Auxiliary Reactions Research Articles

Synthesis and Redox Properties of Iron and Iron Oxide Nanoparticles Obtained by Exsolution from Perovskite Ferrites Promoted by Auxiliary Reactions

Synthesis and Redox Properties of Iron and Iron Oxide Nanoparticles Obtained by Exsolution from Perovskite Ferrites Promoted by Auxiliary Reactions

Assessment of Stoichiometric Autocatalysis across Element Groups.

Autocatalysis has been proposed to play critical roles during abiogenesis. These proposals are at odds with a limited number of known examples of abiotic (and, in particular, inorganic) autocatalytic systems that might reasonably function in a prebiotic environment. In this study, we broadly assess the occurrence of stoichiometries that can support autocatalytic chemical systems through comproportionation. If the product of a comproportionation reaction can be coupled with an auxiliary oxidation or reduction pathway that furnishes a reactant, then a Comproportionation-based Autocatalytic Cycle (CompAC) can exist. Using this strategy, we surveyed the literature published in the past two centuries for reactions that can be organized into CompACs that consume some chemical species as food to synthesize more autocatalysts. 226 CompACs and 44 Broad-sense CompACs were documented, and we found that each of the 18 groups, lanthanoid series, and actinoid series in the periodic table has at least two CompACs. Our findings demonstrate that stoichiometric relationships underpinning abiotic autocatalysis could broadly exist across a range of geochemical and cosmochemical conditions, some of which are substantially different from the modern Earth. Meanwhile, the observation of some autocatalytic systems requires effective spatial or temporal separation between the food chemicals while allowing comproportionation and auxiliary reactions to proceed, which may explain why naturally occurring autocatalytic systems are not frequently observed. The collated CompACs and the conditions in which they might plausibly support complex, "life-like" chemical dynamics can directly aid an expansive assessment of life's origins and provide a compendium of alternative hypotheses concerning false-positive biosignatures.

Radium sorption on biotite; surface complexation modeling study

The sorption of Ra on Olkiluoto biotite in the context of deep geological disposal of spent nuclear fuel was studied with isotherm batch sorption experiments. Ba was used as an analog for Ra in the experiments and modeling studies. A wide concentration range of Ra/Ba was used in the isotherm studies (2.6 × 10−9 M to 1 × 10−3 M) in addition to four different Olkiluoto reference groundwaters with salinity types ranging from fresh to saline. Experimental results show that both in the fresh and saline reference groundwaters, the distribution coefficients of Ra tend to decrease in the higher isotherm concentrations of Ba. With one reference groundwater, the distribution coefficients increased with the concentration of Ba due to significant coprecipitation of Ra. With the fresh reference groundwaters, the distribution coefficients of Ra were consistently approximately one order of magnitude lower than in the saline reference groundwater. A PHREEQC multi-site complexation model coupled with an optimization tool in Python was used to interpret the experimental Ra sorption results. Molecular modeling with CASTEP code implemented into Materials Studio was used to update the PHREEQC model with more realistic biotite sorption site density data. It was observed that while the multi-site model predicts the sorption of Ra well in lower isotherm concentrations, auxiliary reactions of Ra disrupt the model in high Ba isotherm concentrations. The experimental and modeled distribution coefficient data of Ra on biotite can be used in the safety case calculations of the deep geological disposal of spent nuclear fuel in Finland and Sweden.

Sustainable Coproduction of Two Disinfectants via Hydroxide-Balanced Modular Electrochemical Synthesis Using a Redox Reservoir.

Challenges posed by the sacrificial auxiliary reactions and expensive ion-exchange membranes in conventional electrosynthesis necessitate developing new electrochemical processes to enable efficient and sustainable distributed electrochemical manufacturing. Modular electrochemical synthesis (ModES) using a redox reservoir (RR) offers a promising membrane-free approach to improve energy efficiency and reduce waste through the pairing of multiple independent oxidative and reductive half-reactions; however, undesired ion-imbalance and induced pH changes in the existing ModES process limit sustained production. Here we present Ni(OH)2 as a heterogeneous RR that can selectively store and transport the hydroxide ions involved in the target half-reactions by reversible conversion with NiOOH to enable an ion-balanced ModES of two common disinfectants, hydrogen peroxide (H2O2) and sodium hypochlorite (NaClO). This hydroxide-balanced ModES realizes stable operation without appreciable pH swing to accumulate practically useful concentrations of H2O2 and NaClO up to 251 and 481 ppm, respectively. These results illustrate additional design principles for electrosynthesis without sacrificial auxiliary reactions and the need for ion-selective RRs for modular electrochemical manufacturing.

