Evidence for reversible intermediate formation in cyclopentenone cycloaddition

1969 ◽  
Vol 47 (4) ◽  
pp. 711-712 ◽  
Author(s):  
P. de Mayo ◽  
A. A. Nicholson ◽  
M. F. Tchir
Keyword(s):  
Rate Constant ◽  
Intermediate Formation ◽  
Product Formation ◽  
A Value

The usual kinetics for product formation lead to variable values of kd, the rate constant for decay of the responsible triplet. On the assumption that at least one intermediate is formed which gives either products, or reverts to starting materials, a value near 108 s−1 is obtained for kd both from cyclopentene and hex-3-ene cycloadditions.

Lysine164α of protein farnesyltransferase is important for both CaaX substrate binding and catalysis

2001 ◽  
Vol 360 (3) ◽  
pp. 625-631 ◽  
Author(s):  
Kendra E. HIGHTOWER ◽  
Smita DE ◽  
Carolyn WEINBAUM ◽  
Rebecca A. SPENCE ◽  
Patrick J. CASEY
Keyword(s):  
Rate Constant ◽  
Positive Charge ◽  
Substrate Binding ◽  
Product Formation ◽  
Chemical Mechanism ◽  
Α Subunit ◽  
Protein Farnesyltransferase ◽  
Oncogenic Ras ◽  
Tumour Formation ◽  
Mechanism Of Catalysis

Protein farnesyltransferase (FTase) catalyses the formation of a thioether linkage between proteins containing a C-terminal CaaX motif and a 15-carbon isoprenoid. The involvement of substrates such as oncogenic Ras proteins in tumour formation has led to intense efforts in targeting this enzyme for development of therapeutics. In an ongoing programme to elucidate the mechanism of catalysis by FTase, specific residues of the enzyme identified in structural studies as potentially important in substrate binding and catalysis are being targeted for mutagenesis. In the present study, the role of the positive charge of Lys164 of the α subunit of FTase in substrate binding and catalysis was investigated. Comparison of the wild-type enzyme with enzymes that have either an arginine or alanine residue substituted at this position revealed unexpected roles for this residue in both substrate binding and catalysis. Removal of the positive charge had a significant effect on the association rate constant and the binding affinity of a CaaX peptide substrate, indicating that the positive charge of Lys164α is involved in formation of the enzyme (E)·farnesyl diphosphate (FPP)·peptide ternary complex. Furthermore, mutation of Lys164α resulted in a substantial decrease in the observed rate constant for product formation without alteration of the chemical mechanism. These and additional studies provide compelling evidence that both the charge on Lys164α, as well as the positioning of the charge, are important for overall catalysis by FTase.

Kinetic characterization of a cis- and trans-acting M2+-independent DNAzyme that depends on synthetic RNaseA-like functionality — Burst-phase kinetics from the coalescence of two active DNAzyme folds

2007 ◽  
Vol 85 (4) ◽  
pp. 313-329 ◽  
Author(s):  
Richard Ting ◽  
Jason M Thomas ◽  
David M Perrin
Keyword(s):  
Steady State ◽  
Rate Constant ◽  
Acid Catalyst ◽  
Initial Population ◽  
Substrate Complex ◽  
Single Turnover ◽  
A Value ◽  
Burst Phase ◽  
Cis And Trans

This work describes the kinetics of the DNAzyme 925-11, a combinatorially selected, M2+-independent ribophosphodiesterase that is covalently modified with both cationic amines and imidazoles. At 13 °C, cis- and trans-cleaving constructs of 925-11 demonstrate the highest rate constants reported to date for any M2+-independent nucleic acid catalyst, investigated at physiological ionic strength and pH 7.5 (0.3 min–1 for self cleavage and 0.2 min–1 for intermolecular cleavage). In contrast to the cis-cleaving species, single-turnover experiments with the trans-cleaving species exhibit biphasic cleavage data, suggesting the presence of two conformations of the catalyst–substrate complex. Pulse–chase experiments demonstrate that both complexes lead to substrate cleavage. Under multiple-turnover conditions, the higher rate constant appears in a burst phase that decays to a slower steady state exhibiting a rate constant of 0.0077 min–1, a value approximating that of the slow-cleaving phase seen in single-turnover experiments. Slow product release is excluded as the source of the burst phase. An integrated rate equation is derived to describe burst-phase kinetics based on the funneling of the initial population of fast-cleaving conformation into a steady-state population composed largely of the slow-cleaving conformation.Key words: RNase mimics, DNAzymes, ribozymes, kinetics, RNA cleavage.

