pH Dependent Reversible Formation of a Binuclear Ni₂ Metal-Center Within a Peptide Scaffold

A disulfide-bridged peptide containing two Ni²⁺ binding sites based on the nickel superoxide dismutase protein, {Ni₂(SOD^mds)}, has been prepared. At physiological pH (7.4) it was found that the metal sites are mononuclear with a square planar NOS₂ coordination environment with the two sulfur-based ligands derived from cysteinate residues, the nitrogen ligand derived from the amide backbone and a water ligand. Furthermore, S K-edge X-ray absorption spectroscopy indicated that the two cysteinate sulfur atoms ligated to nickel are each protonated. Elevation of the pH to 9.6 results in the deprotonation of the cysteinate sulfur atoms, and yields a binuclear, cysteinate bridged Ni₂²⁺ center with each nickel contained in a distorted square planar geometry. At both pH = 7.4 and 9.6 the nickel sites are moderately air sensitive, yielding intractable oxidation products. However, at pH = 9.6 {Ni₂(SOD^mds)} reacts with O₂ at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate the reduced reactivity at pH = 7.4 is a result of a reduction in S(3p) character and deactivation of the nucleophilic frontier molecular orbitals upon cysteinate sulfur protonation.