Catalysts For Redox Reactions Research Articles

Iron chelators: as therapeutic agents in diseases.

The concentration of iron is tightly regulated, making it an essential element. Various cellular processes in the body rely on iron, such as oxygen sensing, oxygen transport, electron transfer, and DNA synthesis. Iron excess can be toxic because it participates in redox reactions that catalyze the production of reactive oxygen species and elevate oxidative stress. Iron chelators are chemically diverse; they can coordinate six ligands in an octagonal sequence. Because of the ability of chelators to trap essential metals, including iron, they may be involved in diseases caused by oxidative stress, such as infectious diseases, cardiovascular diseases, neurodegenerative diseases, and cancer. Iron-chelating agents, by tightly binding to iron, prohibit it from functioning as a catalyst in redox reactions and transfer iron and excrete it from the body. Thus, the use of iron chelators as therapeutic agents has received increasing attention. This review investigates the function of various iron chelators in treating iron overload in different clinical conditions.

Pyrroloquinoline Quinone (PQQ) Improves Long-term Survival of Fat Grafts by Alleviating Oxidative Stress and Promoting Angiogenesis During the Early Phase After Transplantation.

Reducing absorption after autologous fat grafting is a current challenge. Pyrroloquinoline quinone (PQQ) is the strongest known catalyst for redox reactions, which can scavenge reactive oxygen species (ROS) and alleviate oxidative stress. To establish an in vivo model of PQQ-assisted lipotransfer to clarify the role of PQQ in reducing oxidative stress, alleviating apoptosis, and promoting angiogenesis during the acute hypoxic phase after grafting. Also, to assess whether this intervention would have a positive effect on the improvement of long-term volume retention. Different concentrations of PQQ (low: 10 μM, medium: 100 μM, and high: 1000 μM) were mixed with human adipose tissue and transplanted subcutaneously into nude mice. Meanwhile, a control group of phosphate-buffered saline in an equal volume to PQQ was set up. On the 3rd day after grafting, whole mount fluorescence staining was applied to detect ROS, mitochondrial membrane potential (MMP), apoptosis, adipocyte activity, and angiogenesis. Graft volume retention rate and electron microscopic morphology were evaluated at the 3rd month. Immunohistochemistry and polymerase chain reaction (PCR) were further used to elucidate the mechanism of action of PQQ. PQQ-assisted fat grafting improved the long-term volume retention, promoted the quality and viability of the adipose tissue, and reduced the level of fibrosis. The underlying mechanism was that PQQ could assist in scavenging the accumulated ROS, restoring MMP, enhancing adipocyte viability, alleviating tissue apoptosis, and promoting timely angiogenesis during the hypoxia stress phase. Among them, the most effective concentration of PQQ was 100 μM. Immunohistochemistry and PCR experiments confirmed that PQQ reduced the expression of Bax and cytochrome-c in the mitochondrial apoptotic pathway and increased the level of anti-apoptotic molecule Bcl-2. PQQ could improve the long-term survival of adipocytes by alleviating hypoxic stress and promoting timely angiogenesis in the early phase after lipotransfer.

Insights from Enzymatic Catalysis: A Path towards Bioinspired High‐Performance Electrocatalysts

AbstractEnzymes, especially some metalloenzymes, are exceptional catalysts for redox reactions with technological applications in several fields, including energy conversion systems and producing fuels and value‐added chemicals. Because of that, they have inspired the development of synthetic catalysts to obtain high‐performance materials. However, several fundamental aspects should be better understood to achieve this goal, such as enzyme catalysis and the role of the three‐dimensional catalytic site. Therefore, this review addresses key structural and activity aspects of some remarkable metalloenzymes and discusses three‐dimensional catalysis as an approach to enzyme catalysis. Also, the case of the multicopper oxidase enzymes is shown as an example of how the three‐dimensional coordination environment of the copper ions in the active site can impact catalysis.

MnO Enabling Highly Efficient and Stable Co‐Nx/C for Oxygen Reduction Reaction in both Acidic and Alkaline Media

AbstractIn situ growing transition metals on N‐doped carbon by atomic doping produces a class of promising alternatives to replace Pt‐based catalysts for redox reactions, yet still suffer from unsatisfactory activity and durability in acidic and basic media. Herein, a simple synthetic strategy to fabricate an MnO modifying Co‐Nx/C catalyst with high activity and robust durability is presented. The interphase engineering well controls the Co and N species in the carbon matrix, affording the material with more pyridine N and graphite N; the interaction between Co‐Nx and MnO phase is also well discussed. Accordingly, the obtained Co‐Nx/C‐MnO catalyst exhibits excellent electrocatalytic properties towards oxygen reduction reaction, achieving a half‐wave potential of 0.87 and 0.66 V versus reversible hydrogen electrode in 0.1 m KOH and 0.1 m HClO4 solutions, as well as excellent durability with only −16.9 and −12.2 mV shift after 1000 cycles, respectively. This study provides insights into the design of noble‐metal‐free electrocatalysts from the perspective of active sites and catalyst carriers.

