Paramelaconite‐Enriched Copper‐Based Material as an Efficient and Robust Catalyst for Electrochemical Carbon Dioxide Reduction

A copper‐oxide‐based catalyst enriched with paramelaconite (Cu₄O₃) is presented and investigated as an electrocatalyst for facilitating electroreduction of CO₂ to ethylene and other hydrocarbons. Cu₄O₃ is a member of the copper‐oxide family and possesses an intriguing mixed‐valance nature, incorporating an equal number of Cu⁺ and Cu²⁺ ions in its crystal structure. The material is synthesized using a solvothermal synthesis route and its structure is confirmed via powder X‐ray diffraction, transmission electron microscope based selected area electron diffraction, and X‐ray photoelectron spectroscopy. A flow reactor equipped with a gas diffusion electrode is utilized to test a copper‐based catalyst enriched with the Cu₄O₃ phase under CO₂ reduction conditions. The Cu₄O₃‐rich catalyst (PrC) shows a Faradaic efficiency for ethylene over 40% at 400 mA cm^?2. At ?0.64 versus reversible hydrogen electrode, the highest C₂₊/C₁ product ratio of 4.8 is achieved, with C₂₊ Faradaic efficiency over 61%. Additionally, the catalyst exhibits a stable performance for 24 h at a constant current density of 200 mA cm^?2.     

Structure of Cu2(indomethacin)4 and the reaction with superoxide in aprotic systems

The copper complex of indomethacin (1-(p-chlorobenzoyl)-5-methoxy-2-methyl-indole acetate), a common anti-inflammatory drug, was prepared and characterized. Crystal structure determination revealed the dimeric form of the 1:2 complex, namely Cu₂(indomethacin)₄ · L₂, in the unit cell. Suprisingly, the copper-copper distance (263 pm) was very close to metallic copper (256 pm). The two coordination sites in the copper-copper axis can be readily replaced by superoxide. An intriguing similarity to Cu₂(acetate)₄ was seen.Due to the lipophilic nature of the indomethacin ligand, this copper complex reacted with superoxide in aprotic solvents. The superoxide dismutating activity was successfully demonstrated in Me₂SO/water and acetonitrile/water mixtures using the nitro-blue tetrazolium assay and pulse radiolysis. The second-order rate constant of 6 · 10⁹ M^?1 · s^?1 in strictly aqueous systems dropped only slightly to 1.1 · 10⁹ M^?1 · s^?1 when aprotic solvents were used. This is the fastest rate constant ever observed for a copper-dependent dismutation of superoxide. The KO₂-induced lipid peroxidation in both erythrocytes and liver microsomes was suppressed by 70% in the presence of 1 · 10^?10 mol · ml^?1 of Cu₂(indomethacin)₄. The inhibitory action dropped to 25% when Cu₂Zn₂superoxide dismutase was employed. The formation of copper · indomethacin in rat serum after administration of indomethacin was shown in vitro and in vivo.     

Lipoxygenase-mediated production of superoxide anion in senescing plant tissue

Lipoxygenase activity and superoxide (O^.?₂) production by microsomal membranes and cytosol from bean cotyledons increased in parallel as senescence progressed. Superoxide production was heat denaturable and dependent on the availability of linoleate, the substrate for lipoxygenase. The specific inhibitor of lipoxygenase, U28938, caused a parallel reduction in enzyme activity and the formation of O^?₂. These observations demonstrate that lipoxygenase activity mediates the formation of superoxide anion, and support the contention that membrane senescence is attributable to a sequence of reactions in which lipasederived fatty acids are utilized by lipoxygenase to generate O^?₂ and hydroperoxides.     