The Rate of Recombination of H Atoms in the Presence of NH3

1973 ◽  
Vol 51 (22) ◽  
pp. 3771-3773 ◽  
Author(s):  
L. Teng ◽  
C. A. Winkler
Keyword(s):  
Rate Constant ◽  
Pressure Range ◽  
Flow System ◽  
Carrier Gas ◽  
A Value ◽  
Fast Flow

The rate constant for the homogeneous recombination of H atoms in the presence of NH3, with He as carrier gas, has been determined at 298°K in a fast flow system, over the pressure range 1.50 to 4.55 Torr, using e.s.r. technique. A value of either 4.00 × 1016 or 5.14 × 1016 cm6 mol−2 s−1 was calculated, depending upon the rate constant taken, or estimated, from the literature for the recombination in the presence of helium.

Ruthenium and iron complexes of dipicolinic acid: Synthesis, solution properties and kinetics of electron transfer reactions with ascorbate ions

1983 ◽  
Vol 36 (12) ◽  
pp. 2377 ◽  
Author(s):  
NH Williams ◽  
JK Yandell
Keyword(s):  
Redox Potential ◽  
Rate Constant ◽  
Dipicolinic Acid ◽  
Acid Synthesis ◽  
Equilibrium Mixture ◽  
A Value ◽  
Inert Electrode ◽  
Ph Range ◽  
Kinetics Of ◽  
Kinetics Of Reduction

Standard potentials of the redox couples [bis(pyridine-2,6-dicarboxylate)MIII]-/2- ([M(dipic)2]-/2-, where M = Fe, Ru, Co) have been determined at 25�C, and ionic strength 0.1M (NaClO4 or KNO3). Kinetics of reduction of the oxidized complexes by ascorbate have also been examined under the same conditions. The [Fe(dipic)2]-/2- potential was found to be 355 � 5 mV. Reduction of [Fe(Fe(dipic)2]- in the pH range 4-6 was shown to occur by reaction with ascorbate monoanion (HA-) with a rate constant of (2.2 � 0.2) × 103 1. mol-1 s-1, and ascorbate dianion(A2-) with a rate constant of (7 � 1) × 108 1. mol-1 s-1. K [Ru(dipic)2] has been synthesized. Spectroscopic and analytical evidence suggest that it is a simple six-coordinate species in the solid and in non-aqueous solvents, but that in water it exists as an equilibrium mixture of at least two species. The redox potential for this mixture was found to be 270 � 10 mV. The major component of this mixture is reduced by A2- with a rate constant of (4.7 � 0.1) × 1081.mol-1 s-1. A value of 747 � 5 mV was measured for the redox potential of the cobalt couple, although equilibration of this system with the inert electrode could be achieved only by using [Fe(bpy)2(CN)2] as a mediator. Kinetics of reduction of [Co(dipic)2]- by ascorbate were complex and not reproducible.

Presteady-state kinetics of Bacillus 1,3–1,4-β-glucanase: binding and hydrolysis of a 4-methylumbelliferyl trisaccharide substrate

2001 ◽  
Vol 357 (1) ◽  
pp. 195-202
Author(s):  
Mireia ABEL ◽  
Antoni PLANAS ◽  
Ulla CHRISTENSEN
Keyword(s):  
Steady State ◽  
Rate Constant ◽  
Product Formation ◽  
Lag Phase ◽  
Wild Type ◽  
Induced Fit ◽  
Enzyme Substrate ◽  
Kinetics Of ◽  
Hydrolysis Of ◽  
Presteady State Kinetics

In the present study the first stopped-flow experiments performed on Bacillus 1,3–1,4-β-glucanases are reported. The presteady-state kinetics of the binding of 4-methylumbelliferyl 3-O-β-cellobiosyl-β-d-glucoside to the inactive mutant E134A, and the wild-type-catalysed hydrolysis of the same substrate, were studied by measuring changes in the fluorescence of bound substrate or 4-methylumbelliferone produced. The presteady-state traces all showed an initial lag phase followed by a fast monoexponential phase leading to equilibration (for binding to E134A) or to steady state product formation (for the wild-type reaction). The lag phase, with a rate constant of the order of 100s−1, was independent of the substrate concentration; apparently an induced-fit mechanism governs the formation of enzyme–substrate complexes. The concentration dependencies of the observed rate constant of the second presteady-state phase were analysed according to a number of reaction models. For the reaction of the wild-type enzyme, it is shown that the fast product formation observed before steady state is not due to a rate-determining deglycosylation step. A model that can explain the observed results involves, in addition to the induced fit, a conformational change of the productive ES complex into a form that binds a second substrate molecule in a non-productive mode.