Tumor Microenvironments-Adaptive Apoptotic Effects of Cytidine 5'-monophosphate-Capped Gold Nanoclusters.

In the present work, cytidine 5'-monophosphate capped gold nanoclusters (AuNCs@CMP) are reported as a catalyst for redox reactions, which show both oxidase- and excellent peroxidase-like activity. When employing 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate in the presence of hydrogen peroxide (H2O2), the maximum velocity (Vmax) was 175 × 10-8 M s-1in vitro. Besides, the AuNCs@CMP exhibited high catalytic activity for reactive oxygen species (ROS) generation with H2O2. Particularly, they also displayed excellent catalytic activity for ROS generation in tumor cells, being activated and promoted by the tumor microenvironment (TME). Consequently, the AuNCs@CMP show an excellent antitumor effect on HeLa and SW480 cells as assayed by flow cytometry. The antitumor mechanism of AuNCs@CMP was attributed to the high ROS generation based on the specific environments of the TME. Therefore, the present study provides TME-adaptive AuNCs@CMP with excellent mimetic peroxidase activity, producing significant ROS to kill the tumor cells in TME.

Enhanced mass transfer in nanofluid electrolytes for aqueous flow batteries: The mechanism of nanoparticles as catalysts for redox reactions

Carbon-based nanoparticles are introduced into the electrolytes of vanadium redox flow batteries and their impact on the electrochemical performance of these batteries is experimentally investigated. Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy characterizations reveal defects in the nanoparticles that act as active sites for electrochemical reactions. Cyclic voltammetry analysis reveals that the nanofluidic electrolytes exhibits higher oxidation/reduction kinetics and mass transfers than those of pristine electrolytes. The mass transfers of the VO2+/VO2+ electrolyte ions during oxidation/reduction reactions are 17.2% and 59.8%, respectively, while those of the V2+/V3+ electrolyte ions are 5.6 times and 1.2 times higher than those of the VO2+ and VO2+ electrolytes, respectively. Overall, these nanofluidic electrolytes enhance electron and mass transfers at the active region interface. According to the redox mechanism, the VO2+/VO2+ electrolytes have an increased active area, while the V2+/V3+ electrolytes demonstrate excellent performance by acting as a catalyst to reduce the redox reaction activation energy.

Thermal decomposition derived nano molybdenum nitride for robust counter electrode in dye-sensitized solar cells

The unique category of transition metal nitrides has an immense scope as an electron-driven catalyst in redox reactions. However, synthesizing metal nitrides without contamination is very challenging. The residues present in the catalyst might affect catalytic activity. This work reports a simple synthesis of contamination-free nanoscale molybdenum nitride (Mo2N) powder by integrated wet chemical and thermal decomposition techniques at 800 ̊°C. Systematic structural and morphological studies were done, which shows the spherical shape of γ -Mo2N nanoparticles. Electrochemical and photovoltaic characteristics were studied using cyclic voltammetry, electrochemical impedance spectroscopy (EIS), Tafel polarization and J–V characteristics. As a result of high electrolyte diffusivity, less charge transfer resistance, high electrochemical stability and catalytic activity, the nano Mo2N based DSSCs exhibits 5.3 % efficiency, which is comparable to Pt-based device (6.4 %) fabricated under the similar condition that is 83.7 % of the performance offered by an expensive counter electrode. This simple synthesis method could enable low-cost mass production of Mo2N nanoparticles as counter electrodes in DSSC. The developed counter electrodes may be a suitable alternative for stable, efficient and low-cost DSSCs.

