SOD activities of the copper complexes with tripodal polypyridylamine ligands having a hydrogen bonding site

As a structural mimic of the Arg 141 residue near copper site in native bovine Cu,Zn–superoxide dismutase (Cu,Zn–SOD), four mononuclear copper complexes with tris(2-pyridylmethyl)amine derivatives having a hydrogen bonding site (pivalamido, neopentylamino, or amino groups) at the pyridine 6-position, [Cu(tnpa)(OH)]ClO4 (1), [Cu(tapa)Cl]ClO4 (2), [Cu(tapa)(OH)]ClO4 (3), and [Cu(bppa)](ClO4)2 (4), and two analogous dinuclear copper complexes, [Cu2(tppen)(H2O)2](ClO4)4 (5) and [Cu2(tppen)Cl4] (6), were prepared, and the correlation between the coordination structures of the copper complexes and their superoxide dismutation activities were examined. Their structures in both solution and solid states were characterized by electronic absorption and ESR spectroscopic (for all the complexes) and by X-ray analytical methods (for 1, 2, 5 and 6), respectively. The coordination geometries around the copper ions were determined to be five-coordinate trigonal bipyramidal for 1, 2 and 3 and to be an intermediate of five-coordinate trigonal bipyramidal and square pyramidal for 5 and 6 in both crystal and solution phases, in contrast to four-coordinate square planar structure for 4 reported previously. The cyclic voltammetry measurement of the mononuclear complexes 1–4 showed quasi-reversible redox potentials (Cu(II)/Cu(I) couple) in the range between −330 mV (vs. NHE at pH 7; O2/O2radical dot−) and +890 mV (vs. NHE at pH 7; O2radical dot−/H2O2), which are in the range responsible for superoxide dismutation (SOD) reaction, although the dinuclear copper complexes, 5 and 6, gave only reduction potentials. The SOD activities of complexes 1, 2, and 3 were moderate and those of 5 and 6 were rather high, although 4 showed the lowest activity of all. Those of 1, 2 and 3 with a trigonal bipyramidal structure and hydrogen bonding interaction site are slightly high in comparison with that of the corresponding Cu(II) complex without hydrogen bonding site, [Cu(tpa)(H2O)](ClO4)2 (7). Relatively higher SOD activity observed in complexes 5 and 6 may be explained in terms of higher flexibility in the conformation and cooperativity by dinuclear copper ions.