The base hydrolysis of coordinated fluorophosphate

1984 ◽  
Vol 37 (10) ◽  
pp. 1999 ◽  
Author(s):  
II Creaser ◽  
RV Dubs ◽  
AM Sargeson
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Second Order ◽  
Base Hydrolysis ◽  
Order Rate ◽  
Measured Rate ◽  
Hydrolysis Of

[(NH3)5CoO3PF]+ undergoes base hydrolysis in 0.1-0.3 M NaOH, � = 1.0, at 25�C to generate free FPO32-, F- and [(NH3)4Co(OH)(NH2PO3)] with a second-order rate constant k2 3.3 × 10 mol-1 s-1 where k2 is the composite rate constant for FPO32- release and hydrolysis of F- with the two products being formed in approximately equal amounts. The measured rate constitutes an estimated enhancement of about 1010 over the base hydrolysis of the uncoordinated FPO32- ion under the same conditions when the concentration of the coordinated nucleophile is taken into account.

Studies on Sulphamic Acid and Related Compounds. II. Kinetics and Mechanism of Hydrolysis of Trimethylamine- and of Triethylamine Sulphur Trioxide Addition Compounds

1962 ◽  
Vol 15 (2) ◽  
pp. 251 ◽  
Author(s):  
BE Fleischfresser ◽  
I Lauder
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Aqueous Acetone ◽  
Second Order ◽  
Fluoride Ions ◽  
Order Rate ◽  
Mechanism Of Hydrolysis ◽  
Reaction With Water ◽  
Sulphamic Acid ◽  
Hydrolysis Of

The kinetics of hydrolysis of trimethylamine- and of triethylaminesulphur trioxide addition compounds have been studied in water and in aqueous acetone. Reaction occurs according to the equation,������������� f - + R,N.SO,+H,O-tR,XH+HSO~.The solvolysis reactions are first-order and are not catalyzed by acids. The halide ions, Cl', Br', and 1', show only a normal salt effect on the rate of hydrolysis of + - (CH,),N.SO, but in the presence of fluoride ions, the rate constant for the production + - of acid from (C,H,),N.SO, in water at 95 OC is about one-seventh of that in the absence of fluoride under the same conditions. It is suggested that the fluorosulphonate ion is formed rapidly, and that this ion then undergoes slow hydrolysis :�In the presence of alkali, using water as the solvent, second-order kinetics are observed, the equation for the reaction being,�������������� + - R,N.SO,+~OH-+R,X+SO~= +H,O. Assuming the reaction with water is bimolecular, the ratio of the (bimolecular) rate constants at 35 OC, ko~-/k~,o is approximately lo8 for each complex. In aqueous acetone, at low water concentrations, the hydrolysis reactions of the trialkylaminesulphur trioxide complexes show second-order kinetics. At 35 OC for the hydrolysis of + - (CH,),N.SO, the ratio of the second-order rate constant in aqueous acetone to the + - (calculated) second-order rate constant in water is approximately 550 ; for (C,H,),N.SO, the same ratio is 6900. It is considered that hydrolysis occurs in water and in aqueous acetone via a bi- molecular attack at sulphur.

The hydrolysis of an activated ester by a tris(4,5-di-n-propyl-2-imidazolyl)phosphine-Zn2+ complex in neutral micellar medium as a model for carbonic anhydrase

2002 ◽  
Vol 80 (2) ◽  
pp. 183-191 ◽  
Author(s):  
Terry B Koerner ◽  
R S Brown
Keyword(s):  
Carbonic Anhydrase ◽  
Rate Constant ◽  
Titration Curve ◽  
Order Rate Constant ◽  
Second Order ◽  
Slow Step ◽  
Order Rate ◽  
Brij 35 ◽  
Triton X ◽  
Hydrolysis Of

