Determination Of Relative Rate Constants Research Articles

Determination of Relative Rate Constants for Reactions of 2,2’-Azobis-(2-Amidinopropane) Dihydrochloride Peroxyl Radicals with Dihydroxybenzenes and Their Derivatives

An iodometric method for determining the antiradical activity (ARA) of dihydroxybenzenes in the reaction with 2,2′-azobis-(2-amidinopropane) (AAP) peroxyl radicals (rOO• ) in conditions close to those obtaining in biological systems was developed. A means of calculating relative rate constants of reactions of phenols with rOO• from the kinetics of consumption of an inhibitor added at low concentration was proposed. The greatest levels of ARA were seen with resorcinol, orcinol, and hydroquinone, which are able to dissociate to form highly reactive mono-and bianionic forms of phenolate ions. A method of calculating relative rate constants of these reactions was proposed, based on the AAP - AAP peroxyl radical - phenol kinetic model.

Determination of relative rate constants for in vitro RNA processing reactions by internal competition

Studies of RNA recognition and catalysis typically involve measurement of rate constants for reactions of individual RNA sequence variants by fitting changes in substrate or product concentration to exponential or linear functions. A complementary approach is determination of relative rate constants by internal competition, which involves quantifying the time-dependent changes in substrate or product ratios in reactions containing multiple substrates. Here, we review approaches for determining relative rate constants by analysis of both substrate and product ratios and illustrate their application using the in vitro processing of precursor transfer RNA (tRNA) by ribonuclease P as a model system. The presence of inactive substrate populations is a common complicating factor in analysis of reactions involving RNA substrates, and approaches for quantitative correction of observed rate constants for these effects are illustrated. These results, together with recent applications in the literature, indicate that internal competition offers an alternate method for analyzing RNA processing kinetics using standard molecular biology methods that directly quantifies substrate specificity and may be extended to a range of applications.

Alternative substrate kinetics of Escherichia coli ribonuclease P: Determination of relative rate constants by internal competition

A single enzyme, RNase P, processes the 5′ ends of ptRNAs in cells and organelles that carry out tRNA biosynthesis. This substrate population includes over 80 different competing ptRNAs in Escherichia coli. While the reaction kinetics and molecular recognition of a few individual model substrates of bacterial RNase P have been well described, the competitive substrate kinetics of the enzyme are unexplored. To understand how different ptRNA substrates compete for processing by RNase P, we compared the steady state reaction kinetics of two ptRNAs that differ at sequences that are contacted by the enzyme. For both ptRNAs, substrate cleavage is fast relative to dissociation. As a consequence, V/K, the rate constant for the reaction at limiting substrate concentrations, reflects the substrate association step for both ptRNAs. Reactions containing two or more ptRNAs follow simple competitive alternative substrate kinetics in which the relative rates of processing are determined by ptRNA concentration and their V/K. The relative V/K values for eight different ptRNAs, that were selected to represent a range of structure variation, were determined by internal competition in reactions in which all eight substrates are present simultaneously. The results suggest that rates of ptRNA processing by RNase P are tuned for uniform specificity and consequently optimal coupling to precursor biosynthesis.

Reaction Kinetics of Hydroxyl Radicals with Model Compounds of Fuel Cell Polymer Membranes

AbstractThe chemical stability of perfluorinated and non‐perfluorinated low temperature fuel cell model compounds (MCs) against attack by hydroxyl radicals, HO•, is compared using a competition kinetics approach in aqueous solutions at ambient temperature. HO• radicals were generated in situ by UV photolysis of hydrogen peroxide in the electron spin resonance (ESR) resonator. Acetic acid (AA), trifluoroacetic acid (TFAA), methanesulfonic acid (MSA), trifluorosulfonic acid (TFSA), and perfluoro(2‐ethoxyethane)sulfonic acid (PFEESA) were chosen as MCs, while the rate constants of 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) and methanol (CH3OH) served as reference for the determination of relative rate constants by means of steady state ESR signal amplitudes. In decreasing order the rate constants are: kMSA = (4.8 ± 0.2) × 107 M–1 s–1, kAA = (4.2 ± 0.3) × 107 M–1 s–1, kPFEESA = (3.7 ± 0.1) × 106 M–1 s–1, kTFAA = (7.9 ± 0.2) × 105 M–1 s–1, and kTFSA < 1.0 × 105 M–1 s–1. Applying these results to perfluorinated fuel cell membranes like Nafion®, the main points of attack by HO• are concluded to be the ether groups of the side chains, followed by the remaining carboxyl groups from the manufacturing process of the polymers.

