Identity of the rate-determining step in the gas-phase thermolysis of diborane: a reinvestigation of the deuterium kinetic isotope effect

1989 ◽  
Vol 111 (23) ◽  
pp. 8721-8722 ◽  
Author(s):  
Robert Greatrex ◽  
Norman N. Greenwood ◽  
Susan M. Lucas
Keyword(s):  
Gas Phase ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Rate Determining Step ◽  
Kinetic Isotope

scholarly journals l-mandelate dehydrogenase from Rhodotorula graminis: comparisons with the l-lactate dehydrogenase (flavocytochrome b2) from Saccharomyces cerevisiae

1993 ◽  
Vol 290 (1) ◽  
pp. 103-107 ◽  
Author(s):  
O Smékal ◽  
M Yasin ◽  
C A Fewson ◽  
G A Reid ◽  
S K Chapman
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Lactate Dehydrogenase ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Dimensional Structure ◽  
Striking Difference ◽  
Kinetic Properties ◽  
Proton Abstraction ◽  
Rate Determining Step ◽  
Kinetic Isotope

L-Lactate dehydrogenase (L-LDH) from Saccharomyces cerevisiae and L-mandelate dehydrogenase (L-MDH) from Rhodotorula graminis are both flavocytochromes b2. The kinetic properties of these enzymes have been compared using steady-state kinetic methods. The most striking difference between the two enzymes is found by comparing their substrate specificities. L-LDH and L-MDH have mutually exclusive primary substrates, i.e. the substrate for one enzyme is a potent competitive inhibitor for the other. Molecular-modelling studies on the known three-dimensional structure of S. cerevisiae L-LDH suggest that this enzyme is unable to catalyse the oxidation of L-mandelate because productive binding is impeded by steric interference, particularly between the side chain of Leu-230 and the phenyl ring of mandelate. Another major difference between L-LDH and L-MDH lies in the rate-determining step. For S. cerevisiae L-LDH, the major rate-determining step is proton abstraction at C-2 of lactate, as previously shown by the 2H kinetic-isotope effect. However, in R. graminis L-MDH the kinetic-isotope effect seen with DL-[2-2H]mandelate is only 1.1 +/- 0.1, clearly showing that proton abstraction at C-2 of mandelate is not rate-limiting. The fact that the rate-determining step is different indicates that the transition states in each of these enzymes must also be different.

Efficient oxidation of ethers with pyridine N-oxide catalyzed by ruthenium porphyrins

2015 ◽  
Vol 19 (01-03) ◽  
pp. 411-416 ◽  
Author(s):  
Nobuki Kato ◽  
Yu Hamaguchi ◽  
Naoki Umezawa ◽  
Tsunehiko Higuchi
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Hydrogen Abstraction ◽  
Oxide System ◽  
Cyclic Ethers ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Oxidized Products

We found that oxidation of cyclic ethers with the Ru porphyrin-heteroaromatic N-oxide system gave lactones or/and ring-opened oxidized products with regioselectivity. A relatively high kinetic isotope effect was observed in the ether oxidation, suggesting that the rate-determining step is the first hydrogen abstraction.

Kinetic isotope effect in gas-phase base-induced elimination reactions

1985 ◽  
Vol 107 (9) ◽  
pp. 2818-2820 ◽  
Author(s):  
Veronica M. Bierbaum ◽  
Jonathan Filley ◽  
Charles H. DePuy ◽  
Martin F. Jarrold ◽  
Michael T. Bowers
Keyword(s):  
Gas Phase ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Kinetic Isotope ◽  
Elimination Reactions

Primary kinetic isotope effect in the gas phase photochlorination of ethyl chloride-1-d1

1984 ◽  
Vol 62 (5) ◽  
pp. 899-906 ◽  
Author(s):  
Jan Niedzielski ◽  
T. Yano ◽  
E. Tschuikow-Roux
Keyword(s):  
Activation Energy ◽  
Gas Phase ◽  
Isotope Effect ◽  
Ground State ◽  
Rate Constants ◽  
Kinetic Isotope Effect ◽  
Kinetic Isotope ◽  
Primary Reference ◽  
Hydrogen Deuterium ◽  
Increasing Temperature

