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Irreversible Reactions (irreversible + reaction)
Selected AbstractsCharacterization of the NAD+ binding site of Candida boidinii formate dehydrogenase by affinity labelling and site-directed mutagenesisFEBS JOURNAL, Issue 22 2000Nikolas E. Labrou The 2,,3,-dialdehyde derivative of ADP (oADP) has been shown to be an affinity label for the NAD+ binding site of recombinant Candida boidinii formate dehydrogenase (FDH). Inactivation of FDH by oADP at pH 7.6 followed biphasic pseudo first-order saturation kinetics. The rate of inactivation exhibited a nonlinear dependence on the concentration of oADP, which can be described by reversible binding of reagent to the enzyme (Kd = 0.46 mm for the fast phase, 0.45 mm for the slow phase) prior to the irreversible reaction, with maximum rate constants of 0.012 and 0.007 min,1 for the fast and slow phases, respectively. Inactivation of formate dehydrogenase by oADP resulted in the formation of an enzyme,oADP product, a process that was reversed after dialysis or after treatment with 2-mercaptoethanol (> 90% reactivation). The reactivation of the enzyme by 2-mercaptoethanol was prevented if the enzyme,oADP complex was previously reduced by NaBH4, suggesting that the reaction product was a stable Schiff's base. Protection from inactivation was afforded by nucleotides (NAD+, NADH and ADP) demonstrating the specificity of the reaction. When the enzyme was completely inactivated, approximately 1 mol of [14C]oADP per mol of subunit was incorporated. Cleavage of [14C]oADP-modified enzyme with trypsin and subsequent separation of peptides by RP-HPLC gave only one radioactive peak. Amino-acid sequencing of the radioactive tryptic peptide revealed the target site of oADP reaction to be Lys360. These results indicate that oADP inactivates FDH by specific reaction at the nucleotide binding site, with negative cooperativity between subunits accounting for the appearance of two phases of inactivation. Molecular modelling studies were used to create a model of C. boidinii FDH, based on the known structure of the Pseudomonas enzyme, using the modeller 4 program. The model confirmed that Lys360 is positioned at the NAD+ -binding site. Site-directed mutagenesis was used in dissecting the structure and functional role of Lys360. The mutant Lys360,Ala enzyme exhibited unchanged kcat and Km values for formate but showed reduced affinity for NAD+. The molecular model was used to help interpret these biochemical data concerning the Lys360,Ala enzyme. The data are discussed in terms of engineering coenzyme specificity. [source] Acid decomposition of omeprazole in the absence of thiol: A differential pulse polarographic study at the static mercury drop electrode (SMDE)JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2006Ali M. Qaisi Abstract The reactions of omeprazole, a potent proton pump inhibitor (PPI), were investigated in the absence of a nucleophile. Reactions were monitored, using differential pulse polarography (DPP) at the static mercury drop electrode (SMDE), in solutions buffered to pH values ranging from 2.0 to 8.0. The fast, sensitive, and selective electrochemical technique facilitated to repeat recordings of successive voltammograms [peak current (nA) vs. peak potential (volts vs. Ag/AgCl saturated with 3.0 M KCl)]. The DPP signals of omeprazole and its degradation products, believed to be due to sulfur functional group (the principal site of electrode reaction), gave advantages over the previously employed UV detection technique. The latter primarily relied on pyridine and benzimidazole analytical signals, which are common reaction products of PPI in aqueous acidic solutions. After peak identification, the resulting current (nA)-time (s) profiles, demonstrated that omeprazole undergoes degradation to form two main stable compounds, the first is the cyclic sulfenamide (D+), previously believed to be the active inhibitor of the H+, K+ -ATPase, the second is omeprazole dimer. This degradation is highly dependant on pH. Unlike previous studies which reported that the lifetime of D+ is few seconds, the cyclic sulfenamide (D+) was found to be stable for up to 5,20 min. The results further indicated that omeprazole converts into the cyclic sulfenamide in an irreversible reaction, consequently, D+ and sulfenic acid (an intermediate which rapidly converts into D+) were not interconvertable. The present work suggested that the sulfenic acid is the active inhibitor in vivo. In addition, the omeprazole reactions, in the absence of the thiol, were not as complicated as were previously reported. