Synthesis and characterization of imidazolate-bridged dinuclear complexes as active site models of Cu,Zn-SOD

New dinucleating ligands having two metal-binding sites bridged by an imidazolate moiety, Hbdpi and HMe(4)bdpi (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole, HMe(4)bdpi = 4,5-bis(di(6-methyl -2-pyridylmethyl)aminomethyl)imidazole), have been designed and synthesized as model ligands for copper-zinc superoxide dismutase (Cu,Zn-SOD). The corresponding mononucleating ligand, MeIm(Py)(2) (MeIm(Py)(2) = ((1-methyl-4-imidazolyl)methyl)bis(2-pyridylmethyl)amine), has also been synthesized for comparison. The imidazolate-bridged Cu(II)-Cu(II) homodinuclear complexes represented as [Cu-2(bdpi)(CH3CN)(2)](ClO4)(3). CH3-CN . 3H(2)O (1), [Cu-2(Me(4)bdi)(H2O)(2)](ClO4)(3). 4H(2)O (2), a Cu(II)-Zn(II) heterodinuclear complex of the type of [CuZn(bdpi)(CH3CN)(2)](ClO4)(3). 2CH(3)CN (3), and a Cu(II) mononuclear complex of [Cu(MeIm(Py)(2)(CH3CN)](ClO4)(2). CH3CN (4) have been synthesized, and the structures of complexes 1-4 were determined by X-ray crystallography. The Cu(II)-Zn(II) distance of 6.197(2) Angstrom in 3 agrees well with that of native Cu,Zn-SOD (6.2 Angstrom). All the metals in 1-4 have pentacoordinate geometries with the imidazolate or 1-methylimidazole nitrogen, two pyridine nitrogens, the tertiary amine nitrogen, and a solvent (CH3CN or H2O). The coordination site occupied by a solvent can be susceptible to ligand substitution, providing a binding site for substrate superoxide. Magnetic measurements of the Cu(II)-Cu(II) homodinuclear complexes 1 and 2 have shown an antiferromagnetic exchange interaction with a coupling constant of -2J = 73.4 and 145.9 cm(-1), respectively. The ESR spectra of 1 and 2 exhibited broad signals centered at g congruent to 2.13 due to the spin-spin interaction between two copper ions, while the ESR spectrum of the Cu(II)-Zn(II) heterodinuclear complex 3 showed a signal which is characteristic of mononuclear trigonal-bipyramidal Cu(II) complexes (g(parallel to) = 2.10, g(perpendicular to) = 2.24, /A(parallel to)/= 11.7 mT, and /A(perpendicular to)/= 12.4 mT). The cyclic voltammograms of homodinuclear complexes (1 and 2) in CH3CN gave two reversible waves which correspond to the Cu(I,I)/Cu(I,II) and Cu(I,II)/Cu(II,II) redox processes: E-1/2 = -0.31 and -0.03 V vs Ag/AgCl for 1 and E-1/2 = -0.29 and +0.12 V vs Ag/AgCl for 2, respectively. On the other hand, the Cu(II)-Zn(II) heterodinuclear complex 3 exhibited one Cu(I)/(II) reversible wave, E-1/2 = -0.03 V vs Ag/AgCl, which is shifted in a positive direction (0.21 and 0.19 V) as compared to those of the corresponding Cu(II) mononuclear complexes. All the examined complexes catalyzed the dismutation of superoxide at biological pH; the SOD activity increased in the order 2 < 1 < 3.