On the role of a cuprous ion intermediate in the galactose oxidase reaction

The Cu⁺² electron spin resonance spectrum of galactose oxidase (galactose:O₂ oxidoreductase, E.C. 1.1.3.9) indicates that the metal is in a pseudo-square planar environment. The electron g values are: g_zz = 2.273, g_xx = 2.058 and g_yy = 2.048. The copper nuclear hyperfine constants are (in Gauss): A_zz = 176.5, A_xx = 28.8 and A_yy = 30.1. This spectrum is unaltered in either intensity or g or A values under conditions which cause the inhibition of galactose oxidase by superoxide dismutase. No combination of substrates (galactose and O₂) and oxidant traps (superoxide dismutase and catalase) results in the reduction of the cupric ion resonance. Thus, a Cu⁺¹-enzyme does not appear to be a stable intermediate along this enzyme's reaction path.     

Copper ions and hydrogen peroxide form hypochlorite from NaCl thereby mimicking myeloperoxidase

Sea urchins have elaborated multiple defenses to assure monospermic fertilization. In this work, we have concentrated on a study of the mechanism(s) by which hydrogen peroxide (H₂O₂) prevents polyspermy in Arbacia punctulata. We found that it is not H₂O₂ but probably hypochlorous acid/hypochlorite (HOCl/OCl^?) derived from H₂O₂ that is toxic to the supernumerary sperm. The spermicidal activity of H₂O₂ is potentiated by at least one order of magnitude by cupric ions (Cu²⁺). This increased toxicity is not due to the formation of hydroxyl radicals (·OH) because ·OH scavengers did not counteract the activity of Cu²⁺. More-over, substitution of Cu²⁺ by ferrous ions (Fe²⁺), which are known to cause formation of ·OH from H₂O₂, had no effect on fertilization even at 10²?10³ times higher concentrations. In contrast, 3-amino-1,2,4-triazole (AT), an HOCl/OCl^? scavenger, totally reversed the toxic effects of Cu²⁺. Furthermore, we found that HOCl/OCl^? is generated in solutions of H₂O₂ and Cu²⁺ in the presence of 0.5 M NaCl and that its accumulation is abolished by AT. Thus it is possible that the antifertility properties of copper are due to its ability to mediate formation of HOCl/OCl^?. HOCl/OCl^? generated by Cu²⁺ from H₂O₂ and Cl^?, a low concentration of exogenously added HOCl/OCl^?, or increased concentrations of H₂O₂ has similar inhibitory effects on the fertilization process in sea urchins. Therefore, we suggest that polyspermy is prevented by the action of a myeloperoxidase that affects the formation of HOCl/OCl^? from the Cl^? present in sea water through reaction with H₂O₂ generated by the newly fertilized egg.     

Radical scavenging and electron-transfer reactions in Polyporus Versicolor laccase: A pulse radiolysis study

The interaction of the radicals OH^?, t-BuO^?, e_aq^?, CO₂^XXX and O₂^XXX with the copper oxidase. laccase. from Polyporus, has been studied by the pulse-radiolysis technique. Each of these radicals formed transient adducts with a broad absorption maximum around 310 nm. Analysis of the optical properties and of the very fast rates of formation of these compounds shows that each radical interacts with a limited number of sites on the polypeplide part of the protein amongst R-S-S-R. histidine and aromatic residues. Interaction with the carbonyl group of some of the peptide bonds is also possible. The few target sites are probably hit simultaneously and electron transfer between these sites may also occur. In all cases, in a subsequent step, intramolecular electron transfer from the polypeptide radical adducts leads to a partial reduction of the blue type-1 Cu²⁺ with rates varying between 10³ and 10⁴ s^?1. Further reduction of the type-1 Cu²⁺ occurs through a slow intermolecular reaction between two laccase radical transient adducts. In the case of CO^XXX₂ and O^XXX₂, this slow reduction could alternatively be due to an intermolecular reaction between laccase and CO^XXX₂ or O^XXX₂. The oxidant radicals OH^?. Br^XXX₂ and (SCN)^XXX₂, which formed radical adducts with fully ascorbate-reduced laccase, did not induce any type-1 copper reoxidation.     

