Reductive activation and structural rearrangement in superoxide reductase: a combined infrared spectroscopic and computational study

Superoxide reductases (SOR) are a family of non-heme iron enzymes that limit oxidative stress by catalysing the reduction of superoxide to hydrogen peroxide and, thus, represent model systems for the detoxification of reactive oxygen species. In several enzymes of this type, reductive activation of the active site involves the reversible dissociation of a glutamate from the proposed substrate binding site at the iron. In this study we have employed IR spectroscopic and theoretical methods to gain insights into redox-linked structural changes of 1Fe-type superoxide reductases, focusing on the enzyme from the archaeon Ignicoccus hospitalis. Guided by crystal structure data and complemented by spectra calculation for an active site model, the main IR difference signals could be assigned. These signals reflect redox-induced structural changes in the first coordination sphere of the iron centre, adjacent loop and helical regions, and more remote β-sheets. By comparison with the spectra obtained for the E23A mutant of Ignicoccus hospitalis SOR, it is shown that glutamate E23 dissociates reversibly from the ferrous iron during reductive activation of the wild type enzyme. Moreover, this process is found to trigger a global conformational transition of the protein that is strictly dependent on the presence of E23. Similar concerted structural changes can be inferred from the IR spectra of related SORs such as that from Archaeoglobus fulgidus, indicating a widespread mechanism. A possible functional role of this process in terms of synergistic effects during reductive activation of the homotetrameric enzyme is proposed.DFG, EXC 314, Unifying Concepts in Catalysi

Discriminating changes in intracellular NADH/NAD+ levels due to anoxicity and H2 supply in R. eutropha cells using the Frex fluorescence sensor

The hydrogen-oxidizing “Knallgas” bacterium Ralstonia eutropha can thrive in aerobic and anaerobic environments and readily switches between heterotrophic and autotrophic metabolism, making it an attractive host for biotechnological applications including the sustainable H2-driven production of hydrocarbons. The soluble hydrogenase (SH), one out of four different [NiFe]-hydrogenases in R. eutropha, mediates H2 oxidation even in the presence of O2, thus providing an ideal model system for biological hydrogen production and utilization. The SH reversibly couples H2 oxidation with the reduction of NAD+ to NADH, thereby enabling the sustainable regeneration of this biotechnologically important nicotinamide cofactor. Thus, understanding the interaction of the SH with the cellular NADH/NAD+ pool is of high interest. Here, we applied the fluorescent biosensor Frex to measure changes in cytoplasmic [NADH] in R. eutropha cells under different gas supply conditions. The results show that Frex is well-suited to distinguish SH-mediated changes in the cytoplasmic redox status from effects of general anaerobiosis of the respiratory chain. Upon H2 supply, the Frex reporter reveals a robust fluorescence response and allows for monitoring rapid changes in cellular [NADH]. Compared to the Peredox fluorescence reporter, Frex displays a diminished NADH affinity, which prevents the saturation of the sensor under typical bacterial [NADH] levels. Thus, Frex is a valuable reporter for on-line monitoring of the [NADH]/[NAD+] redox state in living cells of R. eutropha and other proteobacteria. Based on these results, strategies for a rational optimization of fluorescent NADH sensors are discussed

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.This work was supported by the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development), the OMV Group, the EPSRC (DTA studentship for T.E.R.), the European Union’s Horizon 2020 research and innovation program (Marie SklodowskaCurie IF for K.H.L., GAN 701192), and an ERC Consolidator Grant “MatEnSAP” (GAN 682833). I.Z. is indebted to the German Research Foundation (DFG) for financial support within the cluster of excellence EXC 314: Unifying concepts in catalysis, “UniCat”

Water soluble nickel and iron salts for hydroxymethylfurfural HMF and water oxidation the simplest precatalysts?

Electrochemical production of large scale chemicals and fuels is critical to reaching carbon neutrality. However, the required anodic oxidation reactions, namely the oxygen evolution reaction OER or the oxidation of organics into value added products, suffer from large overpotentials. To address this challenge, researchers have been widely investigating non water soluble pre catalysts to operate in the aqueous electrolyte. On the contrary, in this work, we approach a rapid, easy, and green carbon cloth electrode preparation using merely water soluble nitrate precursors and ethanol as chemicals and no heating steps. The drop coated, water soluble transition metal salts reconstruct rapidly into the respective oxyhydroxides under OER conditions, with the oxyanion acting as a beneficial sacrificial reagent. This approach is shown herein for nickel iron catalysts and their successful application for the OER 220 mV overpotential at 10 mA cm amp; 8722;2, long term stability of 40 h at 100 mA cm amp; 8722;2 and the oxidation of 5 hydroxymethylfurfural HMF, quantitative faradaic efficiency . We compare both reactions with both electrodes closely and find that the iron free sample is more active for the HMF oxidation in regimes where mass transport is not the main limiting factor. We anticipate that this simple electrode preparation approach can find wide application in electrocatalysis and beyon

An Intermetallic CaFe6Ge6 Approach to Unprecedented Ca Fe O Electrocatalyst for Efficient Alkaline Oxygen Evolution Reaction

