Autocatalytic Reaction Mechanism Research Articles

Real-Time Monitoring of the Structural Transition of Bombyx mori Liquid Silk under Pressure by Solid-State NMR.

Silk fibroin is stored in the silk glands of Bombyx mori silkworms as a condensed aqueous solution called liquid silk. It is converted into silk fibers at the silkworm's spinnerets under mechanical forces including shear stress and pressure. However, the detailed mechanism of the structural transition of liquid silk to silk fibers under pressure is not well understood. Magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) can exert pressure on liquid samples in a quantitative manner. In this study, solid-state NMR was used to quantitatively analyze the impact of pressure on the structural transition of liquid silk. A combination of 13C DD-MAS and CP-MAS NMR measurements enabled the conformation and dynamics of the crystalline region of the silk fibroin (both before (Silk Ip) and after (Silk IIp) the structural transition) to be detected in real time with atomic resolution. Spectral analyses proposed that the pressure-induced structural transition from Silk Ip to Silk IIp proceeds by a two-step autocatalytic reaction mechanism. The first reaction step is a nucleation step in which Silk Ip transforms to single lamellar Silk IIp, and the second is a growth step in which the single lamellar Silk IIp acts as a catalyst that reacts with Silk Ip molecules to further form Silk IIp molecules, resulting in stacked lamellar Silk IIp. Furthermore, the rate constant in the second step shows a significant pressure dependence, with an increase in pressure accelerating the formation of large stacked lamellar Silk IIp.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species.

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by sulfide species (H2S/HS-) under an argon atmosphere has been studied by a combination of spectroscopic, kinetic, and computational methods. Asymmetric S-shaped time-traces for the formation of MbFeII at varying ratios of excess sulfide were observed at pH 5.3 < pH < 8.0 and 25 °C, suggesting an autocatalytic reaction mechanism. An increased rate at more alkaline pHs points to HS- as relevant reactive species for the reduction. The formation of the sulfanyl radical (HS•) in the slow initial phase was assessed using the spin-trap phenyl N-tert-butyl nitrone. This radical initiates the formation of S-S reactive species as disulfanuidyl/ disulfanudi-idyl radical anions and disulfide (HSSH•-/HSS•2- and HSS-, respectively). The autocatalysis has been ascribed to HSS-, formed after HSSH•-/HSS•2- disproportionation, which behaves as a fast reductant toward the intermediate complex MbFeIII(HS-). We propose a reaction mechanism for the sulfide-mediated reduction of metmyoglobin where only ferric heme iron initiates the oxidation of sulfide species. Beside the chemical interest, this insight into the MbFeIII/sulfide reaction under an argon atmosphere is relevant for the interpretation of biochemical aspects of ectopic myoglobins found on hypoxic tissues toward reactive sulfur species.

Dynamic membrane interaction and amyloid fibril formation of glucagon, melittin and human calcitonin

Glucagon is a 29-amino acid peptide hormone secreted by pancreatic α-cells and interacts with specific receptors located in various organs. Glucagon tends to form gel-like fibril aggregates that are cytotoxic. It is important to reveal the glucagon-membrane interaction to understand activity and cytotoxicity of glucagon and glucagon oligomers. In this review, first glucagon-membrane interactions are described as morphological changes in dimyristoylphosphatidylcholine (DMPC) bilayers containing glucagon in acidic and neutral conditions as compared to the case of melittin. Second, fibril formation by glucagon in acidic solution is discussed in light of morphological and structural changes. Third, kinetic analysis of glucagon fibril formation was performed using a two-step autocatalytic reaction mechanism, as investigated in the case of human calcitonin. The first step is a nuclear formation, and the second step is an autocatalytic fibril elongation. Forth, fibril formation of glucagon inside glucagon-DMPC bilayers in neutral solution under near physiological condition is described.

Analytic solution to the rate law for a fundamental autocatalytic reaction mechanism operating in the “efficient” regime

AbstractIntegrated rate laws in analytic form are scarce in the kinetics literature in cases where a reaction mechanism involves an autocatalytic step. As an example, we have taken up the challenge of integrating the rate law for a mechanism where an initial autocatalytic step competes with a subsequent step in which the catalyst decays unimolecularly to form the product. In addition to its chemical application, this mechanism forms the basis for theories of metabolism and chemical evolution. Even though it is one of the most elementary mechanisms possible, an exact analytic solution to the governing rate law has never been obtained in closed form. Of greatest chemical interest is the case where the autocatalytic step is much faster than the unimolecular step. This situation has the effect of enhancing the yield of the product and is known as the efficient regime. We derive a closed form analytical solution to the rate equation which is accurate in this limit. We use our analytic solution to obtain a formula for the induction time during which the rate of formation of product is accelerating.