Superoxide dismutase activity of macrocyclic polyamine complexes

Copper(II) and nickel(II) complexes of macrocyclic polyamine derivatives possessing partial oligopeptide-like structures are found to suppress the xanthine-xanthine oxidase-mediated reduction of nitroblue tetrazolium and also to suppress formazan formation by potassium superoxide. The activity in the superoxide dismutase assay is dependent on ring size, type and number of donor atoms, metal ion, and substituents on the macrocycles. Some of those are more active than the known O₂^? scavengers such as copper(II)-salicylate and copper(II)-amino acid (or peptide) complexes. Nickel (II)-naphthylmethyl-dioxo-[16]ane N₅, 13, 1 : 1 complex (NiH_?2L) is the most active among the 30 chelates examined.     

Different effects of concanavalin A and its succinylated derivative on superoxide release in peritoneal macrophages

The effects of tetravalent concanavalin A and its succinylated derivative on the intracellular production of superoxide anion (O₂^?) and its release into cell exterior of peritoneal macrophages were observed. Both tetravalent concanavalin A and its succinylated derivative induced marked enhancement of intracellular reduction of nitroblue tetrazolium, which could be inhibited by α-methyl-D-glucoside. The extent of activation of nitroblue tetrazolium reduction induced by both types of the lectin paralleled the activation ratio of oxygen consumption.There was littele difference in the extent of intracellular O₂^? production induced by two types of the lectin. Nitroblue tetrazolium reduction was not affected significantly by pretreatment with colchicine, rotenone or malonate, inhibitors of the cytoskeletal system and of the electron transport system. In contrast, tetravalent concanavalin A induced a higher rate of superoxide release than did succinylated divalent concanavalin A, which lacks the cross-linking activity of surface glycoproteins.These results indicate that superoxide production following oxygen consumption and superoxide release into cell exterior are controlled independently by a separate membrane mechanism and that superoxide production system is not essentially dependent on the involvement of the cytoskeletal system.     

Kinetic study of O-2 DISMUTATION BY BOVINE SUPEROXIDE DISMUTASE. Evidence for saturation of the catalytic sites by O-2.

Dismutation of O₂^? by bovine copper-zinc superoxide $\text{di}$ smutase has been studied at different O₂^? concentrations with a polarographic method. Saturation of the enzyme by the substrate was observed and K_m and Vmax values were calculated. Inhibition by OH^? and CN^? was shown to be of the competitive type. The data support on inner sphere mechanism for the reaction between O₂^? and copper.     

The role of the superoxide anion as a toxic species in the erythrocyte.

The toxic action of the superoxide anion (O₂^?) toward the erythrocyte was investigated with O₂^? generated through the autooxidation of dihydroxyfumaric acid (DHF). A suspension of human red cells exposed to DHF undergoes a rapid breakdown of the cellular hemoglobin to methemoglobin and other green pigments. This hemoglobin breakdown is inhibited by superoxide dismutase (SOD) or catalase (CAT) and is accelerated by lactoperoxidase (LP) added externally to the red cell medium. Associated with the hemoglobin breakdown is a hypotonic hemolysis also inhibited by SOD or CAT and initially accelerated but later inhibited by LP. Conversion of the red cell hemoglobin to carbonmonoxyhemoglobin in an aerated medium results in no hemoglobin breakdown or hypotonic lysis in the presence of DHF, even though O₂^? can be demonstrated in the medium. Although no evidence for membrane sulfhydryl oxidation or lipid peroxidation can be demonstrated in red cells exposed to DHF, the membranes of these cells were found to retain a green pigment. The presence of this green pigment in red cell membranes was inhibited by SOD, CAT, or conversion of the cellular hemoglobin to carbonmonoxyhemoglobin, but was not inhibited by LP. These results have been interpreted as a peroxide-dependent formation of O₂^? by DHF, followed by attack of O₂^? on hemoglobin. The reaction of O₂^? with hemoglobin leads to the formation of a hemoglobin-breakdown product that binds to the red cell membrane, resulting in an increased osmotic fragility of the cell.     

