hydrolysis, and Lactate Dehydrogenase Inhibition and Induces Apoptosis

A symmetrical macrocyclic dizinc(II) complex (1) has been synthesized by using the ligand (L-1) [mu-11,24-dimethyl-4,7,16,19-tetraoxa-3,8,15,20-tetraazatricyclo-[2 0. 3. 1. 1 (1 0 , 1 3)] heptacosa-1 (25),2,7,9,11,13(27),14,20,22(26),23-decaene-26,27-diol]. A series of unsymmetrical macrocyclic dizinc(II) complexes (2-6) has been synthesized by Schiff base condensation of bicompartmental mononuclear complex [ZnL] [mu-3,16-dimethyl-8,11-dioxa-7,12-diazadicyclo-[1.1(14,18)] heptacosa-1,3,5(20),6,12,14,16,18(19)-octacaene-19,20-diolato)zinc(II)] with various diamines like 1,2-diamino ethane (L-2), 1,3-diamino propane (L-3), 1,4-diamino butane (L-4), 1,2-diamino benzene (L-5), and 1,8-diamino naphthalene (L-6). The ligand L-1 and all the zinc(II) complexes were structurally characterized. To corroborate the consequence of the aromatic moiety in comparison to the aliphatic moiety present in the macrocydic ring on the phosphate ester hydrolysis, DNA binding and cleavage properties have been studied. The observed first order rate constant values for the hydrolysis of 4-nitrophenyl phosphate ester reaction are in the range from 2.73 X 10(-2) to 9.86 X 10(-2) s(-1). The interactions of complexes 1-6 with calf thymus DNA were studied by spectroscopic techniques, including absorption, fluorescence, and circular dichroism spectroscopy. The DNA binding constant values of the complexes were found in the range from 1.80 X 10(5) to 9.50 X 10(5) M-1, and the binding affinities are in the following order: 6 > 5 > 1 > 2 > 3 > 4. All the dizinc(II) complexes 1-6 effectively promoted the hydrolytic cleavage of plasmid pBR322 DNA under anaerobic and aerobic conditions. Kinetic data for DNA hydrolysis promoted by 6 under physiological conditions give the observed rate constant (k(obs)) of 4.42 +/- 0.2 h(-1), which shows a 10(8)-fold rate acceleration over the uncatalyzed reaction of ds-DNA. The comparison of the dizinc(II) complexes 1-6 with the monozinc(II) complex [ZnL] indicates that the DNA cleavage acceleration promoted by 1-6 are due to the efficient cooperative catalysis of the two close proximate zinc(II) cation centers. The ligand L-1, dizinc(II) complexes 1, 3, and 6 showed cytotoxicity in human hepatoma HepG2 cancer cells, giving IC50 values of 117, 37.1, 16.5, and 8.32 mu M, respectively. The results demonstrated that 6, a dizinc(II) complex with potent antiproliferative activity, is able to induce caspase-dependent apoptosis in human cancer cells. Cytotoxicity of the complexes was further confirmed by the lactate dehydrogenase enzyme level in HepG2 cell lysate and content media.