Asymptotics and numerical analysis for enzymatic auxiliary reactions

In this paper we study the mathematical model of auxiliary (or coupled) reactions, a mechanism which describes several chemical reactions. In order to apply singular perturbation techniques, we determine an appropriate perturbation parameter $$\epsilon $$ (which is related to the kinetic constants and initial conditions of the model), the inner and outer solutions and the matched expansions of the solutions, up to the first order in $$\epsilon $$ , in the total quasi-steady-state approximation (tQSSA) framework. The contribution of these expansions can be useful for the estimation of the kinetic parameters of the reaction by means of the interpolation of experimental data with the explicit approximations of the solutions. Some numerical results are discussed, showing the high reliability of the tQSSA with respect to the standard QSSA.

Isocyanide Multicomponent Reactions on Solid Phase: State of the Art and Future Application.

Drug discovery efforts largely depend on access to structural diversity. Multicomponent reactions allow for time-efficient chemical transformations and provide advanced intermediates with three or four points of diversification for further expansion to a structural variety of organic molecules. This review is aimed at solid-phase syntheses of small molecules involving isocyanide-based multicomponent reactions. The majority of all reported syntheses employ the Ugi four-component reaction. The review also covers the Passerini and Groebke-Blackburn-Bienaymé reactions. To date, the main advantages of the solid-phase approach are the ability to prepare chemical libraries intended for biological screening and elimination of the isocyanide odor. However, the potential of multicomponent reactions has not been fully exploited. The unexplored avenues of these reactions, including chiral frameworks, DNA-encoded libraries, eco-friendly synthesis, and chiral auxiliary reactions, are briefly outlined.

Efficient Estimation of Formation Enthalpies for Closed-Shell Organic Compounds with Local Coupled-Cluster Methods.

Efficient estimation of the enthalpies of formation for closed-shell organic compounds via atom-equivalent-type computational schemes and with the use of different local coupled-cluster with single, double, and perturbative triple excitation (CCSD(T)) approximations was investigated. Detailed analysis of established sources of uncertainty, inclusive of contributions beyond frozen-core CCSD(T) and errors due to local CCSD(T) approximations and zero-point energy anharmonicity, suggests the lower limit of about 2 kJ·mol-1 for the expanded uncertainty of the proposed estimation framework. Among the tested computational schemes, the best-performing cases demonstrate expanded uncertainty of about 2.5 kJ·mol-1, based on the analysis against 44 critically evaluated experimental values. Computational efficiency, accuracy commensurable with that of a typical experiment, and absence of the need for auxiliary reactions and additional experimental data offer unprecedented advantages for practical use, such as prompt validation of existing measurements and estimation of missing values, as well as resolution of experimental conflicts. The utility of the proposed methodology was demonstrated using a representative sample of the most recent experimental measurements.

Investigation of the ozone formation potential for ethanol using a smog chamber

The ozone formation reactivity of ethanol has been studied using chamber experiments and model simulations. The computer simulations are based on the MCM v3.1 mechanism with chamber-dependent auxiliary reactions. Results show that the MCM mechanism can well simulate C2H5OH-NO x chamber experiments in our experimental conditions, especially on ozone formation. C2H5OH-NO x irradiations are less sensitive to relative humidity than alkane species under our experimental conditions. In order to well simulate the experiments under high relative humidity conditions, inclusion of N2O5+H2O=2HNO3 in the MCM mechanism is necessary. Under C2H5OH-limited conditions, the C2H5OH/NO x ratio shows a positive effect on d(O3-NO)/dt and RO2+HO2. High C2H5OH/NO x ratios enhance the production of organoperoxide radical and HO2 radical concentrations, which leads to a much quicker accumulation of ozone. By using ozone isopleths under typical scenarios conditions, the actual ozone formation ability of ethanol is predicted to be 2.3–3.5 part per billion (ppb) in normal cities, 3.5–146 ppb in cities where ethanol gas are widely used, and 0.2–3.2 ppb in remote areas. And maximum ozone formation potential from ethanol is predicted to be 4.0–5.8 ppb in normal cities, 5.8–305 ppb in cities using ethanol gas, and 0.2–3.8 ppb in remote areas.

