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Showing papers on "Reaction rate constant published in 1991"


Journal ArticleDOI
22 Feb 1991-Science
TL;DR: The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1� to 47�C and reaction free energies from -1.0 to +0.8 electron volts.
Abstract: The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1 degrees to 47 degrees C and reaction free energies from -1.0 to +0.8 electron volts (eV). The ferrocene group was positioned a fixed distance from the gold surface by the self-assembly of a mixed thiol monolayer of (eta(5)C(5)H(5))Fe(eta(5)C(5)H(4))CO(2)(CH(2))(16)SH and CH(3)(CH(2))(15)SH. Rate constants from 1 per second (s(-1)) to 2 x 10(4) s(-1) in 1 molar HClO(4) are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 x 10(4) s(-1) eV(-1). Such self-assembled monolayers can be used to systematically probe the dependence of electron-transfer rates on distance, medium, and spacer structure, and to provide an empirical basis for the construction of interfacial devices such as sensors and transducers that utilize macroscopically directional electron-transfer reactions.

1,179 citations



Journal ArticleDOI
TL;DR: These studies provide the first complete and self-consistent thermodynamic descriptions of DNA polymerase and establish the basis for quantitative assessment of the reactions contributing to its extraordinary fidelity.
Abstract: The elementary steps of DNA polymerization catalyzed by T7 DNA polymerase have been resolved by transient-state analysis of single nucleotide incorporation, leading to the complete pathway: where E, D, N, and P represent T7 DNA polymerase, DNA primer/template, deoxynucleoside triphosphate, and inorganic pyrophosphate, respectively. A DNA primer/template consisting of a synthetic 25/36-mer has been used as a substrate for correct nucleotide incorporation of dTTP in all the experiments. The rate constants and equilibrium constants of each step have been established by direct measurement of individual reactions and fit by computer simulation of the data to obtain a single set of rate constants accounting for all the data. Analysis of the single-turnover kinetics provided measurements of equilibrium dissociation constants for 25/36-mer, dTTP, and PPI equal to 18 nM (koff/ko,), 18 FM (kI/kl), and 2 mM (k5/k5), respectively. The rate-limiting step during single-nucleotide incorporation has been identified as a con- formational change, E.D,-N - E'-D,-N, which occurs at a rate of 300 s-l (k2) upon binding of the correct dNTP. Accordingly, tighter binding of the transition states for the reaction resulting from the conformational change facilitates the phosphodiester bond formation. The chemical step itself was excluded as the rate- limiting step because of the small phosphothioate elemental effect. An observed rate constant of 70 s-l for dTTP(aS) incorporation suggest that the chemical step (k,) occurs at a fast rate, 29000 s-I. Following chemistry, the resulting ternary complex, E'.D,+l-P, undergoes a second conformational change at a rate of 1200 s-l (k4), leading to release of PPI and translocation of the DNA to continue subsequent cycles of polymerization. The rate constants of the reverse steps, 100 (k2), 118000 s-l (k3) and 18 s-l (k4), were derived as fits to the data based upon simulation of single-turnover kinetics of pyrophosphorolysis including measurements of pyrophosphate exchange and the overall equilibrium constant of 1 .O X lo4 for elongation of E-25/36-mer to E-26/36-mer and analysis of the kinetics of the pulse-chase experiment. These studies provide the first complete and self-consistent thermodynamic descriptions of DNA polymerase and establish the basis for quantitative assessment of the reactions contributing to its extraordinary fidelity. The rate constants of the reverse reaction (pyrophosphorolysis) could only be measured by using an exo- nuclease-deficient mutant of gene 5 protein because of the high exonuclease activity of the wild-type enzyme. Thc double mutation D5A,E7A resulted in complete inactivation of the exonuclease activity. The mutant was fully characterized, and all rate constants for the polymerization reaction were shown to be identical with those of the wild-type enzyme. Its excision rate for single-stranded DNA was shown to be reduced by a factor of IO6, while its excision rate for double-stranded DNA was not measurable.

521 citations



Journal ArticleDOI
TL;DR: A substantial data base concerning the rate constants for the gas phase reactions of the NO3 radical with organic compounds is now available, with rate constants having been determined using both absolute and relative rate methods as mentioned in this paper.
Abstract: A substantial data base concerning the rate constants for the gas‐phase reactions of the nitrate (NO3) radical with organic compounds is now available, with rate constants having been determined using both absolute and relative rate methods. To date, the majority of these kinetic date have been obtained at room temperature using relative rate techniques utilizing both the reactions of the NO3 radical with other organic compounds and the equilibrium constant for the NO3+NO2⇄N2O5 reactions as the reference reaction. In this article, the literature kinetic and mechanistic data for the gas‐phase reactions of the NO3 radical with organic compounds (through late 1990) have been tabulated, reviewed and evaluated. While this available data base exhibits generally good agreement and self‐consistency, further absolute rate data are needed, preferably as a function of temperature. Most importantly, mechanistic and product data for the reactions of the NO3 radical with organic compounds need to be obtained.

