Coupled Oxidation Reduction Reactions Research Articles

Green synthesis of bifunctional phthalocyanine-porphyrin cofs in water for efficient electrocatalytic CO2 reduction coupled with methanol oxidation.

Electrocatalytic CO2 reduction (ECR) coupled with organic oxidation is a promising strategy to produce high value-added chemicals and improve energy efficiency. However, achieving the efficient redox coupling reaction is still challenging due to the lack of suitable electrocatalysts. Herein, we designed two bifunctional polyimides-linked covalent organic frameworks (PI-COFs) through assembling phthalocyanine (Pc) and porphyrin (Por) by non-toxic hydrothermal methods in pure water to realize the above catalytic reactions. Due to the high conductivity and well-defined active sites with different chemical environments, NiPc-NiPor COF performs efficient ECR coupled with methanol oxidation reaction (MOR) (Faradaic efficiency of CO (FECO) = 98.12%, partial current densities of CO (jCO) = 6.14mA cm-2 for ECR, FEHCOOH=93.75%, jHCOOH=5.81mA cm-2 for MOR at low cell voltage (2.1V) and remarkable long-term stability). Furthermore, experimental evidences and density functional theory (DFT) calculations demonstrate that the ECR process mainly conducts on NiPc unit with the assistance of NiPor, meanwhile, the MOR prefers NiPor conjugating with NiPc. The two units of NiPc-NiPor COF collaboratively promote the coupled oxidation-reduction reaction. For the first time, this work achieves the rational design of bifunctional COFs for coupled heterogeneous catalysis, which opens a new area for crystalline material catalysts.

Quantifying inorganic sources of geochemical energy in hydrothermal ecosystems, Yellowstone National Park, USA

Combining analytical data from hot spring samples with thermodynamic calculations permits a quantitative assessment of the availability and ranking of various potential sources of inorganic chemical energy that may support microbial life in hydrothermal ecosystems. Yellowstone hot springs of diverse geochemical composition, ranging in pH from <2 to >9 were chosen for this study, and dozens of samples were collected during three field seasons. Field measurements of dissolved oxygen, nitrate, nitrite, total ammonia, total sulfide, alkalinity, and ferrous iron were combined with laboratory analyses of sulfate and other major ions from water samples, and carbon dioxide, hydrogen, methane, and carbon monoxide in gas samples to evaluate activity products for ∼300 coupled oxidation–reduction reactions. Comparison of activity products and independently calculated equilibrium constants leads to values of the chemical affinity for each of the reactions, which quantifies how far each reaction is from equilibrium. Affinities, in turn, show systematic behavior that is independent of temperature but can be correlated with pH of the hot springs as a proxy for the full spectrum of geochemical variability. Many affinities are slightly to somewhat dependent on pH, while others are dramatically influenced by changes in chemical composition. All reactions involving dissolved oxygen as the electron acceptor are potential energy sources in all hot spring samples collected, but the ranking of dominant electron donors changes from ferrous iron, and sulfur at high pH to carbon monoxide, hydrogen, and magnetite as pH decreases. There is a general trend of decreasing energy yields depending on electron acceptors that follows the sequence: O 2(aq) > NO 3 − ≈ NO 2 − ≈ S > pyrite ≈ SO 4 −2 ≈ CO(g) ≈ CO 2(g) at high pH, and O 2(aq) ≈ magnetite > hematite ≈ goethite > NO 3 − ≈ NO 2 − ≈ S ≈ pyrite ≈ SO 4 −2 at low pH. Many reactions that are favorable sources of chemical energy at one set of geochemical conditions fail to provide energy at other conditions, and vice versa. This results in energy profiles supplied by geochemical processes that provide fundamentally different foundations for chemotrophic microbial communities as composition changes.

Preliminary study of coupled oxidation—reduction reactions of included ions in growing ice crystals

Preliminary study of coupled oxidation—reduction reactions of included ions in growing ice crystals

Occurrence and characterization of selenocysteine in proteins

This chapter focuses on the occurrence and characterization of selenocysteine in proteins. Four selenium-dependent enzymes that contain essential selenium in the form of selenocysteine residues have been identified to date. Three of these are of bacterial origin—glycine reductase, certain formate dehydrogenases, and α hydrogenase—and the fourth—glutathione peroxidase—is found in mammals and birds. These four enzymes catalyze coupled oxidation–reduction reactions and can be considered to be typical redox enzymes. The treatment of the glycine reductase selenoprotein with dithiothreitol, the electron donor for in vitro reaction, converts both the selenocysteine and the cysteine residues to forms reactive with alkylating agents. During catalysis with molecular hydrogen as substrate, the selenium-containing hydrogenase is markedly sensitive to inactivation with alkylating agents, and under these conditions, the corresponding alkyl seleno ethers of the selenocysteine residues are formed. Selenocysteine residues in proteins are labile to the procedures commonly employed to cleave proteins to their constituent amino acids. However, if the selenocysteine residues are reduced and alkylated prior to acid hydrolysis or enzymatic digestion, they can be detected and estimated quantitatively.