Curing and rheological study of epoxy molding compound with novel phosphonium silicate latent accelerators

AbstractLatent curing accelerators are significant for curing and molding controlling of epoxy molding compounds. Two novel phosphonium silicate latent accelerators, tetraphenylphosphonium bis(2,3‐dioxy‐naphthalene)methylsilicate (P‐Si‐Me) and tetraphenylphosphonium bis(2,3‐dioxy‐naphthalene)phenylsilicate (P‐Si‐Ph) were synthesized and characterized. The catalytic performance of P‐Si‐Me and P‐Si‐Ph on epoxy/phenolic resin system was evaluated and compared with commercial organophosphine accelerators triphenyl phosphine (TPP) and triphenylphosphine‐1,4‐benzoquinone (TPP‐BQ). Systems with four accelerators displayed similar autocatalytic curing reaction mechanism, and the curing and rheological behavior of four epoxy systems could be well described with the diffusion factor‐incorporated Kamal model, and Castro and Macosko model, respectively. DSC and rheology results showed that the latency of four accelerators followed the order of P‐Si‐Ph ≈ TPP‐BQ &gt;&gt; P‐Si‐Me &gt; TPP, while the catalytic reactivity followed TPP &gt; P‐Si‐Me &gt; P‐Si‐Ph &gt; TPP‐BQ. DMA results exhibited that epoxy systems with phosphonium silicate accelerators had higher Tg than that with TPP and TPP‐BQ. The latency and catalytic performance of phosphonium silicate accelerators were attributed to their ionic complex nature. Moreover, the substituent on silicate anion had obvious impact on the performance of accelerators.

Simulation of Capsule Formation from Amino Acid ThermalHeterocomplex Polymers by Autocatalytic Reaction Mechanism

Simulation of Capsule Formation from Amino Acid ThermalHeterocomplex Polymers by Autocatalytic Reaction Mechanism

Autocatalytic reaction mechanism of nitric acid and formic acid mixtures based on thermal and in situ Raman spectroscopic analyses

The oxidation of organic compounds with nitric acid has the potential to undergo a runaway hazardous reaction and explosion after the thermal–neutral induction period. The purpose of this study is to obtain a better understanding of the oxidation reaction mechanism to clarify the specific conditions under which the exothermic reaction occurs. In this study, the oxidation reactions of formic acid with nitric acid were investigated to represent a typical mixture of organic compound and nitric acid. From the results of thermal analysis, the overall reaction rate in a 15% nitric acid and 10% formic acid aqueous solution was formulated using an autocatalytic reaction model. The apparent activation energy was 75.2 kJ mol−1, and the pre-exponential factor was 7.87 × 108 s−1. The reaction rate increased with either nitric or formic acid concentrations, emphasizing that it is important to treat the mixture at lower concentrations for thermal safety. As a result of the thermal and in situ Raman spectroscopic analyses, it was found that the concentration of nitrous acid increased with the progress of the reaction as nitric acid and formic acid changed into NO2, N2O4, HCO(O)NO, HCO(O)NO2, and so on. The reaction mechanism started from nitrous acid and the oxidation of formic acid with nitric acid is an autocatalytic reaction involving this nitrous acid-initiated cycle.

Cure and thermal decomposition kinetics of a DGEBA/amine system modified with epoxidized soybean oil

In this study, epoxidized soybean oil (ESO) was added to a typical diglycidyl ether of bisphenol A (DGEBA) epoxy resin system with an amine curing agent, and the cure and thermal decomposition kinetics of the epoxy resin system were investigated by thermal analyses. ESO content in the epoxy resin system was changed up to 30%, and the stoichiometric amount of ethylene diamine was used. DSC was used for cure kinetics analysis, and TGA was used for thermal decomposition kinetics analysis. FTIR was used to check the completeness of the polymerization reaction of the cured epoxy resin samples for TGA. With increasing ESO content, cure rate decreased and the beginning temperature of thermal decomposition lowered. The cure and thermal decomposition kinetics of the epoxy resin system could be successfully analyzed by the autocatalytic reaction mechanism and the Ozawa method respectively.

Methanol-Assisted Autocatalysis in Catalytic Methanol Synthesis.

Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO2 and sustainably derived H2 are envisioned to play an important role as energy carriers in a future low-CO2 -emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al2 O3 catalyst is dominated by a methanol-assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO2 although with low absolute activity, but not for Cu/Al2 O3 catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis.

