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

Abstract

A disulfide-bridged peptide containing two Ni2+ binding sites based on the nickel superoxide dismutase protein, {Ni2(SODmds)} 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(SODmds)} reacts with O2 at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate that 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.

Operando Elucidation of Electrocatalytic and Redox Mechanisms on a 2D Metal Organic Framework Catalyst for Efficient Electrosynthesis of Hydrogen Peroxide in Neutral Media

“…The X-ray absorption near edge spectroscopy (XANES) for the Ni–HAB samples under both the O 2 -saturated condition (Figure b) and Ar-saturated condition (Figure c) shows pre-edge peaks that correspond to the 1s–3d transition at ∼8333 eV (peak I) and 1s–4p z transition at ∼8339 eV (peak II), which are consistent with a square-planar Ni complex. ,, The consistent peaks I and II during ORR operation (Figure b) confirm that the Ni nodes maintain their initial +2 oxidation state and square-planar geometry, which is further supported by the lack of significant shift in the edge energy relative to the edge position measured at the open-circuit potential before any potential was applied ( E OC ). Furthermore, the edge position appears independent of the potential, which suggests that Ni is not the active site for ORR.…”

Characterization of Methyl- and Acetyl-Ni Intermediates in Acetyl CoA Synthase Formed during Anaerobic CO2 and CO Fixation

“…Additionally, the data on 4′ are consistent with square-planar geometry, as shown in a comprehensive Ni K-edge XAS study by Colpas et al They demonstrate that square planar complexes have sharp rising-edge features in contrast to tetrahedral complexes which have comparatively featureless rising edges. A more recent study by Shearer and co-workers also demonstrates the stark difference in the rising-edge position of square planar and D 2d distorted binuclear Ni centers . Another important comparison from the Colpas study is between high-spin versus low-spin complexes with comparable S 4 ligand systems.…”

Bioinorganic Chemistry of Nickel

“…The understanding of the biological role of nickel from the inorganic chemistry side is reviewed on a theoretical basis by Siegbahn et al [2], who discuss the enzyme mechanisms, including the canonical mechanism of urease, in view of the recently reported crystal structure of the enzyme-substrate complex [3]. This chemistry is further elucidated by original contributions on the pH dependence of binuclear nickel peptide complexes by Keegan et al [4].…”