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:384,391, 2006 [source] Analysis of the Stability and Degradation Products of TriptolideJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 1 2000YAN PING MAO Triptolide is the major active ingredient of the Chinese herbal remedy Tripterygium wilfordii Hook F. (TwHF). As triptolide content is used to estimate the potency of preparations of TwHF, assessment of its stability is warranted. The accelerated stability of triptolide was investigated in 5% ethanol solution in a light-protected environment at pH 6.9, within a temperature range of 60,90°C. The observed degradation rate followed first-order kinetics. The degradation rate constant (K25°C) obtained by trending line analysis of Arrhenius plots of triptolide was 1.4125 times 10,4 h,1. The times to degrade 10% (t1/10) and 50% (t1/2) at 25°C were 31 and 204 days, respectively. Stability tests of triptolide in different solvents and different pH conditions (pH 4,10) in a light-protected environment at room temperature demonstrated that basic medium and a hydrophilic solvent were the major factors that accelerated the degradation of triptolide. Triptolide exhibited the fastest degradation rate at pH 10 and the slowest rate at pH 6. In a solvent comparison, triptolide was found to be very stable in chloroform. The stability of triptolide in organic polar solvents tested at both 100% and 90% concentration was greater in ethanol than in methanol than in dimethylsulphoxide. Stability was also greater in a mixture of solvent: pH 6 buffer (9:1) than in 100% solvent alone. An exception was ethyl acetate, which is less polar than the other solvents tested, but permitted more rapid degradation of triptolide. Two of the degradation products of triptolide were isolated and identified by HPLC and mass spectroscopy as triptriolide and triptonide. This suggested that the decomposition of triptolide occurred at the C12 and C13 epoxy group and the C14 hydroxyl. The opening of the C12 and C13 epoxy is an irreversible reaction, but the reaction occurring on the C14 hydroxyl is reversible. These results show that the major degradation pathway of triptolide involves decomposition of the C12 and C13 epoxy group. Since this reaction is very slow at 4°C at pH 6, stability is enhanced under these conditions. [source] ,-Cyclodextrin Incorporated Carbon Nanotube-coated Electrode for the Simultaneous Determination of Dopamine and EpinephrineCHINESE JOURNAL OF CHEMISTRY, Issue 3 2005Wang Ge-Yun Abstract An ,-cyclodextrin (,-CD) incorporated carbon nanotube (CNT)-coated electrode was fabricated and applied to the simultaneous determination of dopamine (DA) and epinephrine (EP). It has been found that the modified electrode shows strong catalytic effects on the electro-separation of DA and EP, and the cathodic potential difference between DA and EP is about 390 mV. The reducing peak current is proportional to DA and EP concentrations in the range of 2.0×10 -6,1.0×10 -3 and 1.0×10 -6,1.0×10 -3 mol·L -1, respectively. Their detection limits can reach 1×10 -6 and 5×10 -7 mol·L -1, respectively. Because the oxidation of ascorbic acid (AA) is an irreversible reaction in ,-CD/CNT film, the interference of AA in determination of DA and EP is eliminated. [source] Generalized treatment of NMR spectra for rapid chemical reactionsCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2007Matthew D. Christianson Abstract Application of NMR spectroscopy to fast irreversible reactions (t1/2 < 0.7 s) has been hampered by limitations in instrumentation and general methods for modeling the complicated spectra that result. Analytical descriptions of nuclear spin dynamics during fast reactions, first solved by Ernst and coworkers, are limited to first-order reaction kinetics. We demonstrate that numeric methods enable simulation of NMR spectra for fast reactions having any form of rate law. Simulated stopped-flow NMR spectra are presented for a variety of common kinetic scenarios including reversible and irreversible reactions of first and second-order, multistep reactions, and catalytic transformations. The simulations demonstrate that a wealth of mechanistic information, including reaction rates, rate laws, and the existence of intermediates, is imbedded in a single NMR spectrum. The sensitivity of modern NMR instrumentation along with robust methods for simulating and fitting kinetic parameters of fast reactions make stopped-flow NMR an attractive method for kinetic studies of fast chemical reactions. