A KINETIC ISOTOPE EFFECT STUDY OF THE TSCHUGAEFF REACTION

1961 ◽  
Vol 39 (2) ◽  
pp. 348-358 ◽  
Author(s):  
Richard F. W. Bader ◽  
A. N. Bourns
Keyword(s):  
Isotope Effects ◽  
Strong Support ◽  
Effect Study ◽  
Sulphur Atom ◽  
Carbonyl Carbon ◽  
Cyclic Transition ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Cyclic Transition State ◽  
Step Mechanism

Three different kinetic isotope effects have been measured in the thermal decomposition of S-methyl-trans-2-methyl-1-indanyl xanthate: the S32/S34 effect for the thio-ether sulphur atom, the S32/S34 effect for the thion sulphur, and the C12/C13 effect for the carbonyl carbon. The results of these measurements provide strong support for a two-step mechanism, in which the rate-determining step is the removal through a cyclic transition state of the cis beta hydrogen atom by the thion sulphur atom.

scholarly journals A kinetic-isotope-effect study of catalysis by Vibrio cholerae neuraminidase

1993 ◽  
Vol 294 (3) ◽  
pp. 653-656 ◽  
Author(s):  
X Guo ◽  
M L Sinnott
Keyword(s):  
Vibrio Cholerae ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Effect Study ◽  
K Value ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Leaving Group ◽  
Optimum Ph

Michaelis-Menten parameters for hydrolysis of seven aryl N-acetyl alpha-D-neuraminides by Vibrio cholerae neuraminidase at pH 5.0 correlate well with the leaving-group pKa (delta pK 3.0; beta 1g (V/K) = -0.73, r = -0.93; beta 1g (V) = -0.25; r = -0.95). The beta-deuterium kinetic-isotope effect, beta D2(V), for the p-nitrophenyl glycoside is the same at the optimum pH of 5.0 (1.059 +/- 0.010) as at pH 8.0 (1.053 +/- 0.010), suggesting that isotope effects are fully expressed with this substrate at the optimum pH. For this substrate at pH 5.0, leaving group 18O effects are 18(V) = 1.040 +/- 0.016 and 18(V/K) = 1.046 +/- 0.015, and individual secondary deuterium effects are beta proRD(V) = 1.037 +/- 0.014, beta proSD(V) = 1.018 +/- 0.015, beta proRD(V/K) = 1.030 +/- 0.017, beta proSD(V/K) = 1.030 +/- 0.017. All isotope effects, and the beta 1g(V/K) value are in accord with the first chemical step being both the first irreversible and the rate-determining step in enzyme turnover, with a transition state in which there is little proton donation to the leaving group, the C-O bond is largely cleaved, there is significant nucleophilic participation, and the sugar ring is in a conformation derived from the ground-state 2C5 chair. The apparent conflict between the beta 1g (V) value of -0.25 with all the kinetic-isotope-effect data can be resolved by the postulation of an interaction between the pi system of the aglycone ring and an anionic or nucleophilic group on the enzyme.

Reaction orders and isotope effects in the reversible addition of water to 1,3-dichloroacetone in aqueous dioxan

1971 ◽  
Vol 325 (1560) ◽  
pp. 35-55 ◽  
Keyword(s):  
Isotope Effects ◽  
Initial State ◽  
Cyclic Transition ◽  
Kinetic Isotope ◽  
Reversible Addition ◽  
Cyclic Transition State ◽  
Reaction Orders ◽  
Proton Movement ◽  
Kinetics Of ◽  
Uncatalysed Reaction

The kinetics of the reversible addition of water to 1,3-dichloroacetone have been studied spectrophotometrically in aqueous dioxan with and without the addition of catalysts (hydrogen chloride, perchloric acid, four carboxylic acids and 2,6-dichlorophenol). Reaction orders with respect to water were determined, and also kinetic isotope effects in systems containing 50 and 100 atom % of deuterium. It is concluded that the uncatalysed reaction involves a cyclic transition state containing two extra water molecules, one of which may be replaced by a molecule of catalyst. The dependence of the kinetic orders and isotope effects upon the nature of the catalyst is attributed mainly to differences in the hydration or other properties of the catalyst in the initial state. It is believed that proton movement is partly synchronous with the making or breaking of the carbon-oxygen bonds, but there is no evidence that more than one proton moves simultaneously.

