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.

scholarly journals Kinetics and Mechanism of the Decarboxylation of Pyrrole-2-carboxylic Acid in Aqueous Solution

1971 ◽  
Vol 49 (7) ◽  
pp. 1032-1035 ◽  
Author(s):  
G. E. Dunn ◽  
Gordon K. J . Lee
Keyword(s):  
Aqueous Solution ◽  
Ionic Strength ◽  
Carboxylic Acid ◽  
Isotope Effect ◽  
Anthranilic Acid ◽  
Kinetic Isotope Effect ◽  
Pyrrole Ring ◽  
First Order ◽  
Kinetic Isotope ◽  
Ph Range

The decarboxylation of pyrrole-2-carboxylic acid in aqueous buffers at 50° and ionic strength 1.0 has been found to be first order with respect to substrate at a fixed pH. As the pH is decreased, the rate constant increases slightly in the pH range 3–1, then rises rapidly from pH 1 to 10 M HCl. The 13C-carboxyl kinetic isotope effect is 2.8% in 4 M HClO4 and negligible at pH ~ 3. These observations can be accounted for by a mechanism, previously proposed for the decarboxylation of anthranilic acid, in which the species undergoing decarboxylation is the carboxylate ion protonated at the 2-position of the pyrrole ring. This intermediate can be formed both by ring-protonation of the carboxylate anion and by ionization of the ring-protonated acid. At low acidities ring-protonation is rate determining, but at higher acidities the rate of protonation exceeds that of decarboxylation.

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.

Rate–acidity profiles for exchange of the 4-methyl protons in amino, imino, and keto pyrimidines

1979 ◽  
Vol 57 (20) ◽  
pp. 2783-2789 ◽  
Author(s):  
Ross Stewart ◽  
Stewart J. Gumbley ◽  
R. Srinivasan
Keyword(s):  
Aqueous Solution ◽  
Carbonyl Group ◽  
Principal Components ◽  
Kinetic Isotope Effect ◽  
Activation Parameters ◽  
Methyl Groups ◽  
Imino Group ◽  
Hydroxide Ion ◽  
Kinetic Isotope ◽  
Amino Pyrimidine

The rate of exchange of deuterium for protium in the 4-methyl groups of 2-imino-1,4-dimethyl-1,2-dihydropyrimidine (1), 1,4-dimethyl-2-pyrimidone (2), and 4-methyl-2-amino-pyrimidine (3) has been determined in aqueous solution over an acidity range of some 21 pH(H0) units. The various exchange routes involve attack by base (water, hydroxide ion, buffer anion) on substrate (neutral, singly protonated, doubly protonated) and the identities of the principal components across the acidity spectrum have been established for all three compounds. The Brønsted slope, kinetic isotope effect, and activation parameters for 1 have also been determined. Protonating 1 activates it toward exchange by a factor of 103; addition of a second proton has a further effect of >105. The activating effects of the imino group, the carbonyl group, and the protonated imino group in these compounds are in the ratio 10−1:1:102.

Ketonization equilibria of phenol in aqueous solution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 605-613 ◽  
Author(s):  
Marco Capponi ◽  
Ivo G Gut ◽  
Bruno Hellrung ◽  
Gaby Persy ◽  
Jakob Wirz
Keyword(s):  
Aqueous Solution ◽  
Proton Transfer ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Flash Photolysis ◽  
Equilibrium Constants ◽  
Thermodynamic Cycle ◽  
Acidity Constant ◽  
Kinetic Isotope ◽  
Carbon Acids

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

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.

scholarly journals Solvent kinetic isotope effects of human placental alkaline phosphatase in reverse micelles

1998 ◽  
Vol 330 (1) ◽  
pp. 267-275 ◽  
Author(s):  
Ter-Mei HUANG ◽  
Hui-Chih HUNG ◽  
Tsu-Chung CHANG ◽  
Gu-Gang CHANG
Keyword(s):  
Aqueous Solution ◽  
Isotope Effect ◽  
Reverse Micelles ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Solvent System ◽  
Placental Alkaline Phosphatase ◽  
K Value ◽  
Kinetic Isotope ◽  
Human Placental Alkaline Phosphatase