430 citations


Journal ArticleDOI
TL;DR: The ratio of the velocity of the catalyzed reaction to that of the uncatalyzed reaction increases as the quantity of GSH2/GSSG increases and approaches a constant, limiting value at [GSH]2/[G SSG] greater than 1 mM, suggesting that a reduced, dithiol form of PDI is required for optimum activity.
Abstract: The velocity of the oxidative renaturation of reduced ribonuclease A catalyzed by protein disulfide isomerase (PDI) is strongly dependent on the composition of a glutathione/glutathione disulfide redox buffer. As with the uncatalyzed, glutathione-mediated oxidative folding of ribonuclease, the steady-state velocity of the PDI-catalyzed reaction displays a distinct optimum with respect to both the glutathione (GSH) and glutathione disulfide (GSSG) concentrations. Optimum activity is observed at [GSH] = 1.0 mM and [GSSG] = 0.2 mM. The apparent kcat at saturating RNase concentration is 0.46 +/- 0.05 mumol of RNase renatured min-1 (mumol of PDI)-1 compared to the apparent first-order rate constant for the uncatalyzed reaction of 0.02 +/- 0.01 min-1. Changes in GSH and GSSG concentration have a similar effect on the rate of both the PDI-catalyzed and uncatalyzed reactions except under the more oxidizing conditions employed, where the catalytic effectiveness of PDI is diminished. The ratio of the velocity of the catalyzed reaction to that of the uncatalyzed reaction increases as the quantity [GSH]2/[GSSG] increases and approaches a constant, limiting value at [GSH]2/[GSSG] greater than 1 mM, suggesting that a reduced, dithiol form of PDI is required for optimum activity. As long as the glutathione redox buffer is sufficiently reducing to maintain PDI in an active form [( GSH]2/[GSSG] greater than 1 mM), the rate acceleration provided by PDI is reasonably constant, although the actual rate may vary by more than an order of magnitude. PDI exhibits half of the maximum rate acceleration at a [GSH]2/[GSSG] of 0.06 +/- 0.01 mM.

415 citations




Journal ArticleDOI
TL;DR: In this article, rate constants for the direct reactions of ozone with 45 potential organic drinking water contaminants, including solvents, haloalkanes, esters, aromatics and pesticides (such as alachlor, aldicarb, atrazine, carbofuran, 2,4-dichlorophenoxyacetic acid, endrin, glyphosate), have been measured in water in the presence of hydroxyl radical scavengers to minimize interfering radical chain reactions.

304 citations


Journal ArticleDOI
TL;DR: Correlation analysis based on half-wave oxidation potentials and constants reveals significant quantitative structure-activity relationships (QSARs) that are consistent with the rate-limiting formation of a precursor complex with a small amount of charge-transfer character and can be used to predict additional rate constants for a wide range of environmentally significant substituted phenols.
Abstract: Substituted phenols can be oxidized by singlet oxygen ({sup 1}O{sub 2}), which is formed in sunlit surface waters, and it has been suggested that this reaction may contribute to the environmental fate of phenolic substances. In aqueous solution, the observed rate of phenol disappearance is due to reaction of both the phenolate anion and the undissociated phenol. In order to quantify the effect of substituents on the rates of these reactions, second-order rate constants have been measured for both species for 22 substituted phenols by use of a model system containing the sensitizer rose bengal. Correlation analysis based on half-wave oxidation potentials, E{sub 1/2}, and on {sigma} constants reveals significant quantitative structure-activity relationships (QSARs) for both the undissociated phenols and the phenolate anions. Ortho- and multisubstituted phenols have been included in the correlations. These QSARs are consistent with the rate-limiting formation of a precursor complex with a small amount of charge-transfer character and can be used to predict additional rate constants for a wide range of environmentally significant substituted phenols.

291 citations



Journal ArticleDOI
TL;DR: ESR measurements provide evidence that a caffeine-derived oxygen-centred radical is formed in the reaction of caffeine with .OH and suggest a biochemical basis for the understanding of the reported anticarcinogenic properties of caffeine and related methylxanthine compounds.

Journal ArticleDOI
TL;DR: In this paper, the authors derived the rate constant for the carbonate radical anion with azide, bromide, and hypochlorite ions for pulse radiolysis.


