The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.

Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution

Copper is an essential trace element in living systems, present in the parts per million concentration range. It is a key cofactor in a diverse array of biological oxidation-reduction reactions. These involve either outer-sphere electron transfer, as in the blue copper proteins and the Cu{sub A} site of cytochrome oxidase and nitrous oxide redutase, or inner-sphere electron transfer in the binding, activation, and reduction of dioxygen, superoxide, nitrite, and nitrous oxide. Copper sites have historically been divided into three classes based on their spectroscopic features, which reflect the geometric and electronic structure of the active site: type 1 (T1) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled binuclear copper centers. 428 refs.

Multicopper Oxidases and Oxygenases

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

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Structural and Functional Aspects of Metal Sites in Biology

Even though the well-established Haber–Bosch process has been the major artificial way to “fertilize” the earth, its energy-intensive nature has been motivating people to learn from nitrogenase, which can fix atmospheric N2 to NH3 in vivo under mild conditions with its precisely arranged proteins Here we demonstrate that efficient fixation of N2 to NH3 can proceed under room temperature and atmospheric pressure in water using visible light illuminated BiOBr nanosheets of oxygen vacancies in the absence of any organic scavengers and precious-metal cocatalysts The designed catalytic oxygen vacancies of BiOBr nanosheets on the exposed {001} facets, with the availability of localized electrons for π-back-donation, have the ability to activate the adsorbed N2, which can thus be efficiently reduced to NH3 by the interfacial electrons transferred from the excited BiOBr nanosheets This study might open up a new vista to fix atmospheric N2 to NH3 through the less energy-demanding photochemical process

http://pubs.acs.org/doi/pdf/10.1021/jacs.5b03105

Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets

The high resolution three-dimensional x-ray structure of the metal sites of bovine heart cytochrome c oxidase is reported. Cytochrome c oxidase is the largest membrane protein yet crystallized and analyzed at atomic resolution. Electron density distribution of the oxidized bovine cytochrome c oxidase at 2.8 A resolution indicates a dinuclear copper center with an unexpected structure similar to a [2Fe-2S]-type iron-sulfur center. Previously predicted zinc and magnesium sites have been located, the former bound by a nuclear encoded subunit on the matrix side of the membrane, and the latter situated between heme a3 and CuA, at the interface of subunits I and II. The O2 binding site contains heme a3 iron and copper atoms (CuB) with an interatomic distance of 4.5 A; there is no detectable bridging ligand between iron and copper atoms in spite of a strong antiferromagnetic coupling between them. A hydrogen bond is present between a hydroxyl group of the hydroxyfarnesylethyl side chain of heme a3 and an OH of a tyrosine. The tyrosine phenol plane is immediately adjacent and perpendicular to an imidazole group bonded to CuB, suggesting a possible role in intramolecular electron transfer or conformational control, the latter of which could induce the redox-coupled proton pumping. A phenyl group located halfway between a pyrrole plane of the heme a3 and an imidazole plane liganded to the other heme (heme a) could also influence electron transfer or conformational control.

https://science.sciencemag.org/content/sci/269/5227/1069.full.pdf

Structures of metal sites of oxidized bovine heart cytochrome c oxidase at 2.8 A

Synthesis and Structural Characterization of Unsupported [FeIII-CN-CuII] Bridges Related to That in Cyanide-Inactivated Cytochrome c Oxidase

The planar complexes [NiII(pyN2R2)(OH)]−, containing a terminal hydroxo group, are readily prepared from N,N′-(2,6-C6H3R2)-2,6-pyridinedicarboxamidate(2-) tridentate pincer ligands (R4N)(OH), and Ni(OTf)2. These complexes react cleanly and completely with carbon dioxide in DMF solution in a process of CO2 fixation with formation of the bicarbonate product complexes [NiII(pyN2R2)(HCO3)]− having η1-OCO2H ligation. Fixation reactions follow second-order kinetics (rate = k2′[NiII–OH][CO2]) with negative activation entropies (−17 to −28 eu). Reactions were monitored by growth and decay of metal-to-ligand charge-transfer (MLCT) bands at 350–450 nm. The rate order R = Me > macro > Et > Pri > Bui > Ph at 298 K (macro = macrocylic pincer ligand) reflects increasing steric hindrance at the reactive site. The inherent highly reactive nature of these complexes follows from k2′ ≈ 106 M–1 s–1 for the R = Me system that is attenuated by only 100-fold in the R = Ph complex. A reaction mechanism is proposed based on compu...