Copper Ion from Cu2O Crystal Induces AMPK-Mediated Autophagy via Superoxide in Endothelial Cells

Copper is an essential element required for a variety of functions exerted by cuproproteins. An alteration of the copper level is associated with multiple pathological conditions including chronic ischemia, atherosclerosis and cancers. Therefore, copper homeostasis, maintained by a combination of two copper ions (Cu⁺ and Cu²⁺), is critical for health. However, less is known about which of the two copper ions is more toxic or functional in endothelial cells. Cubic-shaped Cu₂O and CuO crystals were prepared to test the role of the two different ions, Cu⁺ and Cu²⁺, respectively. The Cu₂O crystal was found to have an effect on cell death in endothelial cells whereas CuO had no effect. The Cu₂O crystals appeared to induce p62 degradation, LC3 processing and an elevation of LC3 puncta, important processes for autophagy, but had no effect on apoptosis and necrosis. Cu₂O crystals promote endothelial cell death via autophagy, elevate the level of reactive oxygen species such as superoxide and nitric oxide, and subsequently activate AMP-activated protein kinase (AMPK) through superoxide rather than nitric oxide. Consistently, the AMPK inhibitor Compound C was found to inhibit Cu₂O-induced AMPK activation, p62 degradation, and LC3 processing. This study provides insight on the pathophysiologic function of Cu⁺ ions in the vascular system, where Cu⁺ induces autophagy while Cu²⁺ has no detected effect.     

Cytochrome c reduction by semiquinone radicals can be indirectly inhibited by superoxide dismutase

The inhibition by superoxide dismutase of cytochrome c reduction by a range of semiquinone radicals has been studied. The semiquinones were produced from the parent quinones by reduction with xanthine and xanthine oxidase. Most of the quinones studied were favored over O₂ as the enzyme substrate, and in air as well as N₂, semiquinone radicals rather than superoxide were produced and they caused the cytochrome c reduction. With all but one of the quinones (benzoquinone), cytochrome c reduction in air was inhibited by superoxide dismutase, but the amount of enzyme required for inhibition was up to 100 times greater than that required to inhibit reduction by superoxide. It was highest for the quinones with the highest redox potential. These results demonstrate how superoxide dismutase can inhibit cytochrome c reduction by species other than superoxide. They can be explained by the dismutase displacing the equilibrium: semiquinone + O₂ ? quinone + O₂^? to the right, thereby allowing the forward reaction to out-compete other reactions of the semiquinone. The implication from these findings that superoxide dismutase-inhibitable reduction of cytochrome c may not be a specific test for superoxide production is discussed.     

Superoxide anion permeability of phospholipid membranes and chloroplast thylakoids

The permeability of phospholipid membranes to the superoxide anion (O₂^?) was determined using soybean phospholipid vesicles containing FMN in the internal space. The efflux of O₂^? generated by the illumination of FMN was so slow that more than 90% of the radicals were spontaneously disproportionated within the vesicles before they could react with cytochrome c at the membrane exterior. The amount of diffused O₂^? was proportional to the intravesicular concentration of O₂^? over a range from 1 to 10 μm which was deduced from its disproportionation rate. The permeability coefficient of the phospholipid bilayer for O₂^? was estimated to be 2.1 × 10^?6 cm s^?1 at pH 7.3 and 25 ° C. Superoxide dismutase trapped inside vesicles was not reactive with extravesicular O₂^? unless Triton X-100 was added. O₂^? generated outside spinach chloroplast thylakoids did not interact with superoxide dismutase or cytochrome c which had been enclosed in the thylakoids. Thus, chloroplast thylakoids also showed little permeability to O₂^?.     

Conductive Copper Niobate: Superior Li+‐Storage Capability and Novel Li+‐Transport Mechanism

Niobates with shear ReO₃ crystal structures are remarkably promising anode materials for Li⁺ batteries due to their large capacities, inherent safety, and high cycling stability. However, they generally suffer from limited rate capabilities rooted in their insufficient electronic and Li⁺ conductivities. Here, micrometer‐sized copper niobate (Cu₂Nb₃₄O₈₇) bulk as a new anode material having a high electronic conductivity of 2.1 × 10^?5 S cm^?1 and an impressive average Li⁺ diffusion coefficient of ≈3.5 × 10^?13 cm² s^?1 is exploited, which synergistically leads to an excellent rate capability (184 mAh g^?1 at 10 C) while remaining a large reversible capacity and superior cycling stability. Moreover, the fast Li⁺ transport pathways of grain boundary (micrometer scale) → lattice deformation area (nanometer scale) → (010) crystallographic plane (angstrom scale) are demonstrated in Cu₂Nb₃₄O₈₇. Therefore, these results could pave the way for practical application of Cu₂Nb₃₄O₈₇ in high‐performance Li⁺ batteries.     