31P and 13C solid-state NMR analysis of morphological changes of phospholipid bilayers containing glucagon during fibril formation of glucagon under neutral condition

Glucagon is a 29 amino acid peptide hormone secreted by pancreatic α-cells that interacts with specific receptors located in various organs. Glucagon tends to form gel-like fibrillar aggregates that are cytotoxic due to their activation of apoptotic signaling pathways. To understand the glucagon-membrane interactions, morphological changes in dimyristoylphosphatidylcholine (DMPC) bilayers containing glucagon in neutral solution were investigated by observing 31P NMR spectra. First, lipid bilayers with a DMPC/glucagon molar ratio of 50/1 were observed. One day after preparing the DMPC/glucagon lipid bilayer sample, lipid bilayers were disrupted below the phase transition temperature (Tc). Membrane disruption was reduced 2 days after preparation due to the reduction of glucagon-DMPC interaction, and subsequently increased by 4 days and was reduced again by 7 days. TEM measurements showed that small ellipsoidal intermediates of glucagon were observed inside the small size of lipid bilayer after 4 days, while fibrils grew inside lipid bilayer after 19 days. These results indicate that morphological changes in DMPC/glucagon lipid bilayers are correlated with the evolution of glucagon aggregate state. Particularly, fibril intermediate shows a strong glucagon lipid bilayer interaction. We further investigated the structure and kinetics of glucagon fibril formation inside the DMPC lipid bilayer in a neutral solution using 13C solid-state NMR spectroscopy. α-Helical structures were observed around Gly4 and Ala19 in the monomeric form, which changed to β-sheet structures in the fibril form. The fibrillation process can be explained by a two-step autocatalytic reaction mechanism in which the first step is a homogeneous nuclear formation (k1), and the second step is an autocatalytic heterogeneous fibrillation process (k2).

Autocatalytic Tissue Polymerization Reaction Mechanism in Colorectal Cancer Development and Growth.

The goal of our study was to measure the kinetics of human colorectal cancer (CRC) development in order to identify aberrant mechanisms in tissue dynamics and processes that contribute to colon tumorigenesis. The kinetics of tumor development were investigated using age-at-tumor diagnosis (adenomas and CRCs) of familial adenomatous coli (FAP) patients and sporadic CRC patients. Plots of age-at-tumor diagnosis data as a function of age showed a distinct sigmoidal-shaped curve that is characteristic of an autocatalytic reaction. Consequently, we performed logistics function analysis and found an excellent fit (p < 0.05) of the logistic equation to the curves for age-at-tumor diagnoses. These findings indicate that the tissue mechanism that becomes altered in CRC development and growth involves an autocatalytic reaction. We conjecture that colonic epithelium normally functions as a polymer of cells which dynamically maintains itself in a steady state through an autocatalytic polymerization mechanism. Further, in FAP and sporadic CRC patients, mutation in the adenomatous polyposis coli (APC) gene increases autocatalytic tissue polymerization and induces tumor tissues to autocatalyze their own progressive growth, which drives tumor development in the colon.

The butoxylation of dodecylamine: Reaction mechanism and kinetics

The reaction mechanism and kinetics of the butoxylation of dodecylamine were investigated experimentally using a semi-batch, fully liquid process at temperatures ranging from 120 °C to 150 °C and varying stoichiometric ratios of the reactants. In addition, the catalytic effect of adding 1–3%mol of twelve different functional organic species containing hydroxyl, aldehyde, amine, and/or amide functional groups was studied. It was found that only proton donating groups increase the observed reaction rate. In particular hydroxyl groups in combination with an amine group resulted in a strong acceleration of the reaction. Since the reaction intermediates mono-and dibutoxylated amines have this combination, an auto-catalytic reaction mechanism and corresponding rate law are proposed. The kinetic constants were fitted to the experimental data as a function of temperature, following an Arrhenius type of dependency. The results from the model describe the experimental data with 95% accuracy. Moreover, the results show that the butoxylation of dodecylamine, as a model reactant for a short substituted ethylene oxide, with fatty amines, follows the same mechanism and has similar kinetics as epoxide hardening reactions with amines.