Charge transfer and band gap opening of a ferrocene/graphene heterostructure

The objective of this paper was to study the electrochemical and electronic behavior of pristine graphene used as a catalyst for redox reactions and the charge transport mechanisms that occur in the interface between graphene and covalently attached ferrocene. The covalent grafting and chemical modification of epitaxial graphene by a ferrocene redox marker was performed through electrochemical oxidation of ethylene diamine that provided a low coverage density and a short distance between the ferrocene and graphene. The electrochemical activity, as well as, the electronic and structural properties of the nanomaterials were characterized by Cyclic Voltammetry, Electrochemical Impedance Spectroscopy, X-ray Photoelectron Spectroscopy, RAMAN spectroscopies and Angle-Resolved-Photoelectron Spectroscopy. We show that the proposed modification improved the electronic properties of graphene where p-doping was observed and a band gap opening of about 60 meV was demonstrated. The electrochemical activity and electron transfer ability was inproved. A heterogeneous electron transfer rate of KS = 7 s−1, was obtained thanks to a low density of immobilzed ferrocene and the small spacer provided by ethylene diamine giving a fast tunneling electron transfer. We believe promising perspectives for the development of self-organized graphene electrodes with various electrochemical properties by tailoring the attached redox functional groups.

Sn-Containing Oxide Coatings: Formation, Composition, Electroanalytical and Catalytic Properties

Sn-containing oxide coatings were prepared via plasma electrolytic oxidation (PEO) of Ti plate in the electrolytes with [SnII-EDTA]2- complex anions or SnO2 particles in the anode and anodic-cathodic modes. The coatings formed in electrolyte with SnO2 particles stabilized by SAS contain SnO2 and Sn0. In the electrolyte with [Sn-EDTA]2- complex anions, the SnO2-containing coatings were formed in the anodic mode while Sn0-containing ones were obtained in the anodic-cathodic mode. SnO2-containing structures formed in the electrolytes with [Sn-EDTA]2- anions are shown to be active in catalytic oxidation of CO into CO2 at temperatures above 350 °C. They can be the basis for the preparation of both carriers of catalytically active compounds and catalysts for redox reactions. Potentiometric tests showed that the Sn-containing PEO layers on titanium exhibit the most characteristic pH function for the metal oxide electrodes in the direct potentiometry and acid-base titration.

The Roll of COMMD1 in Membrane Protein Trafficking

Copper is essential for all life due to its role as a catalyst in redox reactions. However, it must be tightly regulated as excess copper can cause oxidative damage to the cell. One example of copper dyshomeostasis is the autosomal recessive condition is known as Wilson disease which can result in liver disease and neurological problems. To maintain copper homeostasis, cells rely on a system of chaperone proteins as well as three copper transporters; CRT1, ATP7A, and ATP7B. ATP7B is a large multidomain membrane‐spanning protein that exports copper from the cytosol and is highly expressed in liver, brain, and kidney cells. ATP7B has a dual role in maintaining copper homeostasis. In the liver, ATP7B functions in the trans‐Golgi network where it pumps copper into the lumen for biosynthesis of secreted cuproproteins. When cellular copper levels are elevated, ATP7B traffics in vesicles towards the apical membrane to move excess copper into bile for excretion from the body. This proves to be a complex decision for the cell as it must retain some ATP7B at the trans‐Golgi for continued biosynthesis of cuproproteins while simultaneously removing the excess. When copper levels are low, ATP7B returns to the Golgi, though it is likely that some is degraded. Although maintaining proper copper homeostasis is important, the mechanism that influences ATP7B distribution is poorly understood. Since being identified as mutated in Bedlington Terriers exhibiting canine copper toxicosis, the protein COMMD1 has been of particular interest as a regulator of cellular copper levels. COMMD1 exhibits both Phosphatidylinositol (PdtIns) effector and Ubiquitin ligase adaptor activity. COMMD1 interacts with a number of membrane proteins including ATP7B. Importantly, COMMD1 downregulation affects cellular copper levels. To investigate the relationship between COMMD1 and ATP7B we used quantitative immunofluorescence microscopy in HepG2 hepatoma cells to analyze the subcellular locations of ATP7B in response to misexpression of COMMD1. The overexpression of the COMMD1 T174M mutant, which was first identified in an atypical Wilson disease patient, significantly decreased the Golgi‐associated fraction of ATP7B and colocalization between ATP7B and lysosomes was also reduced during retrograde trafficking. The T174M mutation may affect PtdIns(4,5)P2 binding by COMMD1, thus we believe that PtdIns(4,5)P2 recruits and facilitates a functional interaction between COMMD1 and ATP7B.Support or Funding InformationNational Science Foundation (MCB‐1411890), NIH 8P20GM103395‐12 (JLB Sub‐Project)

Nanostructured Praseodymium Oxide: Correlation Between Phase Transitions and Catalytic Activity

