Deacylation of Fluorine-substituted trans-Cinnamoyl-α-chymotrypsins

1975 ◽  
Vol 53 (4) ◽  
pp. 513-518 ◽  
Author(s):  
J. T. Gerig ◽  
R. S. McLeod
Keyword(s):  
Rate Constants ◽  
Rate Effect ◽  
Ionization State ◽  
Hydrolysis Rates ◽  
Order Of Magnitude ◽  
Model Reactions

Deacylation of unsubstituted and of o-F-, m-F, p-F, α-F, pentafluoro-, p-methyl-, and p-trifluoromethyl-substituted trans-cinnamoyl- α-chymotrypsins has been studied from pH 4 to 8. The deacylation rate constants were found to depend upon the ionization state of a group on the enzyme with an apparent pK in the range 6.3–7.3. The hydrolysis rates of the correspondingly substituted p-nitrophenylcinnamate esters were determined at pH 10.6. Correlation of the data for these model reactions with the corresponding rates of enzyme deacylation suggests that the p-methyl-substituted acylenzyme is about an order of magnitude more reactive than expected while p-trifluoromethyl substitution results in deacylation at a rate 10 times slower than expected. The remaining substituents exert about the anticipated rate effect on deacylation.

Arrhenius Parameters for Reaction of tert-Butylperoxy Radicals with some Hindered Phenols and Aromatic Amines

1973 ◽  
Vol 51 (22) ◽  
pp. 3738-3745 ◽  
Author(s):  
James Anthony Howard ◽  
Edward Furimsky
Keyword(s):  
Hydrogen Atom ◽  
Rate Constants ◽  
Aromatic Amines ◽  
Activation Parameters ◽  
Peroxy Radical ◽  
Hydrogen Atom Transfer ◽  
Tert Butyl ◽  
Radical Decay ◽  
Order Of Magnitude ◽  
Hindered Phenols

The reaction of tert-butylperoxy radicals with some 2,6-di-tert-butyl-4-substituted phenols, α-naphthol, α-naphthylamine, and N-phenyl-α-naphthylamine has been studied by following the change in radical concentration with time using an e.p.r. spectrometer. Rates of radical decay were first-order in both reactants and were not influenced by the presence of tert-butyl hydroperoxideAbsolute values of the second-order rate constants were measured from −35 to −100° and the preexponential factors and activation energies fell in the range 104–105 M−1 s−1 and 0.5–1.0 kcal mol−1. Rate constants for deuterated phenols (OD) and aromatic amines (ND) were an order of magnitude lower than for the corresponding light compoundsThere was no evidence for quantum mechanical tunneling in these hydrogen atom transfer reactions and it would appear that the activation parameters are low because reaction initially involves the formation of a hydrogen bonded peroxy radical – phenol (or aromatic amine) complex, followed by the transfer of a hydrogen atom within the complex

Faradaic Impedance Study of Mn(II) Reduction Mechanism in NaClO4 and NaCl Solutions on Mercury Electrode

2002 ◽  
Vol 67 (11) ◽  
pp. 1589-1595
Author(s):  
Barbara Marczewska ◽  
Andrzej Persona ◽  
Marek Przegaliński
Keyword(s):  
Electron Transfer ◽  
Rate Constants ◽  
Electrochemical Reaction ◽  
Mercury Electrode ◽  
Reduction Mechanism ◽  
True Rate ◽  
Faradaic Impedance ◽  
Impedance Measurements ◽  
Order Of Magnitude ◽  
Adsorptive Properties

The electrochemical reaction of the Mn(II)/Mn(Hg) system on mercury electrode was studied in 1 M NaClO4 and 1 M NaCl as supporting electrolytes of different complexing and adsorptive properties. The impedance measurements confirmed the two-stage electroreduction of the Mn(II) in investigated solutions. Both the apparent and the true rate constants of the second electron transfer in both supporting electrolytes are lower by one order of magnitude than the rate constant of the first electron transfer. Similar values of corrected rate constants in both electrolytes suggest the similarity in mechanism of the Mn(II) electroreduction.

Elimination rate constants of 36 PCBs in zebra mussels (Dreissena polymorpha) and exposure dynamics in the Lake St. Clair - Lake Erie corridor

1995 ◽  
Vol 52 (12) ◽  
pp. 2574-2582 ◽  
Author(s):  
Heather Morrison ◽  
Rodica Lazar ◽  
G. Douglas Haffner ◽  
Tamara Yankovich
Keyword(s):  
Steady State ◽  
Rate Constants ◽  
Dreissena Polymorpha ◽  
Lake Erie ◽  
Elimination Rate ◽  
Zebra Mussels ◽  
Elimination Kinetics ◽  
Western Lake ◽  
Order Of Magnitude ◽  
Kinetics Of

