Photon-catalyzed unimolecular decomposition with large first-order rate constant

1980 ◽  
Vol 22 (2) ◽  
pp. 614-617 ◽  
Author(s):  
Albert M. F. Lau
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Unimolecular Decomposition ◽  
First Order ◽  
Order Rate

THE THERMAL DECOMPOSITION OF ETHANE: PART II. THE UNIMOLECULAR DECOMPOSITION OF THE ETHANE MOLECULE AND THE ETHYL RADICAL

1966 ◽  
Vol 44 (20) ◽  
pp. 2357-2367 ◽  
Author(s):  
M. C. Lin ◽  
M. H. Back
Keyword(s):  
High Pressure ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Methyl Radicals ◽  
Unimolecular Decomposition ◽  
First Order ◽  
Order Rate ◽  
Ethyl Radical ◽  
Lower Temperature ◽  
Ethyl Radicals

The rate of the elementary dissociation of ethane into two methyl radicals has been measured in its pressure-dependent region at temperatures from 913–999 °K and at pressures from 1–200 mm. The high-pressure first-order rate constant obtained by extrapolation was in agreement with that obtained at lower temperatures,[Formula: see text]Comparison with calculated Kassel curves showed that the best fit of the data was obtained with the Kassel parameter s = 12 ± 1. The high-pressure first-order rate constant for the decomposition of the ethyl radical was obtained by extrapolation of the data reported in Part I, assuming the rate constant for combination of ethyl radicals is independent of temperature.[Formula: see text]From the measured constant for the dissociation of ethane, the rate constant for the combination of methyl radicals was calculated and compared with the values measured in a lower temperature region. Differences in the values of the rate constants and in the shapes of the unimolecular falloff curves are discussed.

The Dephosphorylation of Adenosine 5′ -Triphosphate in a Binary and Ternary Zn2+ Complex

1974 ◽  
Vol 29 (11-12) ◽  
pp. 680-682 ◽  
Author(s):  
Peter Amsler ◽  
David Buisson ◽  
Helmut Sigel
Keyword(s):  
Rate Constant ◽  
Ternary Complex ◽  
Order Rate Constant ◽  
Reactive Species ◽  
Ph Optimum ◽  
First Order ◽  
Order Rate

The dephosphorylation of ATP was characterized by determining the dependence of the first-order rate constant on pH in the absence and presence of Zn2+ and together with Zn2+ and 2,2′-bipyridyl. The Zn2+-accelerated reaction passes through a pH optimum at about 8. The decrease in the rate at higher pH is due to the formation of Zn(ATP) (OH)3-; this species is relatively insensitive towards dephosphorylation. It is concluded that Zn(ATP)2- is the reactive species and that the interaction between N (7) and Zn2+ in this complex is crucial for its reactivity. In the presence of 2,2′-bipyridyl (Bipy) the ternary complex, Zn (Bipy) (ATP)2-, is formed which is rather stable towards dephosphorylation. It is suggested that the described effects of acceleration and inhibition are helpful for understanding the recycled processes in nature.

Kinetics of reconstitution of apotyrosinase by copper

1990 ◽  
Vol 68 (2) ◽  
pp. 476-479
Author(s):  
Donald C. Wigfield ◽  
Douglas M. Goltz
Keyword(s):  
Rate Constant ◽  
Copper Concentration ◽  
Copper Ions ◽  
Copper Ion ◽  
Order Rate Constant ◽  
First Order ◽  
Order Rate ◽  
Pseudo First Order ◽  
Rate Limiting ◽  
Kinetics Of

The kinetics of the reconstitution reaction of apotyrosinase with copper (II) ions are reported. The reaction is pseudo first order with respect to apoenzyme and the values of these pseudo first order rate constants are reported as a function of copper (II) concentration. Two copper ions bind to apoenzyme, and if the second one is rate limiting, the kinetically relevant copper concentration is the copper originally added minus the amount used in binding the first copper ion to enzyme. This modified copper concentration is linearly related to the magnitude of the pseudo first order rate constant, up to a copper concentration of 1.25 × 10−4 M (10-fold excess), giving a second order rate constant of 7.67 × 102 ± 0.93 × 102 M−1∙s−1.Key words: apotyrosinase, copper, tyrosinase.

scholarly journals Inactivation of the endogenous argininosuccinate lyase activity of duck δ-crystallin by modification of an essential histidine residue with diethyl pyrocarbonate

1993 ◽  
Vol 293 (2) ◽  
pp. 537-544 ◽  
Author(s):  
H J Lee ◽  
S H Chiou ◽  
G G Chang
Keyword(s):  
Enzyme Activity ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Histidine Residue ◽  
Argininosuccinate Lyase ◽  
First Order ◽  
Diethyl Pyrocarbonate ◽  
Order Rate ◽  
Lyase Activity ◽  
Pseudo First Order