Fast carbon dioxide fixation by 2,6-pyridinedicarboxamidato-nickel(II)-hydroxide complexes: influence of changes in reactive site environment on reaction rates.

Recently, we have reported the successful use of the GNXAS analysis package to determine reliable metric information (including angles) beyond the first coordination sphere. Of particular present relevance is the significant amplitude enhancement when a backscatter and an intervening atom form a linear or near-linear relationship, such as might occur in an [Fe-X-Cu] bridge. We have used the GNXAS MS protocol to analyze EXAFS data of (1)(ClO[sub 4]) and (3)(ClO[sub 4]) at the Fe K-edge. The EXAFS Fourier transforms (FTs) of the data and of the fit to the data are presented. The fits of both complexes include the two-body contributions Fe-O, Fe-N, and Fe-C[sub [alpha]] and the three-body MS components Fe-N-C[sub [beta]], Fe-O-Cu, and Fe-N-C[sub [gamma]]. The bridge structural differences in linear (1) and nonlinear (3) give rise to dramatic changes in their EXAFS spectra. GNXAS analysis is effective in quantifying the sensitivity of the MS signals to the geometry of the bridging configuration between metal centers. As shown here and for a related case, this technique is relatively sensitive and accurate when the bridge approaches linearity. However, in heme-based Fe-X-Cu bridged assemblies, porphyrin MS effects do not contribute to the Cu EXAFS. 15 refs., 2 figs.

Oxygen-Bridged Iron-Copper Assemblies Pertinent to Heme-Copper Oxidases: Synthesis and Structure of an [FeIII-(OH)-CuII] Bridge and Exafs Multiple-Scattering Effects of Linear Oxo and Nonlinear Hydroxo Bridges

A synthetic analogue of the antiferromagnetically coupled S=2 Fe/Cu binuclear site in oxidized (as-isolated) cytochrome c oxidase has been sought. Initial Cu(II) precursor compound [Cu(Me 6 tren)(OH 2 )](ClO 4 ) 2 containing the trigonal bipyramidal (TBP) complex [1] 2+ upon treatment with methanolic KOH yields [Cu(Me 6 -tren)(OH)] + ([2] + ) with TBP stereochemistry and a terminal hydroxo group. In acetone solution, reaction of [1] 2+ with 2 equiv of the hindered base lithium 2,6-di-tert-butyl-4-methylphenolate and 1 equiv of [Fe(OEP)(OClO 3 3)] affords (via the prsumed intermediate [2 + ]) the bridged assembly [(OEP)Fe-O-Cu(Me 6 tren)] + ([3] 6 ) in 67% isolated yield as the perchlorate salt (3)

Synthesis and characterization of an asymmetric bridged assembly containing the unsupported [FeIII-O-CuII] bridge: an analog of the binuclear site in oxidized cytochrome c oxidase

There exist a limited but growing number of biological metal centers whose properties lie conspicuously outside the realm of known inorganic chemistry. The synthetic analogue approach, broadly directed, offers a powerful exploratory tool that can define intrinsic chemical possibilities for these sites while simultaneously expanding the frontiers of fundamental inorganic chemistry. This speculative application of analogue study is exemplified here in the evolution of synthetic efforts inspired by the cluster chemistry of biological nitrogen fixation.

https://www.pnas.org/content/pnas/100/7/3595.full.pdf

Sonny C. Lee

Papers

Synthesis and Structural Characterization of Unsupported [FeIII-CN-CuII] Bridges Related to That in Cyanide-Inactivated Cytochrome c Oxidase

Fast carbon dioxide fixation by 2,6-pyridinedicarboxamidato-nickel(II)-hydroxide complexes: influence of changes in reactive site environment on reaction rates.

Oxygen-Bridged Iron-Copper Assemblies Pertinent to Heme-Copper Oxidases: Synthesis and Structure of an [FeIII-(OH)-CuII] Bridge and Exafs Multiple-Scattering Effects of Linear Oxo and Nonlinear Hydroxo Bridges

Synthesis and characterization of an asymmetric bridged assembly containing the unsupported [FeIII-O-CuII] bridge: an analog of the binuclear site in oxidized cytochrome c oxidase

Speculative synthetic chemistry and the nitrogenase problem