Betaine: New Oxidant in the Stickland Reaction and Methanogenesis from Betaine and l -Alanine by a Clostridium sporogenes-Methanosarcina barkeri Coculture

Growing and nongrowing cells of Clostridium sporogenes fermented betaine with l-alanine, l-valine, l-leucine, and l-isoleucine as electron donors in a coupled oxidation-reduction reaction (Stickland reaction). For the substrate combinations betaine and l-alanine and betaine and l-valine balance studies were performed; the results were in agreement with the following fermentation equation: 1 R- CH(NH(2))-COOH + 2 betaine + 2 H(2)O --> 1 R-COOH + 1 CO(2) + 1 NH(3) + 2 trimethylamine + 2 acetate. Growth and production of trimethylamine were strictly dependent on the presence of selenite in the medium. With cell suspensions it was shown that C. sporogenes was unable to catabolize betaine as a single substrate. Betaine, however, was reduced to trimethylamine and acetate under an atmosphere of molecular hydrogen. For the reduction of betaine by cell extracts of C. sporogenes, dimercaptans such as 1,4-dithiothreitol could serve as electron donors. No betaine reductase activity was detected in cells grown in a complex medium without betaine. The pH optimum of betaine reductase was at pH 7.3. When C. sporogenes was cocultured with Methanosarcina barkeri strain Fusaro on betaine together with l-alanine, an almost complete conversion of the two substrates to CH(4), NH(3), and presumably CO(2) was observed.

Interactions between amino‐acid‐degrading bacteria and methanogenic bacteria in anaerobic digestion

The degradation of amino acids in anaerobic digestion was examined in terms of the interactions between amino-acid-degrading bacteria and methanogenic bacteria. Certain amino acids were degraded oxidatively by dehydrogenation, with methanogenic bacteria acting as H(2) acceptors. The inhibition of methanogenesis by chloroform also inhibited the degradation of these amino acids and/or caused variations in the composition of volatile acids produced from them. The presence of glycine reduced the inhibitory effect caused by chloroform, probably because glycine acted as an H(2) acceptor in place of methanogenic bacteria. This fact suggested that the coupled oxidation-reduction reactions between two amino acids-one acting as the H(2) donor and the other acting as the H(2) acceptor-may occur in the anaerobic digestion of proteins or amino-acid mixtures. The conversion of some proteins to volatile acids was not affected when methanogenesis was inhibited by chloroform. This suggested that the component amino acids of proteins may be degraded by the coupled oxidation-reduction reactions and that the degradation of proteins may not be dependent on the activity of methanogenic bacteria as H(2) acceptors.

Coupled oxidoreductase activity of horse liver alcohol dehydrogenase

Two assay procedures were used to study ethanol oxidation by crystalline horse liver alcohol dehydrogenase: (A) Acetaldehyde formed by ethanol oxidation (with and without lactaldehyde) was distilled into a semicarbazide solution and the absorbance of the acetaldehyde semicarbazone formed was measured at 224 nm. (B) 1,2-propandiol formed in the coupled oxidation-reduction reaction (CH 2CH 2OH + CH 2· ▪ was measured by a colorimetric method. The K M for ethanol oxidation at pH 7.5 was 0.35 m m. With increasing concentrations of lactaldehyde, both the V M and the K M for ethanol oxidation were increased. At different concentrations of lactaldehyde, the ratio K M V M was found to be constant. The K M for lactaldehyde was 1.1 m m and was not affected by addition of ethanol. The K M for NAD +(H) in the coupled O-R reaction was 4.5 μ m. Propanol and butanol could replace ethanol in the coupled O-R reaction, but were inhibitory at high concentrations. Increasing the lactaldehyde concentration partially relieved this inhibition. In addition to lactaldehyde, glyceraldehyde also stimulated ethanol oxidation. The V M and K M values of three alcohol:aldehyde pairs were compared to determine how the substrate nature controls the efficiency of an alcohol:aldehyde pair in the coupled O-R reaction. Several NAD + analogs were tested in the coupled O-R reaction. It was observed that although these analogs were more active than NAD + in ethanol oxidation in the absence of lactaldehyde, they were less active than NAD + when lactaldehyde was present. A mechanism has been proposed to explain the coupled oxidoreductase activity of liver alcohol dehydrogenase. The mechanism postulates that the rate-limiting step in the coupled oxidoreductase reaction is either the ternary complex interconversions (E-NAD +-RCH 2OH ⇌ E-NADH-RCHO), or the dissociation steps leading to the formation of E-NAD +(H) and product. The mechanism is consistent with the kinetic results described in this paper.