Dynamic mechanical study on the thermal aging of a hydroxyl‐terminated polybutadiene‐based energetic composite

AbstractIn an attempt to determine the aging behavior of hydroxyl‐terminated polybutadiene‐based composite solid propellants, viscoelastic measurements were used to study the effect of thermal aging on this kind of energetic material. Accelerated‐aging tests at 40, 60, and 80°C were performed for 5000 h. The changes in the dynamic mechanical properties, including the storage modulus (E′) and loss factor or damping efficiency (tan δ), with time and temperature were measured to determine the aging rate and likely mechanisms occurring during this process. An Arrhenius analysis based on the determination of relative rate constants showed a linear tendency from tan δ values, whereas a significant curvature was found from E′ values. In addition, the effects of external (surface) and internal (core) sampling in the intensification of the aging process were analyzed. The results confirmed dynamic mechanical analysis as a powerful tool for determining the aging characteristics of composite propellants. This technique allows the evaluation of the actual state of a propellant grain with a small sample and a straightforward measurement. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2397–2405, 2003

Conservation of substrate specificities among coronavirus main proteases.

The key enzyme in coronavirus replicase polyprotein processing is the coronavirus main protease, 3CL(pro). The substrate specificities of five coronavirus main proteases, including the prototypic enzymes from the coronavirus groups I, II and III, were characterized. Recombinant main proteases of human coronavirus (HCoV), transmissible gastroenteritis virus (TGEV), feline infectious peritonitis virus, avian infectious bronchitis virus and mouse hepatitis virus (MHV) were tested in peptide-based trans-cleavage assays. The determination of relative rate constants for a set of corresponding HCoV, TGEV and MHV 3CL(pro) cleavage sites revealed a conserved ranking of these sites. Furthermore, a synthetic peptide representing the N-terminal HCoV 3CL(pro) cleavage site was shown to be effectively hydrolysed by noncognate main proteases. The data show that the differential cleavage kinetics of sites within pp1a/pp1ab are a conserved feature of coronavirus main proteases and lead us to predict similar processing kinetics for the replicase polyproteins of all coronaviruses.

Role of the DNase-I-Binding Loop in Dynamic Properties of Actin Filament

Effects of proteolytic modifications of the DNase-I-binding loop (residues 39–51) in subdomain 2 of actin on F-actin dynamics were investigated by measuring the rates of the polymer subunit exchange with the monomer pool at steady state and of ATP hydrolysis associated with it, and by determination of relative rate constants for monomer addition to and dissociation from the polymer ends. Cleavage of actin between Gly-42 and Val-43 by protease ECP32 resulted in enhancement of the turnover rate of polymer subunits by an order of magnitude or more, in contrast to less than a threefold increase produced by subtilisin cleavage between Met-47 and Gly-48. Probing the structure of the modified actins by limited digestion with trypsin revealed a correlation between the increased F-actin dynamics and a change in the conformation of subdomain 2, indicating a more open state of the filament subunits relative to intact F-actin. The cleavage with trypsin and steady-state ATPase were cooperatively inhibited by phalloidin, with half-maximal effects at phalloidin to actin molar ratio of 1:8 and full inhibition at a 1:1 ratio. The results support F-actin models in which only the N-terminal segment of loop 39–51 is involved in monomer-monomer contacts, and suggest a possibility of regulation of actin dynamics in the cell through allosteric effects on this segment of the actin polypeptide chain.