scholarly journals Photocatalytic Treatment of Paracetamol Using TiO2 Nanotubes: Effect of pH

Processes ◽  
2019 ◽  
Vol 7 (6) ◽  
pp. 319 ◽  
Author(s):  
S. Lozano-Morales ◽  
Graciela Morales ◽  
Miguel López Zavala ◽  
Augusto Arce-Sarria ◽  
Fiderman Machuca-Martínez
Keyword(s):  
Surface Water ◽  
Rate Constant ◽  
Chromatographic Analysis ◽  
Tio2 Nanotubes ◽  
Emerging Contaminants ◽  
Uv Spectroscopy ◽  
Solution Ph ◽  
Effect Of Ph ◽  
A Value ◽  
By Products

Pharmaceuticals are considered among the group of emerging contaminants. Paracetamol is a moderate painkiller, which has been detected in ground and surface water. Photodegradation of paracetamol at a wavelength of radiation of 254 nm with TiO2 nanotubes was studied by UV-spectroscopy, HPLC and measurement of the potential zeta in dependence of the solution pH. The efficiency of the photodegradation of paracetamol (20 mg L−1) was 99% after 100 min exposure. Application of the Langmuir-Hinshelwood equation allowed the evaluation of the rate constant. Non-organic by-products were detected under the conditions of the chromatographic analysis. The photoreaction was faster at pH 6.5, a value at which adsorption was favored, leading to higher efficiency.

Die γ-Radiolyse und Pulsradiolyse des Schwefelkohlenstoffs in wäßriger Lösung / The γ-Radiolysis and Pulse Radiolysis of Carbon Disulfide in Aqueous Solution

1973 ◽  
Vol 28 (1-2) ◽  
pp. 12-22 ◽  
Author(s):  
W. Roebke ◽  
M. Schöneshöfer ◽  
A. Henglein
Keyword(s):  
Rate Constant ◽  
Carbon Disulfide ◽  
Second Order ◽  
Γ Irradiation ◽  
Electrolytic Dissociation ◽  
First Order ◽  
A Value ◽  
Diffusion Controlled ◽  
Polymer Pyrolysis ◽  
Hydrolysis Of

A polymer (CHS2)n and sulfate are formed in the γ-irradiation of deaerated aqueous carbon disulfide solutions. The G-values are 3.6 and 0.41, respectively. In the presence of N,O, G (polymer) is decreased while G(SO4-) is increased. G(SO4-) can be decreased by isopropanol. G(polymer) is increased by H+ ions and reaches a value of 5 below pH = 2. Formic acid, hydrogen sulfide and carbonate are formed in the hydrolysis of the polymer. Pyrolysis at first leads to a red oil consisting of oligomer (HCS2)n and finally to H2S, CS2 plus a residue containing much carbon. The structure of the polymer is discussed.Pulse radiolytic experiments show that CS2 reacts with eaq (3.1 × 1010M-1s-1) and OH(7.4 × 109M-1s-1) in a diffusion controlled manner. The first product of the reaction with OH is SC(OH)S. The pK of the electrolytic dissociationSC(OH)Ṣ ⇄ SC(O-)S + H+is 4.4. The absorption spectra of SC(OH)S and SC(O-)S were measured. SC(OH)S disappears by second order with 2k = 1.6 × 109M-1s-1 at pH = 6. The product is a bivalent acid, the spectrum of which was measured. The second pK of this acid is 5.7, its first pK is lower than 4.Both eāq and H react with CS2 to form SCS-. The absorption spectrum of this radical anion was measured. The pK of the equilibriumSCSH ⇄SCS- + H+is about 1.6. In solutions of low H+-concentration, SCS- disappears by second order with 2k = 6.4 × 109M-1s-1. The structure of dithioformic acid is attributed to the resulting product. In solutions of high H+-concentration, SCS- (or SCSH) disappears by a fast first order process, the rate constant of which increases with H+-concentration. The carbeniat neutralizationis believed to be responsible for this process. The rate constant is 5.1 × 107M-1S-1. The spectrum of SC(H)S was measured. This radical disappears by second order with 2k = 7.4 × 109M-1s-1. The spectrum of the resulting product was also determined.It is concluded that the formation of the polymer and of SO4- occurs in processes in which the first products from the attack of eāq, H and OH on CS2 as well as molecules which were built up from these products are involved.