AbstractPraseodymia gives rise to a rich phase diagram with a large number of phases between the limiting stoichiometries Pr2O3 and PrO2 that differ only slightly in oxygen content (PrnO2n−2). This chemical and crystallographic variability allows the system to release or incorporate lattice oxygen easily at sufficiently high temperatures and thus renders the material interesting as a catalyst for redox reactions according to a Mars–van Krevelen mechanism. Nanostructured praseodymia samples are investigated in this study with respect to their catalytic properties, focusing on methane oxidation and selective NO reduction by CO and CH4. To correlate catalytic activity and crystallographic changes, complementary high‐temperature X‐ray diffraction measurements have been carried out. The determined temperatures of transitions between different oxide phases agree well with peaks in the temperature‐programmed reduction measurements, confirming the direct connection between the availability of lattice oxygen and crystallographic transformations. The catalytic activity for methane oxidation and NO reduction sets in at 450–500 °C, at which temperature the starting material—mainly Pr6O11—transforms into the next oxygen‐depleted phase Pr7O12. With respect to NO reduction, the results show that it is possible to employ both methane and carbon monoxide as reducing agents in the absence of oxygen, in agreement with a Mars–van Krevelen mechanism. Nevertheless, the use of CO instead of CH4 offers considerable advantages, as no deactivation due to carbon residues takes place in this case. Whereas, in an excess of oxygen, NO reduction is inhibited independently of the reducing agent, it is shown that NO reduction can proceed if the O2 concentration remains below a critical concentration.

Composition and structure of iron oxidation surface layers produced in weak acidic solutions

Although oxidation/passivation of iron in basic solution has been extensively investigated, there is very little information on iron corrosion in weak acidic solutions. In this work, iron surface composition and structure, produced in aerobic aqueous solutions ranging from pH 2 to 5, were determined in detail by the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The most striking observation is that at pH 2 and 3 almost all oxidized iron is dissolved in solution, whereas at pH 4 and 5 the product of iron oxidation is deposited on the iron surface in the form of lepidocrocite, gamma-FeOOH. Detailed iron surface and solution analyses allow the proposition of the following overall oxidation reactions: [EQUATION: SEE TEXT]. At pH 2 and 3, only a very thin surface layer consisting of FeO and Fe(OH)2 with polymeric structure is observed on the iron surface. The amounts of these surface species remain almost constant (2-5 nm) from the first minutes to a few hours of reaction, if pH is kept constant. Nevertheless, with time the akaganeite-like, beta-FeOOH structure is also detected. At pH 4 and 5, the amount of lepidocrocite deposited on the iron surface increases with reaction time. Detailed quantitative evaluation of the lepidocrocite produced at pH 5 and its surface distribution on iron was performed based on the comparison of infrared spectroscopic data with spectral simulation results of assumed surface structures. At pH 4 and 5 and a temperature of 40-50 degrees C, in addition to a very large amount of lepidocrocite other oxy-hydroxide surface species such as goethite (alpha-FeOOH) and feroxyhite (delta-FeOOH), were identified. Addition of Cl- ions to solution at 10(-3) M concentration at pH 5 increases the oxidation rate of iron by about 50%, and lepidocrocite remains the only oxidation product. Similarly, an addition of Fe2+ ions to solution at pH 5 very strongly enhances lepidocrocite formation as well as its conductivity. The latter finding is important for the possible application of metallic iron as a catalyst in redox reactions, for example, for decomposition of difficult-to-biodegrade water pollutants.

Crystal Chemistry, Properties and Synthesis of Microporous Silicates Containing Transition Elements

AbstractFor Abstract see ChemInform Abstract in Full Text.

Copper and iron hydroxides as new catalysts for redox reactions in aqueous solutions

Supported or colloidal CuII and FeIII hydroxides catalyse the oxidation of catechol to a muconic acid derivative and of benzene to phenol in aqueous hydrogen peroxide solutions at ambient temperature.

Preparation of Gelatin Layer Film with Gold Clusters in Using Photographic Film

A gelatin layer film with gold clusters is produced by taking advantage of the photosensitivity of silver halide photography. Through exposure silver specks, which are called latent-image specks and are composed of several reduced silver atoms, are formed on the surface of silver halide grains in the photographic film. As the latent-image specks act as a catalyst for redox reaction, reduced gold atoms are deposited on the latent-image specks when the exposed film is immersed in a gold (I) thiocyanate complex solution for 5–20 days. Subsequently, when the silver halide grains are dissolved and removed, the gelatin layer film with gold clusters remains. The film produced by this method is purple and showed an absorption spectrum having a maximum of approximately 560 nm as a result of plasmon absorption. The clusters continued to grow with immersion time, and the growth rate increased as the concentration of the gold complex solution was increased. The cluster diameter changed from 20 nm to 100 nm. By this method, it is possible to produce a gelatin film of a large area with evenly dispersed gold clusters, and since it is produced only on the exposed area, pattern forming is also possible.

