The base-catalysed anomerization of dinitrophenyl glycosides: evidence for a novel reaction mechanism

1990 ◽  
Vol 68 (10) ◽  
pp. 1859-1866 ◽  
Author(s):  
Leise A. Berven ◽  
David Dolphin ◽  
Stephen G. Withers
Keyword(s):  
Reaction Mechanism ◽  
Reaction Rate ◽  
Kinetic Isotope Effect ◽  
Substituent Effects ◽  
Proton Exchange ◽  
Nucleophilic Aromatic Substitution ◽  
Aromatic Substitution ◽  
Proton Abstraction ◽  
Sugar Moiety ◽  
Kinetic Isotope

The mechanism of base-catalysed anomerization of per-O-acetylated 2,4-dinitrophenyl-β-D-glucopyranoside in dimethylsulfoxide has been investigated using a variety of techniques. A mechanism involving proton abstraction at C-1 was eliminated by the absence of proton exchange at that center and the measurement of a secondary deuterium kinetic isotope effect for the 1-deuterio substrate. A mechanism involving phenolate departure and recombination is rendered unlikely on the basis of remote substituent effects on the reaction rate and by the absence of any exchange of the phenyl moiety with added phenolate. A mechanism involving nucleophilic aromatic substitution initiated by an attack of the dimethylsulfinyl anion to generate a glucosyl oxyanion intermediate that anomerizes and recombines with the reactive aryl intermediate is consistent with the observations. This mechanism is further supported by the observation of a purple Meisenheimer complex intermediate and by the observed exchange between the substrate containing a labelled sugar moiety and added unlabelled 2,3,4,6-tetra-O-acetyl-β-D-glucopyranose. Keywords: glycoside, anomerization, reaction mechanism.

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.

scholarly journals Kinetic isotope effects for fast deuterium and proton exchange rates

2016 ◽  
Vol 18 (15) ◽  
pp. 10144-10151 ◽  
Author(s):  
Estel Canet ◽  
Daniele Mammoli ◽  
Pavel Kadeřávek ◽  
Philippe Pelupessy ◽  
Geoffrey Bodenhausen
Keyword(s):  
Exchange Rates ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Proton Exchange ◽  
Kinetic Isotope ◽  
Spin Pairs

By monitoring the effect of deuterium decoupling on the decay of transverse 15N magnetization in D–15N spin pairs during multiple-refocusing echo sequences, we have determined fast D–D exchange rates kD and compared them with fast H–H exchange rates kH in tryptophan to determine the kinetic isotope effect as a function of pH and temperature.

Reactions of Coordinated Ligands. The Bromination of 2,4-dimethyl-3H-imidazole and 3H-imidazole Coordinated (at N1) to Pentaamminecobalt(III)

1991 ◽  
Vol 44 (7) ◽  
pp. 981 ◽  
Author(s):  
AG Blackman ◽  
DA Buckingham ◽  
CR Clark
Keyword(s):  
Aqueous Solution ◽  
Kinetic Isotope Effect ◽  
Ph Dependence ◽  
Acidic Aqueous Solution ◽  
Proton Abstraction ◽  
Kinetic Isotope ◽  
Diffusion Controlled ◽  
Base Form ◽  
Ph Range ◽  
Conjugate Base

The pH dependence of the bromination of R-2,4-Me2ImH3+ [R=(NH3)5Co] by Br2 in acidic aqueous solution, giving R-2,4-Me2-5-BrImH3+, suggests that proton abstraction from a bromine-substituted Wheland intermediate is rate determining for pH < 3 (both OH-- catalysed and spontaneous paths are observed), while in the pH range 3-5, bromine addition is rate determining. For pH > 5 the reaction may be interpreted to occur through rate-determining Br2 addition to the conjugate base from of the reactant. Rate constants for bromine addition to R-2,4-Me2ImH3+ and R-2,4-Me2Im2+ are respectively 1.1×104 and 3.4×1010 dm3 mol-1 s-1 at 25.0°C, I = 1.0 (NaClO4). Bromination of RimH3+, to give R-4-BrImH3+ initially, appears to occur largely via the conjugate base form of the reactant (k2 = 3.6×109 dm3 mol-1 s-1), but for Ph < 2, addition of Br2 to RImH3+ contributes to the observed rate (k1 = 0.68 dm3 mol-1 s-1). Ea = 61�2 kJ mol-1 for the former process, and the primary kinetic isotope effect, kH/kD, increases from 1.3 in 0.05 M H+ to 2.4 in 5.42 M HClO4. These observations are discussed in terms of diffusion-controlled, or preassociation -type mechanisms.

Carbon-13 kinetic isotope effects. V. Substituent effects on k12/k13 for alcoholysis of 1-phenyl-1-bromoethane

1969 ◽  
Vol 47 (13) ◽  
pp. 2506-2509 ◽  
Author(s):  
Jan Bron ◽  
J. B. Stothers
Keyword(s):  
Isotope Effect ◽  
Phenyl Ring ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope ◽  
Factor Governing

As a test of our earlier interpretations of the 13C kinetic isotope effects found for alcoholysis of 1-phenyl-1-bromoethane, we have examined the effect of the p-methyl and p-bromo substituents on the 13C fractionations in ethanol and methanol. Isotopic fractionation at the α-carbon is found to be substituent dependent, and the observed trend is consistent with the proposal that stabilization of the cationic center by the phenyl ring is a major factor governing the isotope effect in these systems. The first example of an inverse primary kinetic isotope effect for carbon (k12/k13 < 1) is described.

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