scholarly journals Errors in the evaluation of Arrhenius and van't Hoff plots

1983 ◽  
Vol 209 (1) ◽  
pp. 277-280 ◽  
Author(s):  
T Keleti
Keyword(s):  
Rate Constant ◽  
Arrhenius Plot ◽  
Product Formation ◽  
Hoff Plot

Errors in the numerical values of activation or normal enthalpies, entropies and free enthalpies calculated from Arrhenius or van't Hoff plots, respectively, are due to the neglect of equidimensionality in equations, or to inappropriate approximations. The logarithmization of dimensioned quantities should be avoided, which demands the use of relative concentrations if a change in mole number occurs in the reaction. The application of the Arrhenius plot to enzymic reactions by using Vmax./ET instead of the rate constant of product formation has meaning only if the reaction follows the simplest Michaelis-Menten mechanism; however, the use of the van't Hoff plot using Km is questionable even in the latter case.

scholarly journals Anionic iodotyrosine residues are required for iodothyronine synthesis

1998 ◽  
pp. 227-231 ◽  
Author(s):  
JJ de Vijlder ◽  
MT den Hartog
Keyword(s):  
Ph Dependence ◽  
Coupling Reaction ◽  
Product Formation ◽  
Hydroxyl Group ◽  
Dissociation Curve ◽  
Anion Formation ◽  
Phenoxy Group ◽  
A Value ◽  
Gradual Loss

Biosynthesis of iodothyronines in thyroglobulin occurs by oxidative coupling of two iodotyrosine residues catalyzed by thyroperoxidase. To study the mechanism of iodothyronine formation, iodine-free thyroglobulin was non-enzymatically iodinated and after removal of non-incorporated iodide, incubated with lactoperoxidase and glucose oxidase between pH 4 and 9. The amount of thyroxine (T4). 3,5,3'-tri-iodothyronine (T3), 3,3',5'-tri-iodothyronine (rT3) and 3,3'-di-iodothyronine (T2) formed was measured by radioimmunoassays after hydrolysis of thyroglobulin. T4 is synthesized out of two di-iodotyrosine (DIT) residues in thyroglobulin. The pH dependence of T4 formation fits the dissociation curve of the DIT phenoxy group (pKa 6.5). The formation of T2, synthesized out of two mono-iodotyrosine (MIT) residues, shows a quite different pH dependence. Below pH 6, T2 synthesis could not be observed, while above pH 7.4 a relatively large increase occurred. The values up to pH 8 fitted the dissociation curve of the MIT-phenoxy group with a pKa of 8.7. The gradual loss in enzymatic activity of peroxidase and oxidase in the reaction made the values obtained above pH 8 unreliable. The importance of the ionization of the phenoxy group for the coupling reaction was further consolidated by showing that the pH-dependent oxidation of 2-methoxy-phenol (guaiacol) had 50% maximal product formation at pH 7, a value concordant with pKa 7.0 for the ionization of the phenoxy group of this agent. T3 and rT3 synthesis followed mainly the ionization curve of the inner-ring hydroxyl group, indicating that this ring has the greatest influence on hormonogenesis. Since anion formation facilitates the removal of an electron under oxidative conditions, the pH dependence agrees with the involvement of phenoxy radicals in iodothyronine synthesis, a process that most likely also occurs in vivo since it is mainly T4 that is formed in thyroglobulin.

The Coordination of Novel, Hard Ligand Systems to Tantalum(V) and Niobium(V) Metal Centres

2014 ◽  
Vol 1019 ◽  
pp. 426-432 ◽  
Author(s):  
Renier Koen ◽  
Andreas Roodt ◽  
Hendrik Visser
Keyword(s):  
Stability Constant ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Formation Reaction ◽  
Step Process ◽  
Kinetic Behaviour ◽  
Rate Determining Step ◽  
A Value ◽  
Rapid Formation ◽  
The Stability

The formation reaction of acetylacetone to solvated tantalum pentachloride ([TaCl5]2) in methanol, is defined by limiting kinetic behaviour and is indicative of a two-step process. This involves the rapid formation of an intermediate, possiblytrans-[TaCl2(OMe)3(η1-acacH)]) with the stability constant,K1, equal to 2.4 ± 0.3 x 103M-1at 25 °C. The second order rate constant,k1, at 25 °C was determined as 3.1 ± 0.1 x 102M-1s-1. A fit of the data obtained from the UV-Vis study, and describing the total reaction, yielded a value of 1.7 ± 0.2 x 103M-1s-1forK1, which is in agreement with the value obtained earlier. Comparison ofk1andk2indicates that the first reaction is roughly six orders of magnitude (106) faster than the second rate determining step.

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