The properties of tris(4,5-di-n-propyl-2-imidazolyl)phosphine–M2+ complexes (3–M2+, M = Zn, Co) in neutral micellar media of Brij-35 and Triton X-100 have been studied in water with respect to their quantitative potenti metric titration, Co2+-visible absorption spectra, and ability of the 3–Zn2+ complex to promote the hydrolysis of the activated ester, p-nitrophenyl acetate (PNPA). Potentiometric titration of the 3–M2+(CIO4–)2 complexes in 20 mM Brij-35 media yields a steep titration curve indicative of the cooperative consumption of two hydroxides, with computed pK1 and pK2 values of 8.75 and 6.25, respectively, and the midpoint of the titration curve (pKapp) being 7.50. A similar titration of the Co2+ complex also indicates cooperative consumption of two HO–, and this is tied to the formation of a 4- or 5-coordinate complex, pKapp ~ 7.3–7.4. The cooperativity is explained in terms of sequential replacement of the two CIO4– ions associated with the 3–M2+ to eventually yield 3–M2+–HO–/(HO–(H2O)n) having the first hydroxide ligated to the metal ion and the second associated as an ion pair. The 3–Zn2+ complex catalyzes the hydrolysis of PNPA in 20 mM Brij-35 and 40 mM Triton X-100. Plots of the observed second order rate constant (k2) vs. pH in Brij-35 increase linearly with pH and plateau to a value of k2max = 0.86 M–1 s–1, with a kinetic pKa of 8.7. These data are analyzed by a process wherein the 3–Zn2+–HO– is kinetically active in the rate-limiting step of the reaction, while the ion-paired (HO–(H2O)n) exists as a spectator to the slow step, possibly promoting rapid breakdown of a tetrahedral intermediate. Analysis of the kinetic data in terms of a model that accounts for the partitioning of PNPA between water and hydrophobic micellar pseudophase indicates that the second-order rate constant of the micelle-bound ester is augmented by 45-fold due to loading of the PNPA substrate into the micelle. Key words: Brij-35, TritonX-100, neutral micelle, carbonic anhydrase model, kinetics, potentiometric titrations, catalysis, p-nitrophenyl acetate hydrolysis.

scholarly journals The reactivities and ionization properties of the active-site dithiol groups of mammalian protein disulphide-isomerase

1991 ◽  
Vol 275 (2) ◽  
pp. 335-339 ◽  
Author(s):  
H C Hawkins ◽  
R B Freedman
Keyword(s):  
Rate Constant ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Second Order ◽  
Native Enzyme ◽  
Liver Protein ◽  
Thiol Groups ◽  
Protein Disulphide Isomerase ◽  
Order Rate ◽  
Reactive Groups

1. The number of reactive thiol groups in mammalian liver protein disulphide-isomerase (PDI) in various conditions was investigated by alkylation with iodo[14C]acetate. 2. Both the native enzyme, as isolated, and the urea-denatured enzyme contained negligible reactive thiol groups; the enzyme reduced with dithiothreitol contained two groups reactive towards iodoacetic acid at pH 7.5, and up to five reactive groups were detectable in the reduced denatured enzyme. 3. Modification of the two reactive groups in the reduced native enzyme led to complete inactivation, and the relationship between the loss of activity and the extent of modification was approximately linear. 4. Inactivation of PDI by alkylation of the reduced enzyme followed pseudo-first-order kinetics; a plot of the pH-dependence of the second-order rate constant for inactivation indicated that the essential reactive groups had a pK of 6.7 and a limiting second-order rate constant at high pH of 11 M-1.s-1. 5. Since sequence data on PDI show the presence within the polypeptide of two regions closely similar to thioredoxin, the data strongly indicate that these regions are chemically and functionally equivalent to thioredoxin. 6. The activity of PDI in thiol/disulphide interchange derives from the presence of vicinal dithiol groups in which one thiol group of each pair has an unusually low pK and high nucleophilic reactivity at physiological pH.