Regulation of Escherichia coli Polynucleotide Phosphorylase by ATP

Polynucleotide phosphorylase (PNPase), an enzyme conserved in bacteria and eukaryotic organelles, processively catalyzes the phosphorolysis of RNA, releasing nucleotide diphosphates, and the reverse polymerization reaction. In Escherichia coli, both reactions are implicated in RNA decay, as addition of either poly(A) or heteropolymeric tails targets RNA to degradation. PNPase may also be associated with the RNA degradosome, a heteromultimeric protein machine that can degrade highly structured RNA. Here, we report that ATP binds to PNPase and allosterically inhibits both its phosphorolytic and polymerization activities. Our data suggest that PNPase-dependent RNA tailing and degradation occur mainly at low ATP concentrations, whereas other enzymes may play a more significant role at high energy charge. These findings connect RNA turnover with the energy charge of the cell and highlight unforeseen metabolic roles of PNPase.

High-performance liquid chromatography-based methods of enzymatic analysis: electron transport chain activity in mitochondria from human skeletal muscle

This study addresses an application of pyridine nucleotide enzymatic analyses to evaluate the activity of the mitochondrial electron transport chain (reduced nicotinamide adenine dinucleotide (NADH) oxidase) and Complexes I and II in samples of human muscle as small as ∼10 mg wet weight. Key aspects in this adaptation are the use of high-performance liquid chromatography with fluorescence detection of NADH and use of alamethicin, a channel-forming antibiotic that enables an unrestricted access of substrates into the mitochondrial matrix. The procedure includes disintegration of tissue by Polytron homogenizer, extraction of myosin from myofibrillar fragments by KCl/pyrophosphate to facilitate release of mitochondria, and preparation of fractions of subsarcolemmal and intermyofibrillar mitochondria. Oxidation of NADH or succinate is assayed in the presence of 40 μg/ml alamethicin and the reaction is terminated by H 2SO 4, which also destroys the remaining NADH. Nicotinamide adenine dinucleotide (NAD) or fumarate concentrations are measured using alcohol dehydrogenase or fumarase plus malic dehydrogenase reactions, respectively. Generation of NADH, assessed in auxiliary reactions in the presence of hydrazine, is strictly proportional to NAD or fumarate content across a concentration range of 1–20 μM. NADH is quantitatively analyzed with a detection limit of 3–5 pmol by HPLC using a reverse-phase Hypersil ODS column connected to a fluorescence detector.

Coupling of ethylbenzene dehydrogenation and benzene hydrogenation reactions in fixed bed catalytic reactors

The application of dual-functionality via a well-mixed catalyst pattern in an integrated fixed bed catalytic reactor for simultaneous production of styrene and cyclohexane has been investigated. A rigorous mathematical model based on the dusty gas model is used for the simulation. Equilibrium shifting is achieved by the drain-off mechanism of hydrogen in dehydrogenation–hydrogenation reaction media. The simulation results show that the fixed bed reactor (FBR) with auxiliary reactions achieves substantial increase in ethylbenzene conversion (≈100%) compared to the fixed bed reactor. Optimal conditions are observed and an effective length criterion for the optimal conditions is presented. The effects of the operating conditions on the reactor performance are examined. It seems that the overall performance of the reactor is a balance between the operating conditions and the rate of hydrogen removal. The results suggest that appreciable energy saving can be achieved by maintaining the reactor beds at different temperatures.

Oxidation ofN-Benzyl Aziridine by Molecular Iodine: Competition of Electron Transfer and Heterolytic Pathways

Excess N-benzyl aziridine (1) reacts with I2 to afford dimer 2, tetramer 3, benzaldehyde (4), and iodoamine 5. The reaction is interpreted as occurring by both electron transfer (ET) and heterolytic mechanisms. An ET mechanism is substantiated for the oxidation by I2 of dimer 2 and tetramer 3, both being substrates easier to oxidise by electron abstraction than 1. Several auxiliary reactions were performed on 1 in order to firmly establish the boundaries to the competition between the ET and heterolytic mechanisms. For the reaction of 1 with 5 a reaction scheme is proposed; in a particular case, a pseudo-first order kinetic law is followed.