Journal ArticleDOI
TL;DR: It is confirmed that thio substitution has no effect on the nonenzymatic alkaline cleavage of RNA and the strong preference of Mg2+ for binding to oxygen rather than sulfur suggest that the pro-RP oxygen atom is not coordinated to Mg 2+ in the E.S complex or in the transition state.
Abstract: The L-21 ScaI ribozyme derived from the intervening sequence of Tetrahymena thermophila pre-rRNA catalyzes a guanosine-dependent endonuclease reaction that is analogous to the first step in self-splicing of this intervening sequence. We now describe pre-steady-state kinetic experiments, with sulfur substituting for the pro-RP (nonbridging) phosphoryl oxygen atom at the site of cleavage, that test aspects of a kinetic model proposed for the ribozyme reaction (Herschlag, D., & Cech, T. R. (1990) Biochemistry 29, 10159-10171). Thio substitution does not affect the reaction with subsaturating oligonucleotide substrate and saturating guanosine ((kcat/Km)S), consistent with the previous finding that binding of the oligonucleotide substrate limits this rate constant. In contrast, there is a significant decrease in the rate of single-turnover reactions of ribozyme-bound (i.e., saturating) oligonucleotide substrate upon thio substitution, with decreases of 2.3-fold for the reaction with guanosine ((kcat/Km)G) and 7-fold for hydrolysis [i.e., with solvent replacing guanosine; kc(-G)]. These "thio effects" are consistent with rate-limiting chemistry, as shown by comparison with model reactions. Nonenzymatic nucleophilic substitution reactions of the phosphate diester, methyl 2,4-dinitrophenyl phosphate monoanion, are slowed 4-11-fold by thio substitution for reactions with hydroxide ion, formate ion, fluoride ion, pyridine, and nicotinamide. In addition, we have confirmed that thio substitution has no effect on the nonenzymatic alkaline cleavage of RNA (Burgers, P. M. J., & Eckstein, F. (1979) Biochemistry 18, 592-596). Considering the strong preference of Mg2+ for binding to oxygen rather than sulfur, the modest thio effect on the chemical step of the ribozyme-catalyzed reaction and the absence of a thio effect on the equilibrium constant for binding of the oligonucleotide substrate suggest that the pro-RP oxygen atom is not coordinated to Mg2+ in the E.S complex or in the transition state. General implications of thio effects in enzymatic reactions of phosphate diesters are discussed.

Journal ArticleDOI
TL;DR: In this article, Fenton's reagent was used to rapidly oxidize polychlorinated biphenyls (PCBs) in aqueous solutions, and the observed product distributions indicated that the reaction proceeds via addition of a hydroxyl group to one of the nonhalogenated sites.
Abstract: Hydroxyl radicals (OH{center dot}s), generated with Fenton's reagent, rapidly oxidized polychlorinated biphenyls (PCBs) in aqueous solutions. The observed product distributions indicated that the reaction proceeds via addition of a hydroxyl group to one of the nonhalogenated sites. The unreactive nature of the halogenated sites accounts for the lower reaction rates observed for the more highly chlorinated congeners. Within homologue groups, chlorine substitution in the meta or para positions results in lower reaction rates observed for the more highly chlorinated congeners. Within homologue groups, chlorine substitution in the meta or para positions results in lower reaction rates due to steric hindrance occurring when hydroxyl radical attacks at the site ortho to the aromatic ring. Estimates of environmental half-lives, based upon kinetic measurements, reported steady-state OH{center dot} concentrations, or estimates of radical production rates, suggest that aqueous-phase oxidation by OH{center dot}s may be an important PCB loss mechanism in cloudwater and surface waters under certain conditions.

Journal ArticleDOI
TL;DR: In this paper, the effects of zeolite structure, acid site density, and reaction temperature on the mode of n-hexane cracking have been studied, and the results can be consistently explained in terms of the occurrence of two different cracking routes, viz. the classical β-scission route and the monomolecular protolytic pathway.