The abstraction of hydrogen/deuterium from CH3CHDCl by ground state chlorine atoms produced photolytically from Cl2 has been investigated at temperatures betwen 280 and 368 K. The relative rates for the internal competition[Formula: see text]are found to conform to an Arrhenius rate law:[Formula: see text]These data, taken together with the external competition results for the C2H5Cl/CH3CHDCl system, in conjunction with the competitive results using CH4 as a primary reference, have yielded the rate constants (cm3 s−1):[Formula: see text]The relatively weak primary kinetic isotope effect, kH/kD, decreases with increasing temperature from 1,855 at 280 K to 1.66 at 365 K. The results are compared with those obtained based on the BEBO method. While both the trend and the magnitude of the kinetic isotope effect are satisfactorily predicted, the activation energy is not.

Intramolecular kinetic isotope effect in gas-phase proton-transfer reactions

1979 ◽  
Vol 101 (8) ◽  
pp. 2242-2243 ◽  
Author(s):  
Keith M. Wellman ◽  
Maria E. Victoriano ◽  
Paulo C. Isolani ◽  
Jose M. Riveros
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Transfer Reactions ◽  
Kinetic Isotope ◽  
Proton Transfer Reactions

ORTHO PARTICIPATION IN THE CONVERSION OF syn-BENZALDOXIME ESTERS TO NITRILES

1965 ◽  
Vol 43 (12) ◽  
pp. 3178-3187 ◽  
Author(s):  
Robert J. Crawford ◽  
Charles Woo
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Alcoholic Solution ◽  
First Order ◽  
Rate Determining Step ◽  
First Order Kinetics ◽  
Kinetic Isotope ◽  
Marked Enhancement

Substituted syn-benzaldoxime esters are transformed, in an alcoholic solution, to the corresponding nitriles according to first-order kinetics. All ortho substituents were observed to accelerate the rate of nitrile formation relative to the corresponding para derivative. While the ko/kp ratios for the bromo, chloro, fluoro, methoxy, and methyl substituents fall within the range of 2 to 9, the iodo and methylthio substituents are 119 and 11 000 respectively. Isotopic replacement of the aldoximino hydrogen by deuterium gives rise to a kinetic isotope effect, kH/kD being 5.21 for syn-o-chlorobenzaldoxime p-toluenesulfonate, 1.22 for syn-o-iodobenzaldoxime p-toluenesulfonate, and 1.23 for syn-o-methylthiobenzaldoxime o-iodobenzoate. The marked enhancement of rate and the absence of an appreciable isotope effect are considered to be associated with sulfur and iodine participation in the rate-determining step. A mechanism which is capable of explaining the results observed is suggested.

The mechanism of the microbial hydroxylation of steroids. Part 4. The C-6 β hydroxylation of androst-4-ene-3,17-dione and related compounds by Rhizopusarrhizus ATCC 11145

1978 ◽  
Vol 56 (5) ◽  
pp. 694-702 ◽  
Author(s):  
Herbert L. Holland ◽  
Peter R. P. Diakow
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Enol Form ◽  
Enol Ethers ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Microbial Hydroxylation ◽  
Related Compounds

The steroid analogue 4,4a,5,6,7,8-hexahydro-2(3H)-naphthalenone was hydroxylated at C-8α, C-8β, and C-4a by Rhizopusarrhizus. Similar products were obtained by peracid oxidation of the corresponding enol ethers: hydroxylation of estr-4-ene-3,17-dione by the same fungus occurred at the analogous C-6 and C-10 positions. These results are consistent with a mechanism of microbial hydroxylation involving the enol form of the Δ4-3-ketone. Data from the incubations with R. arrhizus of androst-4-ene-3,17-dione specifically labelled with deuterium at C-4, C-6α, or C-6β and from those of other deuterium labelled substrates have been interpreted in terms of a mechanism of C-β hydroxylation involving a rate-determining step before enolization of the ketone, followed by rapid enolization and oxidation of the enol to give the 6β-hydroxy-Δ4-3-ketone. The kinetic isotope effect, kH/kD, for the hydroxylation of androst-4-ene-3,17-dione at C-6β has been found to be 1.2 ± 0.1.

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