Corroborative Models of the Cobalt(II) Inhibited Fe/Mn Superoxide Dismutases

Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide
the same set of donor atoms observed in the active site of these enzymes:  KiPr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five
new CoII complexes (1−5) were obtained and characterized by X-ray crystallography, mass spectrometry,
electrochemistry, magnetochemistry, UV−vis, and electron paramagnetic resonance (EPR) spectroscopies. The
crystal structures of 1 and 3−5 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR
data of complexes 3 and 4 agree with those of the CoII-substituted SOD, which strongly support the proposition
that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 1−5
clearly demonstrates the stabilization of the CoII state in the ligand field provided by these ligands. The irreversibility
displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement
of the geometry around the metal center for complexes 1 and 3−5 that probably changes from a trigonal bipyramidal
(high spin, d7) to octahedral (low spin, d6) as CoII is oxidized to CoIII, which is also expected to be accompanied
by a spin-state conversion. As the redox potentials to convert the CoII to CoIII are high, it can be inferred that the
redox potential of the CoII-substituted SOD may be outside the range required to convert the superoxide radical
(O2•-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes
reported here are the first corroborative structural models of the CoII-substituted SOD.