Journal ArticleDOI
TL;DR: Kinetic parameters for triosephosphate isomerase catalysis of the elimination reaction of an equilibrium mixture of dihydroxyacetone phosphate and D-glyceraldehyde-3-phosphates to form methylglyoxal and phosphate ion are reported for the enzyme from rabbit muscle.
Abstract: Kinetic parameters for triosephosphate isomerase catalysis of the elimination reaction of an equilibrium mixture of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde-3-phosphate (DGAP) to form methylglyoxal and phosphate ion are reported for the enzyme from rabbit muscle. Pseudo-first-order rate constants for the disappearance of substrate (kelim) were determined for reactions at [Enzyme] much greater than [Substrate]. The second-order rate constant kEnz = 10.1 M-1 s-1 was determined from a plot of kelim against enzyme concentration. The kinetic parameters, determined from a steady-state kinetic analysis at [Substrate] much greater than [Enzyme], are kcat = 0.011 s-1, Km = 0.76 mM, and kcat/Km = 14 M-1 s-1. The estimated rate-constant ratio for partitioning of the enzyme-bound intermediate between protonation at carbon 2 and elimination, 1,000,000, is much larger than the ratio of 6.5 determined for the reaction of the enediolate phosphate in a loose complex with quinuclidinonium cation, a small buffer catalyst. There is a 10(5)-10(8)-fold decrease in the rate constant for the elimination reaction of the enediolate phosphate when this species binds to triosephosphate isomerase. The kinetic parameters for the elimination reaction catalyzed by the native triosephosphate isomerase and for the reaction catalyzed by a mutant form of the enzyme, which is missing a segment that forms hydrogen bonds with the phosphate group of substrate [Pompliano, D. L., Peyman, A., & Knowles, J. R. (1990) Biochemistry 29, 3186-3194] are similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Journal ArticleDOI
TL;DR: In this paper, the second-order rate constants and activation parameters for the intermolecular Diels-Alder reactions of cyclopentadiene with alkyl vinyl ketones (2a,b) and 5-substituted-1,4-naphthoquinones (3a-c) were determined in water, monohydric alcohols, and highly aqueous solutions containing monoric alcohols and other organic cosolvents.
Abstract: Second-order rate constants and isobaric activation parameters for the intermolecular Diels-Alder reactions of cyclopentadiene with alkyl vinyl ketones (2a,b) and 5-substituted-1,4-naphthoquinones (3a-c) as well as for the intramolecular Diels-Alder reaction of N-furfuryl-N-methylmaleamic acid (4) were determined in water, monohydric alcohols, and highly aqueous solutions containing monohydric alcohols and other organic cosolvents. The intermolecular as well as the intramolecular Diels-Alder reaction are characterized by large rate accelerations upon going from an organic solvent to water as the reaction medium. A model for solvation effects on rate constants for Diels-Alder reactions in aqueous solutions is developed. The typical, huge rate enhancements for Diels-Alder reactions in water and in highly aqueous binary mixtures are significantly sensitive to substituent effects and are discussed in terms of enforced pairwise hydrophobic interactions between diene and dienophile.

Journal ArticleDOI
TL;DR: In this article, a 1 litre stainless steel autoclave at temperatures in the range of 150-180°C was used for wet air oxidation of aqueous solutions of phenol and substituted phenols.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the influence of various parameters on the photodegradation rate, such as the concentrations of the catalyst, dissolved oxygen and organic compound, and the presence of various anions in the dispersion.

Journal ArticleDOI
TL;DR: In this paper, the use of the scanning electrochemical microscope (SECM) in both the steady-state and chronoamperometric feedback modes, is proposed as a new method for measuring the kinetics of following chemical reactions in electrode processes.
Abstract: The use of the scanning electrochemical microscope (SECM), in both the steady-state and chronoamperometric feedback modes, is proposed as a new method for measuring the kinetics of following chemical reactions in electrode processes. Particular attention is given to the E,Ci mechanistic case (first-order irreversible chemical step following reversible electron transfer). Theory for the problem, relating the SECM feedback current to the tip-substrate distance, tip electrode radius, and rate constant for the chemical step, is developed numerically by using the alternating-direction implicit finite-difference scheme. The theoretical results demonstrate that both the chronoamperometric and steady-state feedback modes are sensitive techniques for accurate kinetic determinations, and that homogeneous chemical rate constants in excess of 2 X 104 s-' should be accessible to measurement with current SECM technology. The theoretical predictions are verified experimentally with the measurement of the rate of deamination, in aqueous basic solution (pH 10.2-1 2.4), of oxidized N,N-dimethyl-p-phenylenediamine, produced at a platinum electrode.


