Comparison of the SOD-like activity of hexacoordinate Mn(II), Fe(II) and Ni(II) complexes having isoindoline-based ligands.

Recently, a series of Fe(II) complexes have been published by our group with 3N-donor 1,3-bis(2'-Ar-imino)isoindoline ligands containing various Ar-groups (pyridyl, 4-methylpyridyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl). The superoxide scavenging activity of the compounds showed correlation with the Fe(III)/Fe(II) redox potentials. Analogous, electroneutral chelate complexes with Mn(II) and Ni(II) in 2:1 ligand:metal composition are reported here. Each Mn(II) complex exhibits one reversible redox wave that is assigned as the Mn(III)/Mn(II) redox transition. The E(1/2) spans a 180 mV range from -98 (Ar=3-methylpyridyl) to 82 mV (Ar=thiazolyl) vs. the Fc(+)/Fc depending on the Ar-sidearm. The SOD-like (SOD=superoxide dismutase)activity of all complexes was determined according to the McCord-Fridovich method. The Mn(II) isoindolinates have IC(50) values - determined with 50 μM cytochrome c Fe(III) - that range from (3.22±0.39)×10(-6) (Ar=benzimidazolyl) to (10.80±0.54)×10(-6) M (Ar=N-methylbenzimidazolyl). In contrast with the Fe(II) complexes, the IC(50) concentrations show no significant dependence on the E(1/2) values in this narrow potential range emphasizing that the redox potential is not the governing factor in the Mn(II)-containing scavengers. The analogous Ni(II) compounds show no redox transitions in the thermodynamically relevant potential range (-0.40 to 0.65 V vs. SCE) and accordingly, their superoxide scavenging activity (if any) is below the detection level.