Based on the low cost and relatively high catalytic activity, considerable efforts have been devoted towards developing redox active transition metal TM oxygen electrocatalysts for the alkaline oxygen evolution reaction OER while the role of redox inactive alkaline earth metals has often been neglected in OER. Herein, for the first time, we developed a novel ternary intermetallic CaFe6Ge6 precatalyst, whose surface rapidly transforms into a porous ultrathin Ca amp; 8722;Fe amp; 8722;O heteroshell structure during alkaline OER through the oxidative leaching of surficial Ge. Benefiting from synergistic effects, this highly efficient OER active material with distinct Ca amp; 8722;Fe amp; 8722;O layers has a large electrochemical surface area and more exposed active Fe sites than a Ca free FeOx phase. Also, the presence of Ca in Ca amp; 8722;Fe amp; 8722;O is responsible for the enhanced transport and activation of hydroxyls and related OER reaction intermediate as unequivocally illustrated by a combination of quasi in situ Raman spectroscopy and various ex situ method

A Knowledge Based Molecular Single Source Precursor Approach to Nickel Chalcogenide Precatalysts for Electrocatalytic Water, Alcohol, and Aldehyde Oxidations

The development and comprehensive understanding of nickel chalcogenides are critical since they constitute a class of efficient electro pre catalysts for the oxygen evolution reaction OER and value added organic oxidations. This study introduces a knowledge based facile approach to analogous NiE E S, Se, Te phases, originating from molecular amp; 946; diketiminato [Ni2E2] complexes and their application for OER and organic oxidations. The recorded activity trends for both target reactions follow the order NiSe gt; NiS gt; NiTe. Notably, NiSe displayed efficient performance for both OER and the selective oxidation of benzyl alcohol and 5 hydroxymethylfurfural, exhibiting stability in OER for 11 days under industrially pertinent conditions. Comprehensive analysis, including quasi in situ X ray absorption and Raman spectroscopy, in combination with several ex situ techniques, revealed a material reconstruction process under alkaline OER conditions, involving chalcogen leaching. While NiS and NiSe experienced full chalcogen leaching and reconstruction into NiIII IV oxyhydroxide active phases with intercalated potassium ions, the transformation of NiTe is incomplete. This study highlights the structure activity relationship of a whole series of analogous nickel chalcogenides, directly linking material activity to the availability of active sites for catalysis. Such findings hold great promise for the development of efficient electrocatalysts for a wide range of applications, impacting various industrial processes and sustainable energy solution

In situ Transformation of a Conjugated Nickel Organic Framework into Active Nickel Oxyhydroxide for Electrocatalytic 5 Hydroxymethylfurfural Oxidation

Utilizing electrical energy for the targeted conversion of biomass into valuable molecules is a crucial building block for a future circular economy. Herein, a Nickel Ni based conjugated metal organic framework MOF having salicylaldehydate linkages 1, 3, 5 triformylphloroglucinol Tp was synthesized via a solid state process. The resulting 2D framework Ni Tp demonstrates a highly selective electrocatalytic conversion of 5 hydroxymethylfural HMF to 2, 5 furandicarboxylic acid FDCA with excellent faradaic efficiency 96 4 . In situ Raman and X ray absorption spectroscopy XAS reveal that Ni Tp acts as a precatalyst for uniformly dispersed nickel oxy hydroxide NiOOH in the electrocatalytic organic oxidation reaction OOR process. The combination of efficient electron transport of the Ni Tp and the uniform dispersion of newly formed nickel oxy hydroxide with excellent electrolyte availability leads to redox and potentially catalytic activity of all in situ formed nickel sites. Thus, the Ni Tp is an ideal precatalyst in terms of nickel oxy hydroxide active site exposure. This work demonstrates a cost effective method for synthesizing efficient MOF based electrocatalysts for a relevant catalytic reactio

The In Stability of Heterostructures During the Oxygen Evolution Reaction

The urgent need for efficient oxygen evolution reaction OER catalysts has led to the development and publication of many heterostructured catalysts. The application of such catalysts with multiple phases tremendously increases the material design dimensions, and numerous interface related effects can tune the OER performance. In this regard, multiple of these heterostructured electrodes show remarkable OER activities. However, it is not clear if these carefully designed interfaces remain under prolonged OER conditions. Herein, a molecular approach is used to synthesize four different nickel iron phosphide heterostructured materials and deposit them on fluorine doped tin oxide and nickel foam electrodes. The OER performance of the eight electrodes and the reconstruction of the four materials is investigated by in situ spectroscopy after one day of operation, enabled by a freeze quench approach. The most active electrode is also applied under industrial OER conditions and for the value added oxidation of alcohols to ketones. Before catalysis, this electrode comprises crystalline 4 nm nickel phosphide particles on an amorphous iron phosphide matrix. However, after 24 h, a homogenous nickel iron oxyhydroxide phase has formed. This work questions to which extent the design of heterostructures is a suitable strategy for non noble metal OER catalysi

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction.

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host-guest chemistry