Probing Variable Amine/amide Ligation in Ni(II)N2S2 Complexes Using Sulfur K-edge and Nickel L-edge X-ray Absorption Spectroscopies: Implications for the Active Site of Nickel Superoxide Dismutase

Nickel superoxide dismutase (NiSOD) is a recently discovered metalloenzyme that catalyzes the disproportionation of O2(*-) into O2 and H2O2. In its reduced state, the mononuclear Ni(II) ion is ligated by two cis-cysteinate sulfurs, an amine nitrogen (from the protein N-terminus), and an amide nitrogen (from the peptide backbone). Unlike many small molecule and metallopeptide-based NiN2S2 complexes, S-based oxygenation is not observed in NiSOD. Herein we explore the spectroscopic properties of a series of three Ni(II)N2S2 complexes (bisamine-ligated (bmmp-dmed)Ni(II), amine/amide-ligated (Ni(II)(BEAAM))(-), and bisamide-ligated (Ni(II)(emi))(2-)) with varying amine/amide ligation to determine the origin of the dioxygen stability of NiSOD. Ni L-edge X-ray absorption spectroscopy (XAS) demonstrates that there is a progression in ligand-field strength with (bmmp-dmed)Ni(II) having the weakest ligand field and (Ni(II)(emi))(2-)) having the strongest ligand field. Furthermore, these Ni L-edge XAS studies also show that all three complexes are highly covalent with (Ni(II)(BEEAM))(-) having the highest degree of metal-ligand covalency of the three compounds studied. S K-edge XAS also shows a high degree of Ni-S covalency in all three complexes. The electronic structures of the three complexes were probed using both hybrid-DFT and multiconfigurational SORCI calculations. These calculations demonstrate that the nucleophilic Ni(3d)/S(pi)* HOMO of these NiN2S2 complexes progressively decreases in energy as the amide-nitrogens are replaced with amine nitrogens. This decrease in energy of the HOMO deactivates the Ni-center toward O2 reactivity. Thus, the Ni-S bond is protected from S-based oxygenation explaining the enhanced stability of the NiSOD active-site toward oxygenation by dioxygen.