The formation and decay of the oxyferrous complex of beef adrenocortical cytochrome P-450scc. Rapid-scan and stopped-flow studies

1987 ◽  
Vol 65 (5) ◽  
pp. 486-492 ◽  
Author(s):  
Mohammed A. Kashem ◽  
H. Brian Dunford
Keyword(s):  
Rate Constant ◽  
Acid Group ◽  
Spontaneous Decay ◽  
Stopped Flow ◽  
Oxygen Binding ◽  
First Order Kinetics ◽  
Rapid Scan ◽  
Ph Range ◽  
Kinetics Of ◽  
Cytochrome P 450

The formation and spontaneous decay of the oxyferrous complex of purified beef adrenocortical cholesterol-bound (high spin) cytochrome P-450scc have been studied by means of rapid-scan spectrometry in the Soret region at 4 °C. The oxyferrous complex, the formation of which occurs within 40 ms with a Soret absorption peak at 422 nm, is unstable and decays spontaneously to the ferric cholesterol-bound cytochrome P-450scc. The rapid-scan spectra for both processes were recorded. Isosbestic points occur at the following wavelengths: between ferrous and oxyferrous complex at 418 nm, between oxyferrous complex and ferric cytochrome P-450scc at 411 nm. The kinetics of oxygen binding and spontaneous decay of the oxyferrous complex have also been studied at 4 °C by means of stopped-flow experiments in the pH range 5.1–8.8. The rate constant for oxygen binding is constant at 5.8 × 105 ± 0.8 × 105 M−1∙s−1 over the pH range of the study. On the other hand, the decay process exhibited pH-dependent monophasic first-order kinetics. The rate constant for the decay appears to be influenced by an acid group with a pKa of 7.1 on the oxyferrous complex of cholesterol-bound cytochrome P-450scc.

Kinetic of oxidation of methyl orange by vanadium(V) under conditions VO2+ and decavanadates coexist: Catalysis by Triton X-100 micellar medium

2019 ◽  
Author(s):  
Chem Int
Keyword(s):  
Methyl Orange ◽  
Solution Ph ◽  
Vanadium Concentration ◽  
Limiting Behavior ◽  
First Order ◽  
First Order Kinetics ◽  
Ph Range ◽  
Kinetic Pattern ◽  
Triton X ◽  
Kinetics Of

The kinetics of oxidation of methyl orange by vanadium(V) {V(V)} has been investigated in the pH range 2.3-3.79. In this pH range V(V) exists both in the form of decavanadates and VO2+. The kinetic results are distinctly different from the results obtained for the same reaction in highly acidic solution (pH < 1) where V(V) exists only in the form of VO2+. The reaction obeys first order kinetics with respect to methyl orange but the rate has very little dependence on total vanadium concentration. The reaction is accelerated by H+ ion but the dependence of rate on [H+] is less than that corresponding to first order dependence. The equilibrium between decavanadates and VO2+ explains the different kinetic pattern observed in this pH range. The reaction is markedly accelerated by Triton X-100 micelles. The rate-[surfactant] profile shows a limiting behavior indicative of a unimolecular pathway in the micellar pseudophase.

Oxidation of Nickel(II) and Manganese(II) by Peroxodisulfate in Aqueous Solution in the Presence of Molybdate. Crystal Structure of the (NH4)6[NiMo9O32].6H2O Product

1992 ◽  
Vol 45 (12) ◽  
pp. 1943 ◽  
Author(s):  
SJ Dunne ◽  
RC Burns ◽  
GA Lawrance
Keyword(s):  
Rate Constant ◽  
Linear Dependence ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
X Ray Diffraction ◽  
X Ray ◽  
First Order ◽  
Oxidant Concentration ◽  
Ph Range ◽  
Kinetics Of