Direct Blue Light-Induced Autocatalytic Oxidation of o-Phenylenediamine for Highly Sensitive Visual Detection of Triaminotrinitrobenzene.

o-Phenylenediamine (OPD)-based chromogenic reactions are worthy tools for the development of visual colorimetric assays. The chromogenic reactions are usually triggered by various oxidants, which is not easily tunable and incompatible with some analytes. Herein, we report that direct blue light irradiation can induce the autocatalytic oxidation of OPD to generate 2,3-diaminophenazine (oxidized-state OPD, oxOPD). The autocatalytic photo-oxidation reaction mechanism of OPD is mainly ascribed to the resonant energy transfer between ectronically excited oxOPD and dissolved oxygen to form singlet state oxygen with a high oxidation capacity, which accelerates the oxidation of OPD. We demonstrate that under neutral and alkaline environment, the photoinduced autocatalytic oxidation of OPD is able to be further enhanced by triaminotrinitrobenzene (TATB) explosive because of its inhibition effect on the aggregation caused quenching phenomenon of oxOPD. On this basis, a straightforward visual colorimetric assay for TATB with a tunable dynamic range is developed. This assay is capable of detecting TATB explosive concentrations as low as 2.7 nM. Notably, the obvious color change after addition of TATB enables a naked-eye readout with the lowest detectable TATB concentrations of 60 nM.

Studies on Curing Kinetics and Tensile Properties of Silica-Filled Phenolic Amine/Epoxy Resin Nanocomposite.

In this study, the curing kinetics of the phenolic amine/epoxy resin system were investigated by nonisothermal differential scanning calorimetry (DSC). The model-free isoconversional method of Ozawa–Flynn–Wall reveals a dependence of Eα (activation energy) on conversion (α), which interprets the autocatalytic curing reaction mechanism of the phenolic amine/epoxy resin system. Studies on the effects of nano-SiO2 particles on the tensile properties and tensile fracture face morphology of nanocomposites show that the uniform dispersion of SiO2 nanoparticles plays an important role in promoting the tensile performance of nanocomposites. Additionally, increases of 184.1% and 217.2% were achieved by adding 1.5% weight parts of nano-SiO2 in composites for the tensile strength and tensile modulus, respectively.

Fibrillation mechanism of glucagon in the presence of phospholipid bilayers as revealed by 13C solid-state NMR spectroscopy.

Glucagon is a 29 amino acid peptide hormone secreted by pancreatic α-cells and interacts with specific receptors located in various organs. Glucagon tends to form gel-like fibril aggregates that are cytotoxic because they activate apoptotic signaling pathways. To understand mechanism of fibril formation, we investigated the structure and kinetics of glucagon fibril formation using 13C solid-state NMR spectroscopy. In aqueous acetic acid solution at pH 3.3, distorted α-helical structure appeared around Gly4, Leu14, Ala19 and Leu26 in the monomeric form. In contrast, Gly4 and Ala19 were involved in β-sheet structures in the fibril form. The fibrillation process can be explained by a two-step autocatalytic reaction mechanism in which the first step is a homogeneous nuclear formation (k1), and the second step is an autocatalytic heterogeneous fibrillation process (k2). The rate constants k1 and k2 were separately determined in the acetic acid solution. Fibril formation was further investigated in the presence of lipid bilayers to mimic the physiological condition. We used bicelles which form discoidal nano-particles as the bilayer system and observed that the N-terminal α-helix did not change to β-sheet when fibrils formed in the presence of bicelles. Rate constant k1 became faster and k2 became slower in the presence of bicelles compared to the case in the absence of bicelles. Our findings reveal that the structure and kinetics of fibril formation by glucagon are altered in the presence of lipid bilayers.

Design of a High-Performance Dismantlable Adhesion System Using Pressure-Sensitive Adhesive Copolymers of 2-Hydroxyethyl Acrylate Protected with tert-Butoxycarbonyl Group in the Presence of Cross-Linker and Lewis Acid.

A dismantlable adhesion system satisfies both a strong bonding strength during use and a quick debonding process on demand in response to an external stimulus as a trigger for dismantling. In this study, we synthesized acrylate copolymers consisting of 2-(tert-butoxycarbonyloxy)ethyl acrylate (BHEA), 2-ethylhexyl acrylate (2EHA), and 2-hydroxyethyl acrylate (HEA) as the repeating units and evaluated the properties as dismantlable adhesives. First, the thermal degradation behavior of the obtained polymers was investigated by thermogravimetric analysis and IR spectroscopy. The BHEA-containing polymers were thermally stable during heating at a temperature below 150 °C, but they rapidly degraded, i.e., the deprotection of the tert-butoxycarbonyl groups occurred during heating at 200 °C. The onset temperatures for the deprotection depended on the BHEA and HEA contents and their sequence structures because the hydroxy group in the side chain accelerated the deprotection via an autocatalytic reaction mechanism. Shear holding power and 180° peel tests were carried out with the pressure-sensitive adhesive tapes using the BHEA-containing copolymers as the adhesive materials. The copolymers consisting of the BHEA, 2EHA, and HEA units with 25.7, 35.0, and 39.3 mol %, respectively, exhibited the highest adhesion strength and the subsequent quick reduction of the adhesion strength by heating during the dismantling process. The addition of hexamethylene diisocyanate as the cross-linker and Zn(acac)2 as the Lewis acid to the adhesive polymers was demonstrated to be valid for the design of high-performance dismantlable adhesion systems. A change in the rheological properties during the dismantling process was important for a quick response and selective interfacial failure between the substrate and the adhesive.