Human placental alkaline phosphatase was embedded in a reverse micellar system prepared by dissolving the surfactant sodium bis(2-ethylhexyl) sulphosuccinate (Aerosol-OT) in 2,2,4-trimethylpentane. This microemulsion system provides a convenient instrumental tool to study the possible kinetic properties of the membranous enzyme in an immobilized form. The pL (pH/p2H) dependence of hydrolysis of 4-nitrophenyl phosphate has been examined over a pL range of 8.5-12.5 in both aqueous and reverse micellar systems. Profiles of log V versus pL were Ha-bell shaped in the acidic region but reached a plateau in the basic region in which two pKa values of 9.01-9.71 and 9.86-10.48, respectively, were observed in reverse micelles. However, only one pKa value of 9.78-10.27 in aqueous solution was detected. Profiles of log V/K versus pL were bell-shaped in the acidic region. However, they were wave-shaped in the basic region in which a residue of pKa 9.10-9.44 in aqueous solution and 8.07-8.78 in reverse micelles must be dehydronated for the reaction to reach an optimum. The V/K value shifted to a lower value upon dehydronation of a pKa value of 9.80-10.62 in aqueous solution and 11.23-12.17 in reverse micelles. Solvent kinetic isotope effects were measured at three pL values. At pL 9.5, the observed isotope effect was a product of equilibrium isotope effect and a kinetic isotope effect; at pL 10.4, the log V/K value was identical in water and deuterium. The deuterium kinetic isotope effect on V/K was 1.14 in an aqueous solution and 1.16 in reverse micelles. At pL 11.0 at which the log V values reached a plateau in either solvent system, the deuterium kinetic isotope effect on V was 2.08 in an aqueous solution and 0.62 in reverse micelles. Results from a proton inventory experiment suggested that a hydron transfer step is involved in the transition state of the catalytic reaction. The isotopic fractionation factor (ϕ) for deuterium for the transition state (ϕT) increased when the pH of the solution was raised. At pL 11.0, the ϕT was 1.07 in reverse micelles, which corresponds to the inverse-isotope effect of the reaction in this solvent system. Normal viscosity effects on kcat and kcat/Km were observed in aqueous solution, corresponding to a diffusional controlled physical step as the rate-limiting step. We propose that the rate-limiting step of the hydrolytic reaction changes from phosphate releasing in aqueous solution to a covalent phosphorylation or dephosphorylation step in reverse micelles.

scholarly journals Factors Influencing the Activity of Nanozymes in the Cleavage of an RNA Model Substrate

Molecules ◽  
2019 ◽  
Vol 24 (15) ◽  
pp. 2814 ◽  
Author(s):  
Czescik ◽  
Zamolo ◽  
Darbre ◽  
Mancin ◽  
Scrimin
Keyword(s):  
Gold Nanoparticles ◽  
Metal Complex ◽  
Kinetic Isotope Effect ◽  
Ph Dependence ◽  
The Other ◽  
Kinetic Isotope ◽  
Factors Influencing ◽  
Nitrophenyl Phosphate ◽  
The One ◽  
Guanidinium Cation

A series of 2-nm gold nanoparticles passivated with different thiols all featuring at least one triazacyclonanone-Zn(II) complex and different flanking units (a second Zn(II) complex, a triethyleneoxymethyl derivative or a guanidinium of arginine of a peptide) were prepared and studied for their efficiency in the cleavage of the RNA-model substrate 2-hydroxypropyl-p-nitrophenyl phosphate. The source of catalysis for each of them was elucidated from the kinetic analysis (Michaelis–Menten profiles, pH dependence and kinetic isotope effect). The data indicated that two different mechanisms were operative: One involving two Zn(II) complexes and the other one involving a single Zn(II) complex and a flanking guanidinium cation. The mechanism based on a dinuclear catalytic site appeared more efficient than the one based on the cooperativity between a metal complex and a guanidinium.

Secondary Isotope Effects on the Ionization of 2-Nitropropane

1974 ◽  
Vol 52 (10) ◽  
pp. 1889-1896 ◽  
Author(s):  
A. J. Kresge ◽  
D. A. Drake ◽  
Y. Chiang
Keyword(s):  
Aqueous Solution ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Acid Dissociation ◽  
Methyl Groups ◽  
Anomalous Effect ◽  
Hydroxide Ion ◽  
Kinetic Isotope

The equilibrium isotope effect on the acid dissociation of 2-nitropropane in wholly aqueous solution at 25° was found to be KH/KD = 1.23 ± 0.03 for complete deuteration of both methyl groups; the kinetic isotope effect for reaction of the same substrate with hydroxide ion, kH/kD = 1.09 ± 0.01; and the kinetic isotope effect for reaction with tris-(hydroxymethyl)-methylamine, kH/kD = 1.10 ± 0.01; both of the latter also refer to wholly aqueous solution at 25° and are for complete deuteration of both methyl groups. It is shown that the equilibrium isotope effect is largely, and the kinetic isotope effects probably partly, hyper conjugative in origin, thus supporting a hyper conjugative explanation of the anomalous effect of methyl groups on nitroalkane ionization.

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.

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