Oxidation of Ni2+,aq, by S2O82- to nickel(IV) in the presence of molybdate ion, as in the analogous manganese system, involves the formation of the soluble heteropolymolybdate anion [MMogO32]2- (M = Ni, Mn ). The nickel(IV) product crystallized as (NH4)6 [NiMogO32].6H2O from the reaction mixture in the rhombohedra1 space group R3, a 15.922(1), c 12.406(1) � ; the structure was determined by X-ray diffraction methods, and refined to a residual of 0.025 for 1741 independent 'observed' reflections. The kinetics of the oxidation were examined at 80 C over the pH range 3.0-5.2; a linear dependence on [S2O82-] and a non-linear dependence on l/[H+] were observed. The influence of variation of the Ni/Mo ratio between 1:10 and 1:25 on the observed rate constant was very small at pH 4.5, a result supporting the view that the precursor exists as the known [NiMo6O24H6]4- or a close analogue in solution. The pH dependence of the observed rate constant at a fixed oxidant concentration (0.025 mol dm-3) fits dequately to the expression kobs = kH [H+]/(Ka+[H+]) where kH = 0.0013 dm3 mol-1 s-1 and Ka = 4-0x10-5. The first-order dependence on peroxodisulfate subsequently yields a second-order rate constant of 0.042 dm3 mol-1 s-1. Under analogous conditions, oxidation of manganese(II) occurs eightfold more slowly than oxidation of nickel(II), whereas oxidation of manganese(II) by peroxomonosulfuric acid is 16-fold faster than oxidation by peroxodisulfate under similar conditions.

Open-chain nitrogen compounds. Part XV. A kinetic study of the hydrolysis of 1-aryl-3-aryloxymethyl-3-methyltriazenes and related triazenes

1992 ◽  
Vol 70 (8) ◽  
pp. 2224-2233 ◽  
Author(s):  
Keith Vaughan ◽  
Donald L. Hooper ◽  
Marcus P. Merrin
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Buffer System ◽  
Open Chain ◽  
Nucleophilic Displacement ◽  
Ether Oxygen ◽  
Rate Of Hydrolysis ◽  
Electron Withdrawing Group ◽  
Ph Range ◽  
Kinetics Of

The kinetics of hydyrolysis of a series of 1-aryl-3-aryloxymethyl-3-methyltriazenes, Ar-N=N-NMe-CH2OAr′, was studied over the pH range 2–7.5. Reactions were followed by the change in UV absorbance spectra of the triazenes. The aryloxymethyltriazenes decompose more slowly at pH 7.5 than the hydroxymethyltriazenes, Ar-N=NMe-CH2OH; the hydrolysis is favoured by the presence of an electron-withdrawing group in Ar′. A mixed isopropanol/buffer system was developed in order to improve solubility of the aryloxymethyl triazenes. Lowering the pH caused an increase in the rate of hydrolysis and under strongly acidic conditions an electron-withdrawing group in Ar′ actually slows down the reaction. A Hammett plot of the pseudo-first-order rate constant, kobs, is curved, indicating that two or more mechanisms operate simultaneously and that the contribution of each mechanism is substituent-dependent. A plot of kobs vs. [buffer] is linear; the slope of the plot affords the rate constant, kb for the buffer-catalyzed reaction for each substituent. A Hammett plot of kb vs. σ is linear with ρ = +0.55, suggesting that the buffer-catalyzed reaction involves nucleophilic displacement of the phenoxy group by the buffer anion. Further analysis afforded the specific acid-catalyzed rate constants, [Formula: see text], for each substituent; this component of the reaction has a negative ρ, consistent with a mechanism involving protonation at the ether oxygen. The postulation that specific acid catalysis is a component of the reaction mechanism was confirmed by the observation of a solvent deuterium isotope effect, 2.28 > kH/kD > 1.60. Only the p-NO2 and p-CN phenyloxymethyltriazenes showed any spontaneous decomposition.

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.

