Protein-based models offer mechanistic insight into complex nickel metalloenzymes

A sulfur-templated Ni-Ni' coordination polymer that relies on a polarizable nickel nitrosyl hub

The templating properties of a diaza-nickel-cis-dithiolate towards triphenylphosphine gold(I), yielding a transoid [Ni(N2S2)·2Au(PPh3)] complex (T. A. Pinder, S. K. Montalvo, A. M. Lunsford, C.-H. Hsieh, J. H. Reibenspies and M. Y. Darensbourg, Dalton Trans., 2014, 43, 138-144) suggested that a suitable analogue of d10-Au(I), i.e., {Ni(NO)}10, could generate a tetrahedral nickel node for a [Ni(N2S2)·2Ni(NO)(X)]n coordination polymer. Monomeric precursors, derived from Feltham's [(Ph3P)2Ni(NO)(Cl)] (R. D. Feltham, Inorg. Chem., 1964, 3, 116-119) produced the bidentate/sulfur-chelated [Ni(N2S2)·Ni(NO)(X)] species with loss of PPh3. Exchange of Cl- by azide, N3-, in the {Ni(NO)}10 synthon led to the balance of electrophilicity at Ni(NO) and non-covalent (H-bonding and van der Waals) interactions that stabilized the extended chain of bridging sulfurs, in transoid connectivities, between a square planar NiII and a tetrahedral Ni, the latter within the electronic and spin-delocalized {Ni(NO)}10 system. This study defines a new path that creates coordination polymers using metallodithiolates, the success of which, in this case, depends on the highly polarizable {Ni(NO)}10 unit.

Harnessing Oxidizing Potential of Nickel for Sustainable Hydrocarbon Functionalization

While the oxidative chemistry of transition metals such as iron and copper is a highly developed area of investigation, the study of similar chemistry with nickel is much younger. However, nickel offers rich coordination chemistry with oxygen and other oxidants and is a promising avenue of research for applications such as sustainable hydrocarbon functionalization. Herein, we summarize the progress made recently in nickel coordination chemistry relevant to hydrocarbon functionalization and offer our perspectives on open questions in the field.

Structure-driven development of a biomimetic rare earth artificial metalloprotein

The 2011 discovery of the first rare earth-dependent enzyme in methylotrophic Methylobacterium extorquens AM1 prompted intensive research toward understanding the unique chemistry at play in these systems. This enzyme, an alcohol dehydrogenase (ADH), features a La3+ ion closely associated with redox-active coenzyme pyrroloquinoline quinone (PQQ) and is structurally homologous to the Ca2+-dependent ADH from the same organism. AM1 also produces a periplasmic PQQ-binding protein, PqqT, which we have now structurally characterized to 1.46-Å resolution by X-ray diffraction. This crystal structure reveals a Lys residue hydrogen-bonded to PQQ at the site analogously occupied by a Lewis acidic cation in ADH. Accordingly, we prepared K142A- and K142D-PqqT variants to assess the relevance of this site toward metal binding. Isothermal titration calorimetry experiments and titrations monitored by UV-Vis absorption and emission spectroscopies support that K142D-PqqT binds tightly (Kd = 0.6 ± 0.2 μM) to La3+ in the presence of bound PQQ and produces spectral signatures consistent with those of ADH enzymes. These spectral signatures are not observed for WT- or K142A-variants or upon addition of Ca2+ to PQQ ⸦ K142D-PqqT. Addition of benzyl alcohol to La3+-bound PQQ ⸦ K142D-PqqT (but not Ca2+-bound PQQ ⸦ K142D-PqqT, or La3+-bound PQQ ⸦ WT-PqqT) produces spectroscopic changes associated with PQQ reduction, and chemical trapping experiments reveal the production of benzaldehyde, supporting ADH activity. By creating a metal binding site that mimics native ADH enzymes, we present a rare earth-dependent artificial metalloenzyme primed for future mechanistic, biocatalytic, and biosensing applications.

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

Engineering an Oxygen-Binding Protein for Photocatalytic CO2 Reductions in Water

While native CO2 -reducing enzymes display remarkable catalytic efficiency and product selectivity, few artificial biocatalysts have been engineered to allow understanding how the native enzymes work. To address this issue, we report cobalt porphyrin substituted myoglobin (CoMb) as a homogeneous catalyst for photo-driven CO2 to CO conversion in water. The activity and product selectivity were optimized by varying pH and concentrations of the enzyme and the photosensitizer. Up to 2000 TON(CO) was attained at low enzyme concentrations with low product selectivity (15 %), while a product selectivity of 74 % was reached by increasing the enzyme loading but with a compromised TON(CO). The efficiency of CO generation and overall TON(CO) were further improved by introducing positively charged residues (Lys or Arg) near the active stie of CoMb, which demonstrates the value of tuning the enzyme secondary coordination sphere to enhance the CO2 -reducing performance of a protein-based photocatalytic system.

Rational design of an artificial hydrolytic nuclease by introduction of a sodium copper chlorophyllin in L29E myoglobin

Heme proteins have recently emerged as promising artificial metalloenzymes for catalyzing diverse reactions. In this report, L29E Mb, a single mutant of myoglobin (Mb), was reconstituted by replacing the heme with a sodium copper cholorophyllin (CuCP) to form a new green artificial enzyme (named CuCP-L29E Mb). The reconstituted protein CuCP-L29E Mb was found to exhibit hydrolytic DNA cleavage activity, which was not depending on O₂. In addition, Mg²⁺ ion could effectively promote the DNA cleavage activity of CuCP-L29E Mb. Wild-type (WT) Mb reconstituted with CuCP (named CuCP-WT Mb) did not show DNA cleavage activity under the same conditions. This study suggests that both Mg²⁺ and the ligand Glu29 are critical for the nuclease activity and the artificial nuclease of Mg²⁺-CuCP-L29E Mb may have potential applications in the future.