The argininosuccinate lyase activity of duck delta-crystallin was inactivated by diethyl pyrocarbonate at 0 degrees C and pH 7.5. The inactivation followed pseudo-first-order kinetics after appropriate correction for the decomposition of the reagent during the modification period. The plot of the observed pseudo-first-order rate constant versus diethyl pyrocarbonate concentration in the range of 0.17-1.7 mM was linear and went through the origin with a second-order rate constant of 1.45 +/- 0.1 M-1.s-1. The double-logarithmic plot was also linear, with slope of 1.13, which suggested a 1:1 stoichiometry for the reaction between diethyl pyrocarbonate and delta-crystallin. L-Arginine, L-norvaline or L-citrulline protected the argininosuccinate lyase activity of delta-crystallin from diethyl pyrocarbonate inactivation. The dissociation constants for the delta-crystallin-L-arginine and delta-crystallin-L-citrulline binary complexes, determined by the protection experiments, were 4.2 +/- 0.2 and 0.12 +/- 0.04 mM respectively. Fumarate alone had no protective effect. However, fumarate plus L-arginine gave synergistic protection with a ligand binding interacting factor of 0.12 +/- 0.02. The double-protection data conformed to a random Uni Bi kinetic mechanism. Fluorescence-quenching studies indicated that the modified delta-crystallin had minimum, if any, conformational changes as compared with the native delta-crystallin. Inactivation of the enzyme activity was accompanied by an increasing absorbance at 240 nm of the protein. The absorption near 280 nm did not change. Treatment of the modified protein with hydroxylamine regenerated the enzyme activity to the original level. These results strongly indicated the modification of an essential histidine residue. Calculation from the 240 nm absorption changes indicated that only one histidine residue per subunit was modified by the reagent. This super-active histidine residue has a pKa value of approximately 6.8 and acts as a general acid-base catalyst in the enzyme reaction mechanism. Our experimental data are compatible with an E1cB mechanism [Raushel (1984) Arch. Biochem. Biophys. 232, 520-525] for the argininosuccinate lyase with the essential histidine residue close to the arginine-binding domain of delta-crystallin. L-Citrulline, after binding to this domain, might form an extra hydrogen bond with the essential histidine residue.

The Initial Stages of the Pyrolysis of Dimethyl Ether

1975 ◽  
Vol 53 (18) ◽  
pp. 2742-2747 ◽  
Author(s):  
Philip D. Pacey
Keyword(s):  
Dimethyl Ether ◽  
Rate Constant ◽  
Arrhenius Plot ◽  
Flow System ◽  
Order Rate Constant ◽  
Chain Termination ◽  
Rate Coefficients ◽  
First Order ◽  
Order Rate ◽  
Initiation Reaction

Dimethyl ether was pyrolized in a flow system at 782–936 K and 25–395 Torr with conversions from 0.2–10%. Product analyses were consistent with a simple Rice–Herzfeld mechanism with most chain termination by the recombination of CH3 radicals. The rate coefficients for both the initiation and termination reactions appeared to be slightly pressure dependent. The first-order rate constant for the initiation reaction,[Formula: see text]calculated from the rate of C2H6 formation, was k1 = 1015.0±0.5exp (−318 ± 8 kJ mol−1/RT) s−1, corresponding to ΔHf0(CH3O) = −5 ± 8 kJmol−1. Comparison of CH4 and C2H6 yields enabled calculation of the rate constant for the reaction of CH3 with dimethyl ether. From 373−936 K, the Arrhenius plot for this reaction is a curve.

The homogeneous decomposition reactions of gaseous formic acid

1960 ◽  
Vol 255 (1283) ◽  
pp. 444-455 ◽  
Keyword(s):  
Formic Acid ◽  
Kinetic Parameters ◽  
Rate Constant ◽  
Ion Pairs ◽  
Order Rate Constant ◽  
Second Order ◽  
Unimolecular Decomposition ◽  
Order Component ◽  
First Order ◽  
Homogeneous Decomposition

By use of reaction vessels with specially treated surfaces the homogeneous decomposition of formic acid has been studied kinetically in the range 436 to 532°C. Neither of the two simultaneous reactions ( a ) HCOOH = CO 2 + H 2 , ( b ) HCOOH = CO + H 2 O, is retarded by the usual inhibitors of chain processes. Each appears to be molecular. Reaction ( a ) is of the first order in the range 3 to 650 mm, the first-order rate constant being given by k CO 2 = 10 4⋅8 exp (–30600/ RT )s -1 . It is suggested tentatively that the abnormal kinetic parameters might be explained by regarding the reaction as a decarboxylation of (H + ) (HCOO¯) ion pairs present in minute concentration. Reaction ( b ) shows a pressure dependence most simply explained by a superposition of a predominant second-order component with a small first-order component. The most satisfactory interpretation of the second-order reaction is that it represents the unimolecular decomposition of dimer molecules, known to be present in formic-acid vapour. On this basis the rate constant is given by k CO dimer = 10 13⋅58 exp (–42600/ RT )s -1 , the kinetic parameters thus being in the normal range. The various alternative interpretations are discussed.