Studies on Horseradish Peroxidase. VII. A Kinetic Study of the Oxidation of Iodide by Horseradish Peroxidase Compound II

1971 ◽  
Vol 49 (18) ◽  
pp. 3059-3063 ◽  
Author(s):  
R. Roman ◽  
H. B. Dunford ◽  
M. Evett
Keyword(s):  
Ionic Strength ◽  
Horseradish Peroxidase ◽  
Kinetic Study ◽  
Rate Constant ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Acid Dissociation ◽  
Ph Range ◽  
Compound Ii ◽  
Kinetics Of

The kinetics of the oxidation of iodide ion by horseradish peroxidase compound II have been studied as a function of pH at 25° and ionic strength of 0.11. The logarithm of the second-order rate constant decreases linearly from 2.3 × 105 to 0.1 M−1 s−1 with increasing pH over the pH range 2.7 to 9.0. The pH dependence of the reaction is explained in terms of an acid dissociation outside the pH range of the study.

scholarly journals Study on Kinetics of GD Hydrolysis in HCl aqueous

2021 ◽  
Vol 267 ◽  
pp. 02060
Author(s):  
Shaoxiong Wu ◽  
Hongpeng Zhang ◽  
Ting Miao ◽  
Haiyan Zhu ◽  
Lianyuan Wang ◽  
...  
Keyword(s):  
Rate Constant ◽  
Ph Value ◽  
Negative Ion ◽  
Hydrolysis Rate ◽  
Obvious Effect ◽  
Factors Affecting ◽  
Initial Concentration ◽  
First Order Kinetics ◽  
Kinetics Of ◽  
Positive Ion

Organophosphate neurotoxic agents like Sarin (GB) and Soman (GD) are lethal to person. Except various kinds of decontaminants, they can be also decomposed in natural environment through nucleophilic reaction, where acidic or alkaline substance was to accelerate their hydrolysis. Most of the papers were about GB hydrolysis. Information on GD hydrolysis was relatively small, especially about kinetics of GD in acidic solution. In view of possible effect of positive ion and negative ion on hydrolysis reaction, a relatively simple composes solution, HCl aqueous solutions, was selected to investigate the factors affecting GD hydrolysis rate. Results showed that GD hydrolysis was accorded with the first-order kinetics equation if pH value was kept constant. Its rate constant was independent of GD initial concentration when the amount of H+ was excess than its requested amount. The apparent hydrolysis rate constant (kobs) in pH of 0.90 was about 0.202 min-1 at 20°C, no matter what initial concentration of GD was. The concentration of H+ was the most important factor affecting its rate. The rate constant (kobs) in HCl aqueous as a function of pH value (0.90~2.80) obeyed an equation in 25°C, that is kobs =0.17×10-0.82×pH. Reaction temperature had an obvious effect on hydrolysis rate of GD. Every 10°C increase in temperature, kobs of GD hydrolysis was improved about 2.5 times. The activation energy value (Ea) of GD hydrolysis in HCl aqueous with a pH value of 0.90 was approximately 64.25 kJ/mol.

Kinetics of the Formation of the Blue Complex CrO(O 2 ) 2 Formed by Dichromate and H 2 O 2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

2002 ◽  
Vol 133 (10) ◽  
pp. 1363-1372 ◽  
Author(s):  
Günter Grampp ◽  
Stephan Landgraf ◽  
Tomasz Wesierski ◽  
Beata Jankowska ◽  
Ewa Kalisz ◽  
...  
Keyword(s):  
Stopped Flow ◽  
Acid Solutions ◽  
Rapid Scan ◽  
Flow Investigation ◽  
Kinetics Of

Kinetics of oxidation of thiocyanate by aqueous iodine

1973 ◽  
Vol 26 (9) ◽  
pp. 1863 ◽  
Author(s):  
GT Briot ◽  
RH Smith
Keyword(s):  
Rate Constants ◽  
Activation Energies ◽  
Stopped Flow ◽  
Kinetics Of Oxidation ◽  
Rate Law ◽  
General Base ◽  
Present Rate ◽  
Rapid Equilibrium ◽  
Ph Range ◽  
Kinetics Of