Stereospecific Autocatalytic Surface Explosion Chemistry of Polycyclic Aromatic Hydrocarbons.

Autocatalytic processes are important in many fields of science, including surface chemistry. A better understanding of its mechanisms may improve the current knowledge on heterogeneous catalysis. The thermally induced decomposition of eight different polycyclic aromatic hydrocarbons (PAHs) on a saturated monolayer of atomic oxygen on a Cu(100) surface is studied using temperature-programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). 9-Bromo-heptahelicene decomposes autocatalytically in a narrow temperature range into CO2 and H2O, while non-halogenated heptahelicene decomposes into the same products but does not show autocatalytic behavior. Fixation of the hydrocarbon to the surface via the organometallic bond after elimination of the bromine is identified as a prerequisite for the autocatalytic reaction mechanism. Of all the hydrocarbons studied, only those being sterically overcrowded decompose autocatalytically. Such an observation can be explained by facile dehydrogenation of the overcrowded PAHs. The reaction of such hydrogen with oxygen creates vacancies in the oxygen layer which act as active sites and catalyze further decomposition.

Simulation approach to decomposition kinetics and thermal hazards of hexamethylenetetramine

The thermal stability of HMT under dynamic, isothermal and adiabatic conditions was investigated using differential scanning calorimeter (DSC) and accelerating rate calorimeter (ARC), respectively. It is found from the dynamic DSC results that the exothermic decomposition reaction appears immediately after endothermic peak, a coupling phenomenon of heat absorption and generation, and the endothermic peak and exothermic peak were indentified at about 277–289 and 279–296 °C (Tpeak) with the heating rates 1, 2, 4 and 8 °C min−1. The ARC results reveal that the initial decomposition temperature of HMT is about 236.55 °C, and the total gas production in decomposition process is 6.9 mol kg−1. Based on the isothermal DSC and ARC data, some kinetic parameters have been determined using thermal safety software. The simulation results show that the exothermic decomposition process of HMT can be expressed by an autocatalytic reaction mechanism. There is also a good agreement between the kinetic model and kinetic parameters simulated based on the isothermal DSC and ARC data. Thermal hazards of HMT can be evaluated by carrying out thermal explosion simulations, which were based on kinetic models (Isothermal DSC and ARC) to predict several thermal hazard indicators, such as TD24, TD8, TCL, SADT, ET and CT so that we can optimize the conditions of transportation and storage for chemical, also minimizing industrial disasters.

The Role of Hydrogen Bond in the Mechanism of Autocatalytic Reaction between Acetic Anhydride and tert-Butyl Hydroperoxide

The role of hydrogen bonding in the autocatalytic reaction of acetic anhydride with tert-butyl hydroperoxide in CC14 has been elucidated. The stage-by-stage mechanisms of formation of hydrogen-bonded complexes. Structure and energy parameters of stable conformers of the compounds and energy of the major and transition states have been determined by means of B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods. The contribution of the O···HO hydrogen bonds in the stabilization of the complexes has been determined using the noncovalent interactions index. 1-Hydroxy-1-alkylperoxy compound is the most stable intermediate.

Highly Efficient Synthesis of Hydrogen Storage Material of Formate from Bicarbonate and Water with General Zn Powder

Formate, as an excellent hydrogen-storage material, has recently become increasingly important because formate has low toxicity, is easy to store and transport, and contains relatively high energy density. In this paper, we give an overview of the recent strategies in the conversion of bicarbonate into formate by water splitting with a general metallic Zn powder, which mainly includes: (1) hydrogen production from water with Zn; (2) formate syntheses from bicarbonate with water-derived hydrogen; (3) catalytic role of ZnO and Zn/ZnO interface and reducing role of active intermediate of Zn hydride species (Zn–H); (4) density function theory calculation for formate synthesis from bicarbonate and water with Zn; (5) solar-to-chemical energy conversion efficiency estimation. The novel options of this proposed strategy are mainly the hydrogen being in situ formed from water with metal Zn as a reductant, and the plausible autocatalytic reaction mechanism. When coupled with reducing ZnO into Zn by solar energy, a ...