New enzyme cycling method for determination of oxidized and reduced nicotinamide-adenine dinucleotide

A highly sensitive enzyme cycling method for the measurement of NAD +(H) concentration in tissue extracts has been described. The assay measured propanediol formed during the coupled oxidation-reduction reactions between ethanol and lactaldehyde as catalyzed by liver alcohol dehydrogenase in the presence of catalytic concentrations of NAD +(H). The assay can be used to determine NAD +(H) concentrations in the range of 0.4 to 4 pmoles of coenzyme in a 0.1 ml reaction volume.

A study of formaldehyde dismutation by liver alcohol dehydrogenase with NAD +-analogs

Crystalline horse liver alcohol dehydrogenase catalyzed reduction of acetylpyridine-NAD + in the presence of formaldehyde. The formation of reduced coenzyme was followed spectrophotometrically by recording the absorbance increase at 363 mμ. Formaldehyde oxidation activity and ethanol oxidation activity of the enzyme, both measured by acetylpyridine-NADH formation, responded similarly to the effects of different inhibitors. Acetylpyridine-NAD + was also active in the overall dismutation of formaldehyde and acted catalytically in this process. However, with equivalent concentrations of the coenzymes (0.5 m m) acetylpyridine-NAD + was only 10% as efficient as NAD +. With either NAD + or acetylpyridine-NAD +, addition of alcohols markedly stimulated the dismutation reactions. The pH optimum for formaldehyde dismutation is around 8. In the presence of added alcohols, the pH optimum is shifted to 6; the increased rate at lower pH being due to faster oxidation of hemiacetals of formaldehyde formed between formaldehyde and alcohol. The stimulatory effect of added alcohols on formaldehyde dismutation disappears at higher pH. Above pH 7.5 ethanol markedly inhibits formaldehyde dismutation. At this pH, ethanol undergoes coupled oxidation reduction reaction with formaldehyde and competitively inhibits formaldehyde oxidation. A general mechanism for liver alcohol dehydrogenase has been proposed which explains its (a) alcohol dehydrogenase activity, (b) coupled oxidoreductase activity, and (c) formaldehyde dismutase activity.

Effect of Renal Ischemia on Anaerobic CO2 Production by Dog Kidney in vitro

Summary1) Rates of anaerobic CO2 production by tissue obtained from dog kidneys with evidence of cortical ischemia have been compared with tissue from non-ischemic kidneys. Anaerobic CO2 production rate was consistent with the following coupled oxidation-reduction reaction: α-ketoglutarate + oxaloacetate + GDP + P1 → malate + succinate + CO2 + GTP. 2) Cortical washed homogenates from non-ischemic kidneys produced 14.6 ± 1.8 μmoles CO2/100 mg dry wt hr (M ± SE); cortical homogenates of ischemic kidneys yielded 6.0 ± 3.0 μmoles of CO2 (P<0.05). Similar studies done on slices revealed no such difference between ischemic and non-ischemic kidneys. 3) The activity of α-KG-dehydrogenase was the same in washed cortical homogenates of both ischemic and non-ischemic kidneys. In contrast, malic-dehydrogenase activity was significantly lower in cortical homogenates of ischemic kidneys. 4) Thus, cortical blanching is associated with a specific change in cortical mitochondrial enzymic activity so that both cortex and m...

Factors influencing the oxidation of externally added citrate by cardiac mitochondria

Summary It has been demonstrated that citric acid is oxidized only slowly by guinea-pig heart mitochondrial preparations and that this oxidation is unaccompanied by phosphorylation. When citrate is co-oxidized with pyruvate, there is a marked increase in the oxidation rate which is far greater than with either substrate alone. In addition, there is coupled phosphorylation with this oxidation. Under anaerobic conditions the combination of citrate and pyruvate with cardiac mitochondria, resulted in a coupled oxidation—reduction reaction in which α-ketoglutarate, CO2 and malate are formed. It has been proposed that TPNH generated in the isocitrate dehydrogenase ( D s-isocitrate:TPN+ oxidoreductase (decarboxylating), EC 1.1.1.42), reaction could be utilized to form malate by reductive carboxylation of pyruvate, and evidence has been presented that the activity of malate dehydrogenase (decarboxylating) ( L -malate:TPN+ oxidoreductase (decarboxylating), formerly known as malic enzyme, EC 1.1.1.40) of the mitochondria is adequate to account for such a mechanism. α-Ketoglutarate and malate thus formed from isocitrate and pyruvate on the outside of the mitochondria, could be oxidized further by enzyme systems within the particles, and the oxidation of these substrates should be accompanied by phosphorylation.