Vinyl ether hydrolysis. XIII. The failure of I-strain to produce a change in rate-determining step

1980 ◽  
Vol 58 (2) ◽  
pp. 124-129 ◽  
Author(s):  
Y. Chiang ◽  
W. K. Chwang ◽  
A. J. Kresge ◽  
S. Szilagyi
Keyword(s):  
Vinyl Ether ◽  
Acid Catalysis ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Mineral Acid ◽  
Buffer Solutions ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Acetic Acid Buffer ◽  
Hydrolysis Of

Rates of hydrolysis of 1-ethoxy-3,3,5,5-tetramethylcyclopentene and 1-methoxy-2,3,3,5,5-pentamethylcyclopentene measured in mineral acid and formic and acetic acid buffer solutions show general acid catalysis and give large kinetic isotope effects in the normal direction (kH/kD > 1). This indicates that these reactions proceed by the conventional mechanism for vinyl ether hydrolysis in which proton transfer from the catalyzing acid to the substrate is rate-determining, and that the I-strain in these substrates is insufficiently great to shift the reaction mechanism to rapidly reversible substrate protonation followed by rate-determining hydration of the ensuing cationic intermediate.

scholarly journals Kinetics and Correlation Analysis of Reactivity in the Oxidation of Some α-Hydroxy Acids by Benzimidazolium Dichromate

2018 ◽  
Vol 43 (3-4) ◽  
pp. 300-314
Author(s):  
Dinesh Panday ◽  
Teena Kachawa ◽  
Seema Kothari
Keyword(s):  
Solvent Effect ◽  
Mandelic Acid ◽  
Bond Cleavage ◽  
Hydroxy Acids ◽  
Cyclic Transition ◽  
Hydride Ion ◽  
Rate Determining Step ◽  
Reaction Constant ◽  
Cyclic Transition State ◽  
Hydride Ion Transfer

Kinetic and mechanistic studies of the oxidation of mandelic acid and nine monosubstituted mandelic acids by benzimidazolium dichromate (BIDC) in dimethyl sulfoxide are discussed with an emphasis on correlation of structure and reactivity. The reactions were of first order with respect to BIDC. However, Michaelis-Menten type kinetics were observed with respect to hydroxy acids. The reactions are catalysed by protons. The deuterium isotope effect for the oxidation of mandelic acid ( kH/ kD = 5.91 at 298 K) indicated an α-C-H bond cleavage in the rate-determining step. An analysis of the solvent effect showed that the role of cationsolvation is major. The reaction showed an excellent correlation with the Hammett σ values, the reaction constant being negative. Based on the kinetic data, analysis of the solvent effect and results of structure-reactivity correlation along with some non-kinetic parameters, a mechanism involving rate-determining oxidative decomposition of the complex through hydride-ion transfer via a cyclic transition state to give the corresponding oxoacid is suggested.

scholarly journals Tyr-143 facilitates interdomain electron transfer in flavocytochrome b2

1992 ◽  
Vol 285 (1) ◽  
pp. 187-192 ◽  
Author(s):  
C S Miles ◽  
N Rouvière-Fourmy ◽  
F Lederer ◽  
F S Mathews ◽  
G A Reid ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Isotope Effects ◽  
Mutant Enzyme ◽  
Stopped Flow ◽  
Wild Type ◽  
Wild Type Enzyme ◽  
Flavocytochrome B2 ◽  
Rate Determining Step ◽  
Kinetic Isotope

The role of Tyr-143 in the catalytic cycle of flavocytochrome b2 (L-lactate:cytochrome c oxidoreductase) has been examined by replacement of this residue with phenylalanine. The electron-transfer steps in wild-type and mutant flavocytochromes b2 have been investigated by using steady-state and stopped-flow kinetic methods. The most significant effect of the Tyr-143----Phe mutation is a change in the rate-determining step in the reduction of the enzyme. For wild-type enzyme the main rate-determining step is proton abstraction at the C-2 position of lactate, as shown by the 2H kinetic-isotope effect. However, for the mutant enzyme it is clear that the slowest step is interdomain electron transfer between the FMN and haem prosthetic groups. In fact, the rate of haem reduction by lactate, as determined by the stopped-flow method, is decreased by more than 20-fold, from 445 +/- 50 s-1 (25 degrees C, pH 7.5) in the wild-type enzyme to 21 +/- 2 s-1 in the mutant enzyme. Decreases in kinetic-isotope effects seen with [2-2H]lactate for mutant enzyme compared with wild-type, both for flavin reduction (from 8.1 +/- 1.4 to 4.3 +/- 0.8) and for haem reduction (from 6.3 +/- 1.2 to 1.6 +/- 0.5) also provide support for a change in the nature of the rate-determining step. Other kinetic parameters determined by stopped-flow methods and with two external electron acceptors (cytochrome c and ferricyanide) under steady-state conditions are all consistent with this mutation having a dramatic effect on interdomain electron transfer. We conclude that Tyr-143, an active-site residue which lies between the flavodehydrogenase and cytochrome domains of flavocytochrome b2, plays a key role in facilitating electron transfer between FMN and haem groups.

