Isolation and characterization of a bis(dithiolene)-supported tungsten-acetylenic complex as a model for acetylene hydratase.

Acetylene hydratase is currently the only known mononuclear tungstoenzyme that does not catalyze a net redox reaction. The conversion of acetylene to acetaldehyde is proposed to occur at a W(IV) active site through first-sphere coordination of the acetylene substrate. To date, a handful of tungsten complexes have been shown to bind acetylene, but many lack the bis(dithiolene) motif of the native enzyme.

Class Ib Ribonucleotide Reductases: Activation of a Peroxido-MnMn to Generate a Reactive Oxo-MnMn Oxidant.

In the postulated catalytic cycle of class Ib Mn ribonucleotide reductases (RNRs), a Mn core is suggested to react with superoxide (O) to generate peroxido-MnMn and oxo-MnMn entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [Mn(BPMP)(OAc)](ClO) (, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-MnMn () in the presence of superoxide anion that converted to (μ-O)(μ-OH)MnMn () via the addition of an H-donor (TsOH) or (μ-O)MnMn () upon warming to room temperature.

Oxo-Free Hydrocarbon Oxidation by an Iron(III)-Isoporphyrin Complex.

Metal-halides that perform proton coupled electron-transfer (PCET) oxidation are an important new class of high-valent oxidant. In investigating metal-dihalides, we reacted [Fe(Cl)(T(OMe)PP)] (, T(OMe)PP = -tetra(4-methoxyphenyl)porphyrinyl) with (dichloroiodo)benzene. An Fe--chloro-isoporphyrin complex [Fe(Cl)(T(OMe)PP-Cl)] () was obtained.

High-Valent d Ni versus d Cu Oxidants in PCET.

Oxygenases have been postulated to utilize d Fe and d Cu oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-electron count can hold over the PCET reactivity, two metastable high-valent metal-oxygen adducts, [Ni(OAc)(L)] () and [Cu(OAc)(L)] (), L = ,-(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate, were prepared from their low-valent precursors [Ni(OAc)(L)] () and [Cu(OAc)(L)] (). The complexes /-/ were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, X-ray diffraction, and absorption spectroscopies and mass spectrometry.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase.

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity.

Hydrogen Atom Transfer by a High-Valent Nickel-Chloride Complex.

Oxo-metal-halide moieties have often been implicated as C-H bond activating oxidants with the terminal oxo-metal entity identified as the electrophilic oxidant. The electrophilic reactivity of metal-halide species has not been investigated. We have prepared a high-valent nickel-halide complex [Ni(Cl)(L)] (2, L = N,N'-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide) by one-electron oxidation of a [Ni(Cl)(L)] precursor.

Mimicking Class I b Mn -Ribonucleotide Reductase: A Mn Complex and Its Reaction with Superoxide.

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn ) active site in class I b RNRs that requires superoxide anion (O ), rather than dioxygen (O ), to access a high-valent Mn oxidant. Complex 1 ([Mn (O CCH )(N-Et-HPTB)](ClO ) , N-Et-HPTB=N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O at -40 °C resulting in the formation of a metastable species (2).

Design and reactivity of Ni-complexes using pentadentate neutral-polypyridyl ligands: Possible mimics of NiSOD.

Superoxide plays a key role in cell signaling, but can be cytotoxic within cells unless well regulated by enzymes known as superoxide dismutases (SOD). Nickel superoxide dismutase (NiSOD) catalyzes the disproportion of the harmful superoxide radical into hydrogen peroxide and dioxygen. NiSOD has a unique active site structure that plays an important role in tuning the potential of the nickel center to function as an effective catalyst for superoxide dismutation with diffusion controlled rates.

Tuning the Reactivity of Terminal Nickel(III)-Oxygen Adducts for C-H Bond Activation.

Two metastable Ni complexes, [Ni(OAc)(L)] and [Ni(ONO)(L)] (L = N,N'-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate, OAc = acetate), were prepared, adding to the previously prepared [Ni(OCOH)(L)], with the purpose of probing the properties of terminal late-transition metal oxidants. These high-valent oxidants were prepared by the one-electron oxidation of their Ni precursors ([Ni(OAc)(L)] and [Ni(ONO)(L)]) with tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Fascinatingly, the reaction between any [Ni(X)(L)] and NaOCl/acetic acid (AcOH) or cerium ammonium nitrate ((NH)[Ce(NO)], CAN), yielded [Ni(OAc)(L)] and [Ni(ONO)(L)], respectively.

Characterization and Reactivity Studies of a Terminal Copper-Nitrene Species.

High-valent terminal copper-nitrene species have been postulated as key intermediates in copper-catalyzed aziridination and amination reactions. The high reactivity of these intermediates has prevented their characterization for decades, thereby making the mechanisms ambiguous. Very recently, the Lewis acid adduct of a copper-nitrene intermediate was trapped at -90 °C and shown to be active in various oxidation reactions.

Crystal structure of Clostridium difficile toxin A.

Clostridium difficile infection is the leading cause of hospital-acquired diarrhoea and pseudomembranous colitis. Disease is mediated by the actions of two toxins, TcdA and TcdB, which cause the diarrhoea, as well as inflammation and necrosis within the colon. The toxins are large (308 and 270 kDa, respectively), homologous (47% amino acid identity) glucosyltransferases that target small GTPases within the host.

Crystal structure of toxin A.

infection is the leading cause of hospital-acquired diarrhoea and pseudomembranous colitis. Disease is mediated by the actions of two toxins, TcdA and TcdB, which cause the diarrhoea, as well as inflammation and necrosis within the colon. The toxins are large (308 and 270 kDa, respectively), homologous (47% amino acid identity) glucosyltransferases that target small GTPases within the host.

Chlorobenzene Poisoning and Recovery of Platinum-Based Cathodes in Proton Exchange Membrane Fuel Cells.

The platinum electrocatalysts found in proton exchange membrane fuel cells are poisoned both reversibly and irreversibly by air pollutants and residual manufacturing contaminants. In this work, the poisoning of a Pt/C PEMFC cathode was probed by a trace of chlorobenzene in the air feed. Chlorobenzene inhibits the oxygen reduction reaction and causes significant cell performance loss.

Reduction of hexavalent chromium by the thermophilic methanogen .

Despite the significant progress on iron reduction by thermophilic microorganisms, studies on their ability to reduce toxic metals are still limited, despite their common co-existence in high temperature environments (up to 70°C). In this study, , an obligate thermophilic methanogen, was used to reduce hexavalent chromium. Experiments were conducted in a growth medium with H/CO as substrate with various Cr concentrations (0.

Characterization of the Copper(II) Binding Sites in Human Carbonic Anhydrase II.

Human carbonic anhydrase (CA) is a well-studied, robust, mononuclear Zn-containing metalloprotein that serves as an excellent biological ligand system to study the thermodynamics associated with metal ion coordination chemistry in aqueous solution. The apo form of human carbonic anhydrase II (CA) binds 2 equiv of copper(II) with high affinity. The Cu(2+) ions bind independently forming two noncoupled type II copper centers in CA (CuA and CuB).