The elimination kinetics of 36 PCB congeners, ranging in log octanol–water partition coefficients (log Kow) from 5.60 to 7.50, were determined in zebra mussels (Dreissena polymorpha) with shell lengths from 1.0 to 1.5 cm. Elimination rate constants, based on lipid-normalized data, ranged from 0.172 to 0.042 day−1 and exhibited a significant negative regression with log Kow. Time to 95% steady state ranged from 17.5 to 71.0 days and was used to determine the period over which mussels integrated exposure concentrations. Bioavailable PCB congener concentrations, calculated with a steady-state model, were determined from mussels collected offshore of Middle Sister Island in western Lake Erie. Chemical concentrations in water, estimated using mussels, were within an order of magnitude of direct measurements for congeners with log Kow < 6.6. The rapid elimination kinetics of zebra mussels suggests that these organisms can closely track temporal fluctuations in ambient chemical concentrations, and therefore have the potential to regulate contaminant cycling in aquatic ecosystems.

Reactions of 1,3-diacylthioureas with methoxide ion and with amines

1987 ◽  
Vol 52 (1) ◽  
pp. 120-131 ◽  
Author(s):  
Jaromír Kaválek ◽  
Josef Jirman ◽  
Vojeslav Štěrba
Keyword(s):  
Carbonyl Group ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Equilibrium Constants ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Order Of Magnitude ◽  
Acetyl Group ◽  
Methoxide Ion ◽  
Rate Limiting

Rate constants of base-catalyzed methanolysis and dissociation constants in methanol have been determined for benzoylthiourea (II), 1,3-diacetylthiourea (III), 1,3-dibenzoylthiourea (IV), and 1-acetyl-3-benzoylthiourea (V). With the diacyl derivatives III and IV, the reaction of methoxide ion with the neutral substrate is accompanied by that of methoxide with the substrate anion (at higher alkoxide concentrations). Above 0.1 mol l-1 CH3O(-), the rate constants are also affected by medium. The rate of the reaction of neutral diacyl derivative is decreased, and that of the reaction of methoxide with the substrate anion is rapidly increased. The dissociation constant of II is higher than that of acetylthiourea (I) by about one order of magnitude, but the attack of methoxide on the carbonyl group of II is about three times slower than that in I. The benzoyl group at the N1 nitrogen exhibits a greater activating influence (in both the rate and the equilibrium constants) on the other NHCOR group than the acetyl group does. With V the ratio of methanolysis rate constants is 9 : 1 in favour of the acetyl group. The reaction of diacetyl derivative III with 1-butanamine has been followed in butanamine buffers. At the lowest butanamine concentrations, the reaction is second order in the amine, and the rate-limiting step is the proton transfer from the intermediate to the second amine molecule. At the highest butanamine concentrations the reaction becomes first order in the amine, and the rate-limiting step changes to the attack of butanamine on the carbonyl group of diacetyl derivative III.

Kinetics and mechanism of methanolysis of benzoyl derivatives of substituted phenylureas and phenylthioureas

1984 ◽  
Vol 49 (9) ◽  
pp. 2103-2110 ◽  
Author(s):  
Jaromír Kaválek ◽  
Said El Bahaie ◽  
Vojeslav Štěrba
Keyword(s):  
Nitrogen Atom ◽  
Steric Hindrance ◽  
Rate Constants ◽  
Dissociation Constants ◽  
Kinetics And Mechanism ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Order Of Magnitude ◽  
Rate Limiting ◽  
Derivatives Of

The methanolysis rate constants and dissociation constants have been measured of benzoyl derivatives of substituted phenylureas and phenylthioureas. The dissociation constants of the thio derivatives are higher by 1 order of magnitude and the rate constants are higher by 2 orders of magnitude than the respective values of the oxygen analogues. Logarithms of the rate and dissociation constants have been correlated with the Hammet σ constant; the ρ constant of the methanolysis of the oxygen derivatives is almost 2x higher than that of the thio derivatives, which is explained by a change in the rate-limiting step. Methylation of the phenyl nitrogen atom increases the acidity by almost 2 orders of magnitude. This effect is due obviously to steric hindrance to the conjugation with the adjacent carbonyl or thiocarbonyl group.

scholarly journals Evolution of enzyme catalytic power. Characteristics of optimal catalysis evaluated for the simplest plausible kinetic model

1977 ◽  
Vol 163 (1) ◽  
pp. 111-116 ◽  
Author(s):  
Keith Brocklehurst
Keyword(s):  
Equilibrium Constant ◽  
Rate Constants ◽  
Catalytic Efficiency ◽  
Kinetic Mechanism ◽  
Enzyme Concentration ◽  
Forward Rate ◽  
Power Characteristics ◽  
Evolutionary Changes ◽  
Order Of Magnitude ◽  
Km Value