Inhibitors of superoxide dismutases: A cautionary tale

An extensive search resulted in the identification of pamoic acid as an inhibitor of superoxide dismutases. Pamoic acid appeared to rapidly and reversibly inhibit all types of superoxide dismutases and did so in both the cytochrome c reduction and in the dianisidine photooxidation assays, used to measure this activity. It could nevertheless be shown that pamoic acid did not at all inhibit superoxide dismutase but rather diminished the sensitivity of the assays. The mechanism proposed to account for this effect involved oxidation of pamoate, by O₂^?, to yield a pamoate radical which can then reduce cytochrome c or oxidize pyrogallol. Pamoate thus competes with superoxide dismutase for the available O₂^?, without affecting the observable effects of that O₂^? upon cytochrome c or upon pyrogallol. It consequently makes these assays less responsive to superoxide dismutase, while appearing to be without effect in the absence of superoxide dismutase. Several of the predicted consequences of this proposal were affirmed. Other workers, interested in finding inhibitors for superoxide dismutases, are hereby forwarned of this subtle snare.     

A new Cu/Zn carboxylato-bridged 1D polymer: Direct synthesis, X-ray structure and magnetic properties

A new complex [Cu₂Zn(O₂CMe)₆(NH₃)₂]_n was isolated as an unexpected product in an open-air synthesis of a mixed-metal compound using zero-valent copper, zinc oxide and ammonium acetate in the presence of 2-diethylaminoethanol in acetonitrile solution. Its structure consists of Cu₂(O₂CMe)₄ units situated on crystallographic inversion centres and Zn(NH₃)₂(O₂CMe)₂ units on crystallographic 2-fold axes. One O atom of each of the O₂CMe groups is attached via Zn bridges to the Cu atoms of the dimeric Cu₂(O₂CMe)₄ unit forming a 1D polymer in the bc direction. The polymer is not linear, with the dihedral angles between successive Cu-Cu vectors being 48.9°. Within the Cu₂(O₂CMe)₄ units the Cu···Cu distance is 2.675(2) Å and the angles between the O₂CMe planes are 88.8(5)°. The magnetic properties have been analyzed using the Hamiltonian ? = J?₁?₂ with J = 286 cm⁻¹ and g = 2.13. High-field EPR spectra showed both the exchange-coupled copper pair and non-interacting copper(II) ions. The presence of the latter species was explained as an effect of zinc atoms occupying a fraction of the copper sites.     

Oxidation of manganous pyrophosphate by superoxide radicals and illuminated spinach chloroplasts.

The oxidation of Mn²⁺-pyrophosphate to Mn³⁺ by superoxide (O₂^?) was quantitative as evidenced from the formation of Mn³⁺-pyrophosphate and hydrogen peroxide and from the inhibition by superoxide dismutase. Using the competitive relation between Mn²⁺-pyrophosphate and superoxide dismutase for the O₂^?, the rate constant of Mn²⁺ oxidation was estimated to be about 6 × 10⁶m^?1 s^?1. The oxidation of Mn²⁺-pyrophosphate by illuminated chloroplasts was also indicated to be stoichiometrically induced by O₂^?. In the presence of saturating amounts of the Mn²⁺, a double enhancement of hydrogen peroxide production and triple uptake of oxygen were found, as expected from the oxidation of Mn²⁺-pyrophosphate by O₂^?. Anaerobiosis or superoxide dismutase annuled these increments. We propose that the O₂^? generated as the sole initial step of the Mehler reaction oxidized Mn²⁺-pyrophosphate, and we discuss the role of free manganese in chloroplasts.     