Journal ArticleDOI
TL;DR: In this article, the rate of asymmetric epoxidation of allylic alcohols is shown to be first order in substrate and oxidant, and inverse second order in inhibitor alcohol, under pseudo-first-order conditions in catalyst.
Abstract: The rate of titanium-tartrate-catalyzed asymmetric epoxidation of allylic alcohols is shown to be first order in substrate and oxidant, and inverse second order in inhibitor alcohol, under pseudo-first-order conditions in catalyst. The rate is slowed by substitution of electron-withdrawing substituents on the olefin and varies slightly with solvent, CH 2 Cl 2 being the solvent of choice. Assymetric induction suffers when the size of the alkyl hydroperoxide is reduced. Kinetic resolution of secondary allylic alcohols is shown to be sensitive to the size of the tartrate ester group and insensitive to the steric nature of inhibitor alcohol. Most importantly, the species containing equimolar amounts to Ti and tartrate is shown to be the most active catalyst in the reaction mixture, mediating reaction at much faster rates than titanium tetraalkoxide alone


Journal ArticleDOI
TL;DR: In this article, aluminum clusters were produced using laser vaporization and reacted under thermal conditions with oxygen in a flow tube reactor, and an etching reaction was observed and bimolecular rate constants were reported for Al+n, n=1-33, and Al−n n=5-37.
Abstract: Aluminum clusters, both anion and cation, are produced using laser vaporization and reacted under thermal conditions with oxygen in a flow tube reactor. An etching reaction is observed and bimolecular rate constants are reported for Al+n, n=1–33, and Al−n, n=5–37. For certain clusters, namely Al+7, Al−13, and Al−23, no apparent reactivity is observed (they are found to be produced from larger species). Interestingly, these correspond to predicted jellium shell closings with 20, 40, and 70 electrons, respectively. Besides these exceptions, and a small odd/even alternation in reaction rates, the overall reactivity is relatively insensitive to cluster size, and is found to range between about 1 × 10−12 and 5 × 10−12 cm3/s.

Journal ArticleDOI
TL;DR: The integrated reaction progress curve was rigorously interpreted in terms of a minimal model involving the Michaelis-Menten reaction sequence in the interface, and most of the individual rate and equilibrium constants for the catalytic cycle were determined.
Abstract: Hydrolysis of vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM) by pig pancreatic phospholipase A2 (PLA2) occurs in a highly processive "scooting" mode, and the rate is comparable to or exceeds the rates observed with detergent-dispersed mixed micelles under optimal conditions. A complete kinetic description of the steady-state time course of the hydrolysis is developed. The analysis covers the whole Michaelis-Menten space: it emphasizes the key features of interfacial catalysis by a detailed theoretical analysis, describes the experimental protocols to determine the values of the kinetic and equilibrium constants for interfacial catalysis, and provides an interpretation of the effect of calcium, substrate, products, apparent activators, and competitive inhibitors on the reaction progress curve by a single set of rate and equilibrium parameters. In this paper, the integrated reaction progress curve was rigorously interpreted in terms of a minimal model involving the Michaelis-Menten reaction sequence in the interface: E* + S in equilibrium E*S----E*P in equilibrium E* + P, and most of the individual rate and equilibrium constants for the catalytic cycle were determined. This rigorous description of interfacial catalysis was made experimentally possible by examining the action of PLA2 in the scooting mode under conditions of at most one enzyme per vesicle, where it hydrolyzed all of the substrate in the outer monolayer of vesicles without leaving the surface. Other experimentally verified constraints for this analysis include the following: all enzyme was bound to vesicles; the integrity of vesicles was maintained during the course of hydrolysis; and the substrate, enzyme, and products did not exchange between vesicles nor did they exchange across the bilayer. The mechanistic significance of the rate constants is discussed in the accompanying papers.


Journal ArticleDOI
TL;DR: In this paper, a detailed kinetic analysis of the propagation of the catalytic chain and its competition with the biphenyl chain is presented. Butts et al. identified all the key steps of the chain and their rate constants.
Abstract: In the presence of stoichiometric amounts of carbon dioxide, and catalytic amounts of Ni II (dppe)Cl 2 , electrolysis of bromobenzene results in the nearly quantitative formation of benzoic acid with negligible production of benzene or biphenyl. Based on a detailed kinetic analysis of the propagation of this catalytic chain and of its competition with the biphenyl chain, all the key steps of the catalytic chain are identified and their rate constants determined

Journal ArticleDOI
TL;DR: In this paper, a pressure-corrected elementary reaction model for gas-phase combustion was applied to the oxidation of methane in supercritical water, and the activation energy over the temperature range 560-650 o was 42.8±4.3 kcal/mol.
Abstract: The oxidation kinetics of methane in supercritical water were determined in an isothermal, plug flow reactor over the temperature range 560-650 o C at 245.8 bar. The oxidation rate was found to be first order in methane concentration and 2/3-order in oxygen concentration. The activation energy over the temperature range 560-650 o was 42.8±4.3 kcal/mol. A pressure-corrected elementary reaction model for gas-phase combustion was applied to the oxidation of methane in supercritical water