1. Evolutionary changes in the structure of an enzyme that provide an increase in its Km value are considered. Provided that Km increases as a result of increases in the forward rate constants of the catalysis relative to the reverse rate constants, the enzyme catalyses the conversion of a fixed concentration of its substrate more rapidly when its structure provides that Km>[S] than when Km<[S]. 2. Catalytic efficiency of enzymes is discussed in terms of the simplest plausible model, the Haldane [(1930) Enzymes, Longmans, London] reversible three-step model: [Formula: see text] The rate equation for the forward reaction of this model (formation of P) may be written in the simple form: [Formula: see text] Keq. is the equilibrium constant (=[P]eq./[S]eq.), and kcat.=V/[E]T, where [E]T is the total enzyme concentration. 3. To assess the effectiveness of an enzyme, it is necessary only to determine the extent to which the constraints of a particular kinetic mechanism permit v2 (v when Km»[S]) to approach vd (the diffusion-limited rate). 4. The value of the optimal rate of catalysis (vopt., the maximal value of v2) is dictated by the equilibrium constant for the reaction, Keq.; v2=vd/a, where [Formula: see text] when k+1 is assumed equal to k−3, and vopt.=vd/amin.. When Keq.≥1, it is necessary that k+2»k−1 for a to take its minimum value, amin.; when Keq.«1, it is necessary only that k+2»Keq.·k−1, i.e. a can equal amin. even if k+2<k−1. When Keq.»1, vopt.=vd; when Keq.=1, vopt.=vd/2, and when Keq.«1, vopt.=Keq.·vd. 5. The analysis, together with predicted effects of evolutionary pressure, suggests that in practice the rates of the fastest enzyme-catalysed freely reversible reactions might be expected to be lower than the value of k+1[E]T[S] by about an order of magnitude, particularly if Keq.<1. 6. The existing literature suggests that, in general, appropriate values of Km have evolved for the provision of high rates of catalysis but that many values of kcat. are not large enough to provide optimal rates of catalysis unless the value of k+1in vivo is lower than its value in free solution.

scholarly journals The role of peroxide in haem degradation. A study of the oxidation of ferrihaems by hydrogen peroxide

1978 ◽  
Vol 174 (3) ◽  
pp. 901-907 ◽  
Author(s):  
S B Brown ◽  
H Hatzikonstantinou ◽  
D G Herries
Keyword(s):  
Hydrogen Peroxide ◽  
Catalytic Activity ◽  
Linear Dependence ◽  
Rate Constants ◽  
Experimental Error ◽  
Ph Dependence ◽  
Monomeric Species ◽  
Order Of Magnitude ◽  
Computer Based

The oxidation of ferrihaems by H2O2 was studied as a model for haem catabolism. Rates of ferrihaem oxidation were evaluated by using a new computer-based method that measures the loss in catalytic activity of the ferrihaem during oxidation. For protoferrihaem, deuteroferrihaem, coproferrihaem and mesoferrihaem, oxidation proceeded via the monomeric species and no dimer contribution was detectable. The pH-dependence of oxidation was studied in the range 6.5–11. Within experimental error, the data were compatible with an inverse linear dependence on [H+]. This was interpreted in terms of attack by HO2- on monomeric ferrihaem. The specific second-order rate constants for oxidation of monomeric species by HO2- were of the same order of magnitude for all the ferrihaems, and were in the sequence coproferrihaem greater than protoferrihaem greater than mesoferrihaem congruent to deuteroferrihaem. A model is suggested involving formation of a ferrihaem monomerperoxide complex, which may either dissociate with the formation of a peroxidatic intermediate or be involved in an intramolecular oxidation of the ferrihaem. Haem catabolism may occur via the same or a similar intermediate.

scholarly journals Catalase model systems. Decomposition of hydrogen peroxide catalysed by mesoferrihaem, deuteroferrihaem, coproferrihaem and haematoferrihaem

1978 ◽  
Vol 174 (3) ◽  
pp. 893-900 ◽  
Author(s):  
H Hatzikonstantinou ◽  
S B Brown
Keyword(s):  
Mechanism Of Action ◽  
Rate Constants ◽  
Inverse Relationship ◽  
Ph Dependence ◽  
Catalytic Decomposition ◽  
Order Rate Constant ◽  
Model Systems ◽  
Ph Variation ◽  
Order Of Magnitude ◽  
Decomposition Of H2o2