On the Interaction Between Human Plasma Kallikrein and C1-Esterase Inhibitor

1983 ◽  
Vol 49 (03) ◽  
pp. 193-195 ◽  
Author(s):  
Torbjörn Nilsson
Keyword(s):  
Dissociation Constant ◽  
Human Plasma ◽  
Rate Constant ◽  
Enzyme Inhibitor ◽  
Order Rate Constant ◽  
Second Order ◽  
Plasma Kallikrein ◽  
First Order ◽  
Order Rate ◽  
Esterase Inhibitor

SummaryThe kinetics of the reaction between human plasma kallikrein and CĪ-esterase inhibitor was studied in a purified system. By monitoring the inhibition reaction for extended periods of time, it was found to proceed in two consecutive steps, a fast reversible second-order binding step followed by a slower, irreversible first-order transition. The rate constants in this reaction model were determined, as well as the dissociation constant of the initial, reversible enzyme-inhibitor complex. Thus, at 37° C the second-order rate constant was found to be 5 · 104 M -1 · s-1, the first order rate constant was 5 · 10-4 s-1 and the dissociation constant K was 1.5 · 10-8 M. Heparin (28 U/ml) and 6-aminohexanoic acid (10 mM) had no effect on the k1 of the of the reaction.

Kinetics of Reactions of 8-Quinolinol and Acetate with Hydroxyaluminum Species in Aqueous Solutions. 1. Polynuclear Hydroxyaluminum Cations

1971 ◽  
Vol 49 (10) ◽  
pp. 1683-1687 ◽  
Author(s):  
R. C. Turner ◽  
Wan Sulaiman
Keyword(s):  
Acetic Acid ◽  
Rate Constant ◽  
Empirical Equation ◽  
Decomposition Reaction ◽  
Order Rate Constant ◽  
Acetate Concentration ◽  
First Order ◽  
Order Rate ◽  
Pseudo First Order ◽  
Kinetics Of

The effect of varying 8-quinolinol and acetate concentration on the rate of decomposition of poly-nuclear hydroxyaluminum cations was studied. It was found that the concentration of the undissociated 8-quinolinol and acetic acid molecules determined the magnitude of the first order rate constant for the decomposition of the polynuclear hydroxyaluminum cations, except when the acetate concentrations were relatively high. With high acetate concentrations, it appeared that polynuclear acetate species were involved in the reactions. An empirical equation was developed showing the effect of 8-quinolinol and acetic acid molecule concentrations on the pseudo first order rate constant for the decomposition reaction.

Reversible ring opening in the hydrolysis of spiro ortho esters

1985 ◽  
Vol 63 (10) ◽  
pp. 2673-2678 ◽  
Author(s):  
Robert A. McClelland ◽  
Claude Moreau
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Ring Opening ◽  
Lone Pair ◽  
Order Rate Constant ◽  
Product Analysis ◽  
Ortho Ester ◽  
First Order ◽  
Order Rate ◽  
Ortho Esters

Hydrolysis kinetics are reported for four spiro ortho esters: 3,4-dihydro-6-methoxy-1H-2-benzopyran-1-spiro-2′-1′,3′-dioxolane (13), its 1′,3′-dioxane analog (14), and the 6-unsubstituted versions of each (11 and 12). For comparison, also included are the diethoxy analogs: 1,1-diethoxy-3,4-dihydro-6-methoxy-1H-2-benzopyran (10) and the 6-unsubstituted compound (9). Product analysis implicates an initial opening of the dioxolane or dioxane ring in the spiro ortho esters, as expected on the basis of stereoelectronic considerations. The intermediate dialkoxycarbocations can be observed in HCl solutions. A detailed analysis has been carried out for the 6-methoxy systems to provide the rate constants k1, the second-order rate constant for H+-catalyzed formation of the cation from the ortho ester, k2, the first-order rate constant for water addition to the cation, and k−1, the first-order rate constant for ring closing of the cation to reform the ortho ester. The two spiro ortho esters are shown in this analysis to undergo reversible ring opening in their hydrolysis, in that values of k−1, are greater than k2. The differences, however, are not large, k−1/k2 being 1.2 (dioxolane, 13) and 3.8 (dioxane, 14). Comparison with the diethoxy ortho ester also reveals that the ring opening process (k1, rate constants) is inherently more difficult with the dioxolane, although not with the dioxane. An argument involving lone pair orientation is advanced to explain this.

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