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 165,183, 2007. [source] Polyphase evolution and reaction sequence of compositional domains in metabasalt: a model based on local chemical equilibrium and metamorphic differentiationGEOLOGICAL JOURNAL, Issue 3-4 2000T. M. Toóth Abstract Eclogitic garnet amphibolite samples from the Southern Steep Belt of the Central Alps show evidence of several stages of metamorphic evolution and exhumation. A method for unravelling this evolution is presented and applied to these samples. It involves a combination of detailed petrographic analysis and microchemical characterization with quantitative models of the thermodynamically stable phase relations for specific compositional domains of each sample. Preserved mineral relics and textural evidence are compared to model predictions to identify the important irreversible reactions. The interpretation of the exhumation history is thus based on the consistency of a wide spectrum of observations with predicted phase diagrams, leading to robust reconstruction of a pressure,temperature (P,T) path even where the mineralogical relics in samples are insufficient, due to retrogression, to warrant application of multi-equilibrium thermobarometric techniques. The formation of compositionally different domains in the metabasalt samples studied is attributed to prograde growth of porphyroblasts (e.g. garnet, plagioclase, zoisite) in the matrix, implying substantial metamorphic differentiation at the scale of a few millimetres. Chemical interaction among different domains during the subsequent P,T evolution is shown to have been very limited. This led to different reaction sequences during exhumation, in which relics preserved in different domains reflect a range of continually changing P,T conditions. For samples from a single outcrop, we deduce a Barrovian prograde path to eclogite facies (23,±,3,kbar, 750,±,50°C), followed by (rapid) decompression to 8,±,1,kbar and 675,±,25°C, and a final heating phase at similar pressures reaching 750,±,40°C. This evolution is attributed to the Alpine cycle involving subduction,collision and slab breakoff,extrusion of tectonic fragments that make up the Southern Steep Belt of the Central Alps. Copyright © 2000 John Wiley & Sons, Ltd. [source] Tuning the Amphiphilicity of Building Blocks: Controlled Self-Assembly and Disassembly for Functional Supramolecular Materials,ADVANCED MATERIALS, Issue 28 2009Yapei Wang Abstract Amphiphilicity is one of the molecular bases for self-assembly. By tuning the amphiphilicity of building blocks, controllable self-assembly can be realized. This article reviews different routes for tuning amphiphilicity and discusses different possibilities for self-assembly and disassembly in a controlled manner. In general, this includes irreversible and reversible routes. The irreversible routes concern irreversible reactions taking place on the building blocks and changing their molecular amphiphilicity. The building blocks are then able to self-assemble to form different supramolecular structures, but cannot remain stable upon loss of amphiphilicity. Compared to the irreversible routes, the reversible routes are more attractive due to the good control over the assembly and disassembly of the supramolecular structure formed via tuning of the amphiphilicity. These routes involve reversible chemical reactions and supramolecular approaches, and different external stimuli can be used to trigger reversible changes of amphiphilicity, including light, redox, pH, and enzymes. It is anticipated that this line of research can lead to the fabrication of new functional supramolecular assemblies and materials. [source] Development and testing of a comprehensive chemical mechanism for the oxidation of methaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2001K. J. Hughes A comprehensive chemical mechanism to describe the oxidation of methane has been developed, consisting of 351 irreversible reactions of 37 species. The mechanism also accounts for the oxidation kinetics of hydrogen, carbon monoxide, ethane, and ethene in flames and homogeneous ignition systems in a wide concentration range. It has been tested against a variety of experimental measurements of laminar flame velocities, laminar flame species profiles, and ignition delay times. The highest sensitivity reactions of the mechanism are discussed in detail and compared with the same reactions in the GRI, Chevalier, and Konnov mechanisms. Similarities and differences of the four mechanisms are discussed. The mechanism is available on the Internet as a fully documented CHEMKIN data file at the address http://www.chem.leeds.ac.uk/Combustion/Combustion.html. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 513,538, 2001 [source] | |||||||||||||