The catalytic decomposition of H2O2 by deuteroferrihaem, mesoferrihaem, coproferrihaem and haematoferrihaem was studied as a model for the mechanism of action of catalase. For haematoferrihaem, anomalous but reproducible results were obtained, which could not be adequately explained. For each of the other ferrihaems studied, both monomeric and dimeric species catalysed decomposition, although the activity of monomer (aM) was much greater than that of dimer (aD). The pH variation of aD in the range 6.5–11 was consistent with an inverse dependence on [H+]1/2. The molecular mechanism whereby such a dependence could be achieved is not apparent. A study of the pH-dependence of aM in the range 6.5–11 revealed a linear inverse relationship with [H+]. This is interpreted in terms of attack by HO2- on ferrihaem monomer. The specific pH-independent rate constants for this reaction were in the order coproferrihaem greater than protoferrihaem greater than or equal to mesoferrihaem congruent to deuteroferrihaem. The order of magnitude of these rate constants is the same as that for catalysis by Fe(H2O)63+ and the second-order rate constant for decomposition of H2O2 by catalase. The implications on the mechanism of action of catalase are discussed.

scholarly journals Фракто- и фотолюминесценция кварца при разрушении

2021 ◽  
Vol 63 (8) ◽  
pp. 1120
Author(s):  
В.И. Веттегрень ◽  
А.Г. Кадомцев ◽  
И.П. Щербаков ◽  
Р.И. Мамалимов
Keyword(s):  
Rate Constants ◽  
Time Interval ◽  
Quartz Surface ◽  
Micro Cutting ◽  
Pl Spectra ◽  
Order Of Magnitude ◽  
Average Size

The fractoluminescence (FL) spectra at the destruction of the quartz surface by "micro-cutting" of diamond micro crystals and impact on its surface with a steel striker and photoluminescence (PL) spectra after "micro-cutting" are obtained. Band of 2.12 eV in the FL spectra was attributed to excited Si-O● radicals. FL consist from a set of signals whose duration was ≈ 50 ns, and their intensity varies by an order of magnitude. The time interval between the signals varies from ≈ 0.1 to several µs. Signals generated at impact contain five, and at "micro-cutting" contain four superimposed maxima. It is assumed that the FL signals appear when the barriers that prevent the movement of dislocations along the sliding planes breaks, and the smallest "primary" cracks are formatted. The rate constants of growth the cracks and fading of FL after the cracks stops are determined. The average size of the "primary" cracks surfaces after impact is ≈ 5 nm2, and after "micro-cutting" is ≈ 25 nm2. In the PL spectra after the fracture, a band of 2.4 eV was observed, which is attributed to non excited ≡Si-O- radicals on the cracks surface after their stop.

Dynamics of silicate exchange in highly alkaline potassium silicate solutions

1998 ◽  
Vol 76 (2) ◽  
pp. 183-193
Author(s):  
Eva Vallazza ◽  
Alex D Bain ◽  
Thomas W Swaddle
Keyword(s):  
Rate Constants ◽  
Relaxation Times ◽  
Complete Analysis ◽  
True Rate ◽  
Line Shape Analysis ◽  
Cyclic Trimer ◽  
Selective Inversion ◽  
Order Of Magnitude ◽  
Kinetics Of ◽  
Link Formation

The problem of measuring the kinetics of Si exchange between aqueous silicate species by 29Si NMR has been revisited, using highly alkaline KOH solutions (2.8 mol SiIV per kg solvent, [SiIV]/K2O = 0.43) at 60-90°C to minimize the number of silicate species present. Longitudinal 29Si relaxation times T1 and apparent rate constants estimated from line-shape analysis (LSA) varied markedly with the degree of purity of the KOH used, but rate constants k obtained by selective inversion-recovery (SIR) using the CIFIT data-fitting program were independent of the source of KOH and were smaller than those obtained from LSA by at least an order of magnitude. Although only four kinetically significant silicate anions (monomer M, dimer D, linear trimer L, and cyclic trimer C) were present, overlap of the D and L resonances prevented complete analysis of the SIR data. True rate constants could therefore be obtained only for the M-D exchange (for formation of D, k1 (90°C) = 0.13 ± 0.01 kg mol -1 s-1, Δ H1dagger = 67.4 kJ mol-1, Δ S1dagger = -78 J K-1 mol-1; for dissociation of D, k-1 (90°C) = 1.4 ± 0.1 s-1, Δ H-1dagger = 64.7 kJ mol-1, and Δ S1dagger = -66 J K-1 mol-1). Models that included L as the precursor of C (MDLC mechanism) showed, within the limitations imposed by D-L band overlap, that the reactivities of M, D, L, and C in Si-O-Si link formation or dissociation were all roughly comparable. Good fits of the experimental data, however, and equally reliable rate constants for the M-D exchange, could be obtained with models that ignored the presence of L entirely (MDC mechanism). The simple MDC model also provides consistent apparent rate constants kC and k-C for the overall formation of C from M + D and the reverse process, respectively, by SIR of either M or C ( Δ HCdagger = 76.5 kJ mol-1, Δ SCdagger = -57 J K-1 mol-1; Δ H-Cdagger = 88.6 kJ mol-1, and Δ S-Cdagger = -7 J K-1 mol-1).Key words: kinetics, silicates, 29Si NMR.

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