The kinetics of oxidation of thiocyanate to sulphate by aqueous iodine in the pH range 9.2-12.5 have been studied using a spectrophotometric stopped flow technique. The reaction is general base-catalysed, having the rate law ��������������������� -d[I2]a/dt = ([SCN-][I3-]/[I-]2)Σ kB[B] where [I2]a is the total analytical concentration of iodine, [B] is the concentration of base, and where the summation is taken over all bases present. Rate constants, kB, and activation energies have been measured for the bases, OH-, PO43- and CO32-. ��� A mechanism involving the initial steps ����������������� I2+SCN- ↔ ISCN+I- �����������������(rapid equilibrium) ������������� ISCN+H2O+B → HOSCN+I- + HB+ �����������(rate determining) followed by rapid reactions of HOSCN with itself or with iodine is proposed.

The kinetics of fluoride binding by lactoperoxidase

1968 ◽  
Vol 46 (12) ◽  
pp. 1471-1474 ◽  
Author(s):  
Raymond Segal ◽  
H. Brian Dunford ◽  
Martin Morrison
Keyword(s):  
Dissociation Rate ◽  
Specific Rate ◽  
Accurate Analysis ◽  
Binding Reaction ◽  
Reversible Binding ◽  
First Order Kinetics ◽  
Flow Apparatus ◽  
Absorbance Changes ◽  
Ph Range ◽  
Kinetics Of

The kinetics of the reversible binding of fluoride by ferric lactoperoxidase has been studied over the pH range 3.8–5.4 by means of a stopped-flow apparatus. Because of small absorbance changes upon the binding of fluoride by lactoperoxidase, the reaction was driven to completion by addition of a large excess of fluoride. Under these conditions the binding reaction obeys pseudo-first-order kinetics and an accurate analysis of the dissociation rate data is not possible. The results are consistent with a mechanism in which HF binds to the active site which contains no groups capable of ionization in the pH range of the present study. The specific rate constant for the binding reaction of HF to lactoperoxidase is (9.7 ± 0.4) × 102 M−1s−1.

scholarly journals Photodegradation of levofloxacin in aqueous and organic solvents: A kinetic study

2013 ◽  
Vol 63 (2) ◽  
pp. 223-229 ◽  
Author(s):  
Iqbal Ahmad ◽  
Raheela Bano ◽  
Muhammad Ali Sheraz ◽  
Sofia Ahmed ◽  
Tania Mirza ◽  
...  
Keyword(s):  
Dielectric Constant ◽  
Degradation Rate ◽  
Hplc Method ◽  
Ph Profile ◽  
Degree Of Ionization ◽  
First Order ◽  
First Order Kinetics ◽  
Anionic Species ◽  
Ph Range ◽  
Kinetics Of

The kinetics of photodegradation of levofloxacin in solution on UV irradiation in the pH range 2.0-12.0 has been studied using a HPLC method. Levofloxacin undergoes first-order kinetics in the initial stages of the reaction and the apparent first-order rate constants are of the order of 0.167 to 1.807×10-3 min-1. The rate-pH profile is represented by a curve indicating the presence of cationic, dipolar and anionic species during the reaction. The singly ionized form of the molecule is non-fluorescent and is less susceptible to photodegradation. The increase in the degradation rate in the pH range 5.0-9.0 may be due to greater reactivity of the ionized species existing in that range. The rate appears to vary with a change in the degree of ionization of the species present in a particular pH range and their susceptibility to photodegradation. Above pH 9, the decrease in the rate of photodegradation may be a result of deprotonation of the piperazinyl group. The levofloxacin molecule is more stable in the pH range around 7, which is then suitable for formulation purposes. The photodegradation of levofloxacin was found to be affected by the dielectric constant and viscosity of the medium

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