Elimination of Molecular Hydrogen and Methane from Collision-Activated Alkoxide Negative Ions in the gas Phase. An ab initio and Isotope Effect Study

1985 ◽  
Vol 38 (8) ◽  
pp. 1197 ◽  
Author(s):  
RN Hayes ◽  
JC Sheldon ◽  
JH Bowie ◽  
DE Lewis
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Gas Phase ◽  
Molecular Hydrogen ◽  
Isotope Effects ◽  
Negative Ions ◽  
Kinetic Isotope Effects ◽  
Effect Study ◽  
Negative Ion ◽  
Kinetic Isotope

Ab initio calculations indicate that the collisional induced losses of molecular hydrogen from the ethoxide negative ion and methane from the t- butoxide negative ion to be stepwise processes in which the key intermediates are [H-… MeCHO ] and [Me-…Me2CO] respectively. Deuterium kinetic isotope effects observed for these and other alkoxide negative ions are in accord with the operation of a stepwise reaction.

scholarly journals KINETIC HYDROGEN ISOTOPE EFFECTS IN AROMATIC BROMODEPROTONATION

1966 ◽  
Vol 44 (3) ◽  
pp. 379-386 ◽  
Author(s):  
B. T. Baliga ◽  
A. N. Bourns
Keyword(s):  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Salt Effect ◽  
Isotope Effects ◽  
Effect Study ◽  
Ion Concentration ◽  
Bromide Ion ◽  
Halogenation Reaction ◽  
Hydrogen Isotope Effect ◽  
Step Mechanism

The bromodeprotonation of sodium p-methoxybenzenesulfonate is an ordinary halogenation reaction in which a hydrogen ortho to the methoxy group is replaced by bromine. The kinetic data for this reaction do not distinguish between a one- and a two-step mechanism with Br2 as the brominating species. The two-step mechanism was confirmed by the observation of a variation in the kinetic hydrogen isotope effect, kH/kD, with changing bromide ion concentration. The observed isotope effects are 1.01, 1.07, 1.18, and 1.31 at bromide ion concentrations 0, 0.5, 1, and 2 M, respectively.The isotope effect study strongly establishes that in this reaction the small depression in rate produced by bromide ion, beyond that due to Br3− formation, arises mainly from a salt effect, and only a small amount of this depression is caused by return of the intermediate to reactants.

Mechanistic and kinetic implications on the ORR on a Au(100) electrode: pH, temperature and H–D kinetic isotope effects

2014 ◽  
Vol 16 (27) ◽  
pp. 13762-13773 ◽  
Author(s):  
Dong Mei ◽  
Zheng Da He ◽  
Yong Li Zheng ◽  
Dao Chuan Jiang ◽  
Yan-Xia Chen
Keyword(s):  
Alkaline Solution ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Rate Determining Step ◽  
Kinetic Isotope

The potential determining reaction and the rate-determining step for the ORR on Au(100) in acidic and alkaline solution have been clarified.

scholarly journals Mechanism and Origin of Remote Stereocontrol in the Organocatalytic C(sp2)-H Alkylation using Nitroalkanes as Alkylating Agents

2021 ◽  
Author(s):  
Sharath Chandra Mallojjala ◽  
Rahul Sakar ◽  
Rachael W. Karugu ◽  
Madhu Sudan Manna ◽  
Santanu Mukherjee ◽  
...  
Keyword(s):  
Alkylating Agent ◽  
Isotope Effects ◽  
Alkylating Agents ◽  
Bond Formation ◽  
Strong Support ◽  
Kinetic Isotope Effects ◽  
Energy Profile ◽  
Bond Forming ◽  
Kinetic Isotope ◽  
Carbon Carbon Bond

ABSTRACT: Experimental 13C kinetic isotope effects (KIEs) and DFT calculations are used to evaluate the mecha-nism and the origin of enantioselectivity in the C(sp2)‒H alkylative desymmetrization of cyclopentene-1,3-diones using nitroalkanes as the alkylating agent. An unusual combination of an inverse (~0.980) and a normal (~1.030) KIE is observed on the bond-forming carbon atoms of the cyclopentene-1,3-dione and nitroalkane, respectively. These data provide strong support for a mechanism involving reversible carbon-carbon bond-formation followed by rate- and enantioselectivity-determining nitro-group elimination. The theoretical free energy profile and predicted KIEs indicate that this elimination event occurs via an E1cB pathway. The origin of remote stereocontrol is evaluated by distortion-interaction and SAPT0 analyses of the enantiomeric E1cB transition states.

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