Acid and Base Catalysis in the Oxidation of 6,7,8-Trimethyllumazine. II. Kinetics and Mechanism

1974 ◽  
Vol 52 (23) ◽  
pp. 3884-3894 ◽  
Author(s):  
Ross Stewart ◽  
Kiyotaka Oyama
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Base Catalysis ◽  
Methyl Group ◽  
General Base ◽  
Kinetic Isotope ◽  
Conjugate Acid ◽  
Acid And Base ◽  
Rate Plot ◽  
Kinetics Of

The kinetics of the oxidation of 6,7,8-trimethyllumazine, 1, by permanganate in aqueous solution is examined. The pH–rate plot shows a minimum at pH 3.5 and there is catalysis by buffers. Above pH 3.5 the 7-keto compound, 5, is produced by a general base catalyzed route the rate of which is identical to that of the reaction that causes exchange at the 7-methyl group. The kinetic isotope effect for oxidation of the 7-CD3 compound is 6.89 at 31.4°.Below pH 3.5 scission of the pyrazine ring occurs, there is a small isotope effect, and the rate is somewhat faster than that observed for exchange of the 7-methyl group. There appear to be three routes leading to product; one, a permanganate-independent reaction catalyzed by general acids, a second which is also permanganate independent and involves hydration of the substrate, and a third which is permanganate dependent and involves attack by permanganate on the conjugate acid of the substrate.The utility of oxidation as a means of locating hydration sites in heterocyclic compounds is discussed and some comments are made on the mechanism of the enzymic oxidation of the ribityl analog of 1.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

RSC Advances ◽  
2016 ◽  
Vol 6 (42) ◽  
pp. 36096-36108 ◽  
Author(s):  
Feng-Yang Bai ◽  
Xu Wang ◽  
Yan-Qiu Sun ◽  
Rong-Shun Wang ◽  
Xiu-Mei Pan
Keyword(s):  
Temperature Dependence ◽  
Isotope Effect ◽  
Atmospheric Chemistry ◽  
Theoretical Basis ◽  
Kinetic Isotope Effect ◽  
Kinetic Isotope ◽  
Kinetics Of ◽  
Sound Theoretical Basis

Mechanisms and kinetics of the reaction of CF3CX2CX2CX2OX with OX (X= H, D) radical are investigated on a sound theoretical basis.

Aspects of the hydrolysis of formamide: revisitation of the water reaction and determination of the solvent deuterium kinetic isotope effect in base

2002 ◽  
Vol 80 (10) ◽  
pp. 1343-1350 ◽  
Author(s):  
H Slebocka-Tilk ◽  
F Sauriol ◽  
Martine Monette ◽  
R S Brown
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Base Hydrolysis ◽  
Kinetic Isotope ◽  
Acid And Base ◽  
Acid Reaction ◽  
Ph Range ◽  
Hydrolysis Of

A study of the hydrolysis of formamide is reported with the aims of isolating the water reaction for hydrolysis from the acid and base hydrolysis terms and determining the solvent deuterium kinetic isotope effect (dkie) on base-catalyzed hydrolysis. Respective activation parameters (ΔH‡ and ΔS‡) of (17.0 ± 0.4) kcal mol–1 and (–18.8 ± 1.3) cal mol–1 K–1 for the acid reaction and (17.9 ± 0.2) kcal mol–1 and (–11.1 ± 0.5) cal mol–1 K–1 for the base reaction were determined from Eyring plots of the second-order rate constants over the range of 27–120°C. Kinetic studies at the minima of the pH/rate profiles in the pH range from 5.6 to 6.2 in MES buffers at 56°C, and in the pH range of 4.25–6.87 in acetate and phosphate buffers at 120°C are reported. At 56°C the available data fit the expression k56obs = 0.00303[H3O+] + 0.032[HO–] + (3.6 ± 0.1) × 10–9, while at 120°C the data fit k120obs = (0.15 ± 0.02)[H3O+] + (3.20 ± 0.24)[HO–] + (1.09 ± 0.29) × 10–6. Preliminary experimental estimates of Ea (ln A) of 22.5 kcal mol–1 (15.03) for the water rate constant (kw) are calculated from an Arrhenius plot of the 56 and 120°C data giving an estimated kw of 1.1 × 10–10 s–1 (t1/2 = 199 years) at 25°C. Solvent dkie values of kOH/kOD = 1.15 and 0.77 ± 0.06 were determined at [OL–] = 0.075 and 1.47 M, respectively. The inverse value is determined under conditions where the the first step of the reaction dominates and is analyzed in terms of a rate-limiting attack of OL–.Key words: formamide, activation parameters, water reaction, acid and base hydrolysis, solvent kinetic isotope effect.

Allosteric control of pyrimidine transport in Hymenolepis diminuta: an unusual kinetic isotope effect

Parasitology ◽  
1983 ◽  
Vol 87 (2) ◽  
pp. 289-293 ◽  
Author(s):  
G. L. Uglem ◽  
R. K. Dupre ◽  
J. P. Harley
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Pyrimidine Ring ◽  
Hymenolepis Diminuta ◽  
Absorption Kinetics ◽  
Allosteric Control ◽  
Kinetic Isotope ◽  
Sigmoidal Kinetics ◽  
Kinetics Of ◽  
Initial Uptake

SUMMARYA thymine–uracil transport system is present in the plasma membrane brush-border of the tapeworm, Hymenolepis diminuta. The relation between initial uptake and substrate concentration of either thymine or uracil was sigmoidal when 2-14C-labelled substrates were used. In contrast, absorption kinetics of methy 1-14C- and 3H-labelled substrates were hyperbolic. Since there was no metabolism of the labelled substrates during the incubation period, these differences indicate that the sigmoidal kinetics of pyrimidine transport in this organism is an isotope effect associated with the presence of the 14C label at the 2 position in the pyrimidine ring.

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