Microcalorimetric study of the enzymatic hydrolysis of starch: An α-amylase catalyzed reaction

Microcalorimetric studies of enzyme activities have been made on the hydrolysis of starch catalyzed by α-amylase. The best conditions for the hydrolysis of starch in the presence of α-amylase have been pointed out and the α-amylase activity was determined by isothermal batch microcalorimetry. The heat changes involved during the hydrolytic reaction can be related to the starch concentrations in a linear way and the heights of the calorimetric curves can be directly related to the amylase activities. Calibration curves were obtained. The new method for starch assay and α-amylase activity determination is direct, simple, accurate and it needs no pretreatment of the sample or auxiliary reactions. The α-amylase catalytic activity has also been investigated by microcalorimetry in the presence of some metal ions.

Hexose metabolism in pancreatic islets: Succinate dehydrogenase activity in islet homogenates

Succinate dehydrogenase activity was measured in rat pancreatic islet homogenates incubated in the presence of [1,4-14C]succinate, the reaction velocity being judged through the generation of 14CO2 in the auxiliary reactions catalysed by pig heart fumarase and chicken liver NADP-malate dehydrogenase. In the presence of 1.0 mM succinate, the reaction velocity averaged 5.53 +/- 0.44 pmol min-1 microgram-1 islet protein. The Km for succinate was close to 0.4 mM and the enzymic activity was restricted to mitochondria. These kinetic results indicate that, under the present experimental conditions, the activity of succinate dehydrogenase does not vastly exceed that of either NAD-isocitrate dehydrogenase or the 2-ketoglutarate dehydrogenase complex, at least when the latter enzymes are activated by ADP and/or Ca2+. Nevertheless, the activity of succinate dehydrogenase is sufficient to account for the increase in O2 uptake evoked in intact islets by the monomethyl ester of succinic acid. It could become a rate-limiting step of the Krebs cycle in models of B-cell dysfunction.

Subcellular Location of NADPH-Dependent Hydroxypyruvate Reductase Activity in Leaf Protoplasts of Pisum sativum L. and Its Role in Photorespiratory Metabolism

Protoplasts purified from pea (Pisum sativum L.) leaves were lysed and fractionated to assess the subcellular distribution of NADPH-dependent hydroxypyruvate reductase (NADPH-HPR) activity. Rate-zonal centrifugation and sucrose-gradient experiments demonstrated that most (about 70%) of the NADPH-HPR activity was located in the supernatant or cytosol fraction. Detectable, but relatively minor activities were associated with the chloroplast fraction (up to 10% on a chlorophyll basis when compared to the lysate) and with peroxisomes. The minor NADPH-HPR activity in the peroxisomes could be fully accounted for by the secondary NADPH-dependent activity of NADH-dependent HPR. The subcellular distribution of NADPH-HPR followed closely that previously determined for NADPH-dependent glyoxylate reductase (NADPH-GR), an enzyme localized predominantly in the cytosol of pea leaf protoplasts (CV Givan et al. 1988 J Plant Physiol 132: 593-599). Low activities of both NADPH-HPR and NADPH-GR were also found in purified chloroplasts prepared by mechanical homogenization of Pisum and Spinacia leaves. In pea and spinach chloroplasts, rates of both NADPH-HPR and NADPH-GR were lower than the activity of the NADH-dependent GR. The results are discussed in relation to a possible role for NADPH-HPR in the oxidative carbon pathway of photorespiration. Both NADPH-HPR and the GRs could function as auxiliary reactions to photorespiration, utilizing hydroxypyruvate and/or glyoxylate ;leaked' or otherwise exported from peroxisomes. NADPH-HPR function might be especially significant under conditions of limiting NADH supply to peroxisomes, with extraperoxisomal reduced pyridine nucleotide acting as the reductant.

Standard enthalpies of formation of tellurium compounds III. Cs 2TeO 3, Cs 2Te 2O 5, Cs 2Te 4O 9, and Cs 2TeO 4

The enthalpies of solution of Cs 2TeO 3, Cs 2Te 2O 5, Cs 2Te 4O 9, and Cs 2TeO 4 in 3.11 mol·dm −3 (3.0H 2SO 4 + 0.1K 2Cr 2O 7 + 0.01MnSO 4)(aq) have been measured calorimetrically. Combination with values from auxiliary reactions gave for the standard enthalpies of formation: Δ f H m o(s, 298.15 K)/(kJ·mol −1): Cs 2TeO 3, −(993.2 ± 3.4); Cs 2Te 2O 5, −(1352.4 ± 6.4); Cs 2Te 4O 9, −(2033.3 ± 12.8); Cs 2TeO 4, −(1118.9 ± 4.8).