Inhibition of bovine plasma amine oxidase by superoxide dismutase active Cu(II) complexes

Aqueous Cu²⁺ and Cu(II) complexes of salicylate, lysine, and tyrosine decrease the rate of benzylamine oxidation by bovine plasma amine oxidase. Bissalicylato Cu(II) and Cu²⁺ inhibit non-competitively with respect to benzylamine. Lysine, tyrosine, Cu(EDTA)^2?, Zn²⁺, and Co²⁺ do not inhibit, and erythrocyte Cu, Zn superoxide dismutase shows only slight inhibition of the amine oxidase. The data are most consistent with an inhibitory mechanism involving dismutation of O₂^? by the Cu(II) complexes within a site relatively inaccessible to the enzyme superoxide dismutase. Excess lysine significantly decreases inhibition by the bis-lysine complex of Cu(II).     

Effects of Free Radicals on the Fluidity of Myocardial Membranes

Free radicals, including superoxide anions (O₂^?_?), hydroxyl radical (HO'), and hypohalite radical (OCl'), as well as oxidants such as hydrogen peroxide (H₂O₂) and hypochlorous acid (HOCl), have been indicated in the pathogenesis of myocardial ischemic and reperfusion injury. In this report, we compared the integrity of the myocardial membrane when exposed to these free radicals/oxidants. Isolated rat heart membrane preparations were exposed to chemically generated free radicals with or without their respective scavengers. Membrane fluidity was monitored by fluorescence polarization using the diphenylhexatriene probe, as well as by electron spin resonance (ESR) spectroscopy using 2,2,6,6-tetramethyl piperidine-n-oxyl as the spin labeling agent. HO', H₂O₂, and OCl' + HOCl increased the fluorescence polarization (FP) and microvis-cosity significantly by 1.7-fold, 1.8-fold, and 1.7-fold, respectively, as compared to an only 1.2– fold increase in FP by O₂^?_? O₂^?_? did not alter the fatty acid profiles of the membrane phospholipids. However, HO' and H₂O₂ reduced the arachidonic acid contents in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). These radicals also stimulated the lipid peroxidation by several-fold, while that by O₂^?_? was only insignificant. These results suggest that HO' and H₂O₂ decreased the membrane fluidity and induced lipid peroxidation by releasing the arachidonic acid from PC, PE. and PI.     

Hydrogen peroxide mediated tolerance to copper stress in the presence of 28-homobrassinolide in Vigna radiata

Hydrogen peroxide (H₂O₂) in minute quantity serves as a signalling molecule. However, the role of H₂O₂ in combination with brassinosteroids (stress regulators) in plants under toxic levels of copper, is poorly understood. With an aim to explore and elaborate their role in plants subjected to abiotic stress, the surface sterilized seeds of mung bean (Vigna radiata) were sown in earthen pots filled with soil and manure enriched with different levels of Cu²⁺ (50 or 100 mg kg^?1 of soil) and allowed to grow under natural environmental conditions. At 15 and 20 days stage, the plants were sprayed with H₂O₂ (2.5 mM) and/or 28-homobrassinolide (HBL, 10^?5 mM), respectively. At 45 days stage, the analysis of the plants revealed that the presence of copper in the soil caused a significant decrease in growth characteristics, activity of carbonic anhydrase and nitrate reductase, relative water content, chlorophyll content and the rate of photosynthesis whereas, the activity of antioxidant enzymes (catalase, peroxidase and superoxide dismutase) and the proline accumulation in leaves increased in Cu stressed plants. However, the exogenously applied HBL and/or H₂O₂, in the absence of Cu-stress strongly favoured the growth, photosynthetic parameters and also improved the activity of antioxidant enzymes and the proline content. Furthermore, the combined application of HBL and H₂O₂ to the foliage of the stressed plants neutralized the toxic impact of all copper regimes. Therefore, we are of the opinion that these chemicals somehow maintained the homeostasis of the metal in the plants that exhibit healthy growth.