Porphyrins, porphyrin metabolism and porphyrias. I. Update

The biosynthesis of porphyrins is one of the most conserved parthways known, about the same sequence of reactions taking place in all species. By associating different metals, porphyrins give rise to the ?pigments of life?: chlorophyll, haem and cobalamin. The unique tetrapyrrolic structure enables it to function in an array of reactions as a single electron carrier and as a catalyst for redox reactions. In this capacity, it constitutes the prosthetic group of enzymes participating in cellular respiration, in conversion reactions involving steroids and lipophilic xenobiotics, in protective mechanisms directed against oxidative stress and in pathways providing central messenger molecules. The formation of haem is accomplished by a sequence of eight dedicated enzymes encoded by different genes, some being active in ubiquitous as well as in erythroid isoforms. Large differences between the participating enzymes with regard to catalytic power, with low capacity steps positioned early in the catalytic chain, constitute a bar against substrate overloading of enzymes processing porphyrins, thus preventing accumulation in the body of these phototoxic compounds under physiological conditions. Most of the haem in the body is produced by the liver and bone marrow, but the mechanisms applied for the control of the synthesis differ between the two organs. The extremely potent hemeprotein enzymes formed in the liver are rapidly turned over in response to current metabolic needs. They have half-lives in the order of minutes or hours and are restored by fast-acting mechanisms for the de novo synthesis, when needed. Uninterrupted and instant availability of the compound is secured by acute deinhibition of the initial enzyme of the synthetic chain, ubiquitous 5-aminolevulinate synthase (ALAS-1), in response to drain of the free cellular haem pool caused by prevailing demands for hemeproteins or by increased catabolism of the compound. In contrast, in the erythroid progenitor cell the haem synthetic machinery is designed for uninterrupted production of huge amounts of haem for combination with globin chains to form hemoglobin at a steady rate. In the erythron the synthesis of the enzymes participating in the formation of haem is under control of erythropoietin, formed under hypoxic conditions. In the absence of iron, to be incorporated in the porphyrin formed in the last step of the synthesis, the mRNA of erythroid 5-aminolevulinate synthase (ALAS-2) is blocked by attachment of an iron-responsive element (IRE) binding cytosolic protein, and transcription of this key enzyme is inhibited. In humans, the genes for each of the haem synthetic enzymes may become the target of mutations that give rise to impaired cellular enzyme activity. Seven of the enzyme deficiencies are associated with accumulation of toxic intermediaries and with disease entities termed porphyrias. The acute porphyrias are characterized by attacks of neuropsychiatric symptoms, which may be due to a toxic surplus of the porphyrin presursor 5-aminolevulinic acid, or a consequence of a deficit of vital hemeproteins resulting from impaired synthesis of haem. In the cutaneous porphyrias, impairment of enzymatic steps where porphyrins are processed gives rise to solar hypersensitivity due to accumulation of phototoxic porphyrins in the skin. Early diagnosis, information to the patient regarding the nature of the illness and counselling aimed at avoidance of triggering factors are cornerstones in the handling of the porphyric diseases. Gene analysis is of incomparable diagnostic reliability in carrier detection, but biochemical methods must be applied in the important task of monitoring porphyric disease activity. In most forms of porphyria the gene carriers run the risk of development of associated diseases in liver or kidneys, a circumstance that prompts application of well-structured surveillance programs.

Kinetic modeling studies of transition metal-exchanged zeolites as catalysts for redox reactions

A kinetic modeling program based on a regenerative oxidation-reduction mechanism was used to simulate experimental data on the catalytic activities of Fe-Y and Fe-mordenite zeolites for the oxidation of CO with O 2, NO, and N 2O. The model was also used to simulate catalytic activities of Y-zeolites exchanged with different first row transition metals (Cr, Mn, Fe, Co, Ni, Cu) for the oxidation of CO with NO and the decomposition of N 2O into its elements. It was found that the model could predict the relative qualitative activities of the catalysts by assuming only that the energy of the bond between base exchange cations and adsorbed extralattice oxygen was a function of the base exchange metal. Since this model simulates successfully most of the general catalytic trends of transition metal-exchanged zeolites for these redox reactions, it is proposed that it be used as a guide to future experimental work for related reactions. As an example, the model was used to estimate activities of transition metal-exchanged zeolites for the partial oxidation of methane. The simulations predict that high yields of methanol and formaldehyde are more likely to be obtained with zeolites exchanged with cations that cannot be easily oxidized to higher oxidation states, such as Co(II).