Reactions of ground state chlorine atoms with fluorinated methanes and ethanes

The reactions of photochemically generated Cl(2PJ) atoms with a number of fluorohydrocarbons have been investigated in the temperature range 8–95 °C by the competitive photochlorination technique using CH4 as a primary standard. Relative and absolute rate parameters are reported for CH3F, CH2F2, CH3CH2F, CH2FCH2F, CH3CHF2, CH3CF3, CH2FCHF2, CHF2CHF2, and several auxiliary reactions including CH3Cl, C2H6, and C3H8. The internal competition for hydrogen abstraction in asymmetric fluorethanes is examined in detail. The reactivity trends are discussed and it is found that the activation energies in the fluoromethane series correlate with the known C–H bond dissociation energies. The hydrogen reactivity in the fluoroethane series for which a sufficient data base of DH°(C–H) values is not available is best rationalized in terms of inductive effects and resonance interactions.

Standard enthalpies of formation of uranium compounds VIII. UCl 3, UCl 5, and UCl 6

Enthalpies of solution of UCl 3 in H 2SO 4(aq) have been measured calorimetrically. In the same way enthalpies of solution of UCl 5 and UCl 6 have been measured in (HCl + FeCl 3)(aq) and HCl(aq), respectively. Together with data for the auxiliary reactions, the standard enthalpies of formation of UCl 3(s), UCl 5(s), and UCl 6(s) have been derived. The results obtained are as follows: ΔH f o(s, 298.15 K)/(kJ · mol −1) UCl 3 −(862.1 ± 3.2) UCl 5 −(1041.6 ± 1.9) UCl 6 −(1066.5 ± 1.9)

Präparative organische Chemie unter Verwendung einfach herzustellender Katalysatoren / A Simple Method for Preparation of Catalysts for Preparative Organic Chemistry

Abstract There are numerous simple chemical reactions in which reaction products arise, which compared with the materials at the outset, possess a considerably greater surface area. A significant example is the reaction of aluminium alcoholates with water. Reactions of this kind can, to a certain extent, be used as auxiliary reactions, in order to distribute other materials homogenously. In the chosen example, a material dissolved beforehand in the alcoholate, e.g. a solution of a metal salt, would be transferred in the course of the distributing reaction into the newly-developing structure of the reaction product. In this publication the application of some catalysts thus prepared to some classical organic chemical reactions is described by way of example

Mechanism of Pigeon Liver Malic Enzyme: FORMATION OF l-LACTATE FROM l-MALATE, AND EFFECTS OF MODIFICATION OF PROTEIN THIOL GROUPS ON MALIC ENZYME, OXALACETATE, AND PYRUVATE REDUCTASE ACTIVITIES

Abstract Crystalline pigeon liver malic enzyme (malate dehydrogenase (decarboxylating), EC 1.1.1.40) catalyzes the slow, magnesium-dependent reduction of oxalacetate and conversion of l-malate to l-lactate in the presence of TPNH and TPN, respectively, with TPN acting in a catalytic manner. Pyruvate reductase activity associated with this enzyme has a pH optimum of 6.5 to 6.8 and a Michaelis constant of 5.52 mm for pyruvate. The activity ratio of oxidative decarboxylation of l-malate, reduction of pyruvate, and reduction of oxalacetate catalyzed by the enzyme is approximately 7.3:0.31:1. Titration of 4 sulfhydryl groups by 5,5'-dithiobis(2-nitrobenzoic acid) induced a reversible loss of malic enzyme activity and an increase in pyruvate reductase activity, while the oxalacetate reductase activity remained constant. The modified enzyme exhibited a new activity ratio of 0.3:0.6:1. These observations are consistent with both reductases being auxiliary reactions that could be expected from a sequential, ordered mechanism with oxalacetate as the enzyme-bound intermediate in a central complex. It is postulated that alteration of thiol groups led to a reversible blockage of the CO2 step, which was responsible for the observed changes in the diverse activities of the enzyme.