Probing the Antioxidant Action of Selenium and Sulfur Using Cu(I)-Chalcogenone Tris(pyrazolyl)methane and -borate Complexes

Hydroxyl radical generated from the reaction of Cu+ with hydrogen peroxide results in oxidative DNA damage, and this damage is implicated in aging, cancer, and many other diseases. Selenium- and sulfur-containing compounds can act as antioxidants, and coordination of selenium and sulfur to copper is one explanation for this antioxidant activity. To determine how copper coordination results in antioxidant activity, biologically relevant tris(pyrazolyl)methane and borate Cu+ complexes of the formulas Tp*Cu(L) and [Tpm(R)Cu(L)](+), where (L = N, W-dimethylimidazole selone, dmise; N, N'-dimethylimidazole thione, dmit; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; Tpm(R) = tris(pyrazolyl)methane, R = H; Tpm, R = Me; Tpm*, R = /Pr; TpmPr), have been synthesized and characterized. The structures of complexes Tp*Cu(Dmit), Tp*Cu(dmise), [Tpmtu(dmise)][BF(4)1, and [TpmRCu(Dmit)][BF4] (where R = H; Tpm, R = Me; Tpm*, R = /Pr; Tpm(Pr)) were determined by X-ray crystallography. All the Cu+ centers adopt distorted tetrahedral coordination geometry, and Cu-Se and Cu-S distances for all the complexes are approximately 2.30 angstrom, and 2.20 angstrom, respectively. The effects of counterion and steric bulk at the 3 and 5 positions of the pyrazolyi ring on the structural and spectroscopic properties are discussed. Selone or thione coordination to copper significantly alters the Cu+/2+ redox potential: Cu-selone complexes have Cu2+/+ potentials from 283 to 390 mV, whereas those of Cuthione complexes range from 70 to 232 mV versus NHE. The Cu-selone complexes have Cu2(+/+) potentials near or below that of the cellular reductant NADH (-324 mV). Thus, selenium and sulfur coordination to copper in biological systems may prevent the Cu2+ reduction by NADH required for the catalytic formation of damaging hydroxyl radical.