The crocin assay for the determination of relative rate constants of alkoxyl radical reactions with the pyridoindole stobadine and with other antioxidants

The water-soluble carotenoid crocin is bleached in aqueous solutions by tert-butyloxy radicals (t-BuO(.)) photolytically generated from tert-butyl hydroperoxide. Crocin bleaching as measured at 440 nm can be competitively inhibited by antioxidants. This method therefore allows the determination of the relative reaction rates of these compounds with the t-BuO(.) radicals. In this test system the scavenging effect of the pyridoindole stobadine, a new antioxidant with potential for the treatment of tissue injury caused by oxidative stress, was compared with other known antioxidants. Generally good agreement was observed in two different approaches for the evaluation of the experimental data. The relative rate constants of the antioxidants tested increased in the order: stobadine, chlorpromazine, isoascorbic acid, trolox, ascorbic acid. Since it has been shown previously that stobadine can scavenge alkoxyl radicals also in a non-polar environment, this antioxidant property may be important in the inhibition of lipid peroxidation.

Analysis of kinetic data accompanied by catalyst fouling with application to olefin hydrogenation

Measurements of kinetic data in experimental catalytic reactors are often plagued by non-constant catalytic activities, for example, due to unexpected fouling of catalysts, even during a relatively short time interval. The purpose of the present work is to illustrate how the data analysis in the composition space can be conveniently employed to derive appropriate kinetic information from such ill-defined rate data. The catalytic reaction considered is the hydrogenation of propene, butene, and their mixtures at 3.5 MPa over a presulfided Co-Mo catalyst supported on gamma-alumina, in which volatile polymer formation of butene at low reaction temperatures and depolymerization at higher temperatures are observed to occur. The present work reports the data analysis methods employed, which eventually have led to consistent reaction pathways and to the determination of relative rate constants associated with them. The results reported herein may be useful for designing and operating commercial hydrotreaters where various C3/C4 olefin mixtures are hydrogenated. The methodology adopted here may be extended to any complex reaction system such as catalytic reforming of naphtha.

Determination of relative rate constants of reactions on polymers from the triad distribution data

Journal of Polymer Science: Polymer Physics EditionVolume 22, Issue 3 p. 525-527 Note Determination of relative rate constants of reactions on polymers from the triad distribution data Y. Merle, Y. Merle Laboratories de Chimie Macromoléculaire ERA 471, Faculté des Sciences, Université de Haute-Normandie, 76130 Mont Saint Aignan, FranceSearch for more papers by this author Y. Merle, Y. Merle Laboratories de Chimie Macromoléculaire ERA 471, Faculté des Sciences, Université de Haute-Normandie, 76130 Mont Saint Aignan, FranceSearch for more papers by this author First published: March 1984 https://doi.org/10.1002/pol.1984.180220314Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume22, Issue3March 1984Pages 525-527 RelatedInformation

Determination of relative rate constants of hydrolysis of poly[(dimethylamino)ethyl methacrylate

Determination of relative rate constants of hydrolysis of poly[(dimethylamino)ethyl methacrylate

Quenching of O(21D2) by Atmospheric Gases

Laboratory investigations of electronically excited oxygen atoms in the O(21D2) state are central to any detailed consideration of atmospheric chemistry, for this species is a photolysis product of the gases O3(λ<310 nm), O2(λ<180 nm) and CO2(λ<167 nm) (ref. 1) following excitation by solar radiation. The O(21D2) state, 1.967 eV above the O(23P2) ground state2, is known to be highly reactive; this, combined with the low probability for the magnetic dipole-allowed spontaneous emission at λ=630 nm (21D2→23P2, Anm = 6.9×10−3 s−1) (ref. 3) makes laboratory studies of this radiation extremely difficult4. Hence a number of indirect measurements on the kinetic behaviour of O(21D2) have employed spectroscopic marker techniques which monitor a product of the reaction of this atomic state with an added molecule. Measurements of the electronic absorption of OH(A2Σ+←X2π) (ref. 5) and the emission from O2(b1Σ+g→X3Σ−g) (ref. 6) provide examples of such methods. Analyses of stable products have also been used to obtain kinetic data on this state7. These methods usually lead to the determination of relative rate constants. Several recent reviews on laboratory and aeronomical studies of O(21D2) summarize the rate data derived from such investigations8–12.