The mechanisms of hydrolysis of alkyl N-alkylthioncarbamate esters at 100 °C

2005 ◽  
Vol 83 (9) ◽  
pp. 1483-1491 ◽  
Author(s):  
Eduardo Humeres ◽  
Maria de Nazaré M. Sanchez ◽  
Conceição ML Lobato ◽  
Nito A Debacher ◽  
Eduardo P. de Souza
Keyword(s):  
Rate Constant ◽  
Order Rate Constant ◽  
Base Catalysis ◽  
Hydrolysis Rate ◽  
Negative Slope ◽  
General Base ◽  
Order Rate ◽  
Basic Hydrolysis ◽  
Rate Profile ◽  
Hydrolysis Of

The hydrolysis of ethyl N-ethylthioncarbamate (ETE) at 100 °C was studied in the range of 7 mol/L HCl to 4 mol/L NaOH. The pH–rate profile showed that the hydrolysis occurred through specific acid catalysis at pH < 2, spontaneous hydrolysis at pH 2–6.5, and specific basic catalysis at pH > 6.5. The Hammett acidity plot and the excess acidity plot against X were linear. The Bunnett–Olsen plot gave a negative slope indicating that the conjugate acid was less hydrated than the neutral substrate. It was concluded that the acid hydrolysis occurred by an A1 mechanism. The neutral species hydrolyzed with general base catalysis shown by the Brønsted plot with β = 0.48 ± 0.04. Water acted as a general base catalyst with (pseudo-)first-order rate constant, kN = 3.06 × 10–7 s–1. At pH > 6.5 the rate constants increased, reaching a plateau at high basicity. The basic hydrolysis rate constant of ethyl N,N-diethylthioncarbamate, which must react by a BAc2 mechanism, increased linearly at 1–3 mol/L NaOH with a second-order rate constant, k2 = 2.3 × 10–4 (mol/L)–1 s–1, which was 10 times slower than that expected for ETE. Experiments of ETE in 0.6 mol/L NaOH with an excess of ethylamine led to the formation of diethyl thiourea, presenting strong evidence that the basic hydrolysis occurred by the E1cb mechanism. In the rate-determining step, the E1cb mechanism involved the elimination of ethoxide ion from the thioncarbamate anion, producing an isothiocyanate intermediate that decomposed rapidly to form ethylamine, ethanol, and COS.Key words: alkylthioncarbamate esters, ethyl N-ethylthioncarbamate, ethyl N,N-diethylthioncarbamate, hydrolysis, mechanism.

Interactions of Heparin and a Covalently Linked Antithrombin Heparin Complex with the Components of the Fibrinolytic Pathway.

Blood ◽  
2012 ◽  
Vol 120 (21) ◽  
pp. 2211-2211
Author(s):  
Ankush Chander ◽  
Helen M Atkinson ◽  
Leslie R. Berry ◽  
Anthony K.C. Chan
Keyword(s):  
Enzyme Activity ◽  
Rate Constant ◽  
Serine Proteases ◽  
Conflicts Of Interest ◽  
Order Rate Constant ◽  
Second Order ◽  
Coagulation Cascade ◽  
Simultaneous Addition ◽  
Order Rate ◽  
Significant Difference

Abstract Abstract 2211 Introduction: Unfractionated heparin (UFH) is used for the prophylaxis and treatment of thromboembolic diseases. UFH catalyzes inhibition by antithrombin (AT) of the serine proteases in the coagulation cascade. Additionally, UFH has been shown to interact with components of the fibrinolytic pathway in vitro. However UFH has several limitations which impact its utility as a therapeutic agent. Our lab has developed a novel covalent antithrombin-heparin complex (ATH) which inhibits most serine proteases of the coagulation pathway significantly faster when compared to non covalent mixtures of AT and UFH. However, the interactions of ATH with the components of the fibrinolytic pathway have not been studied before. Thus, the present study investigates possible serpin-heparin interactions of AT + UFH vs ATH within the fibrinolytic pathway. Methods: Discontinuous second order rate constant assays under pseudo-first order conditions were carried out to obtain second order rate constant (k2) values for the inhibition of plasmin by AT+UFH versus ATH. Briefly, at specific time intervals 20 nM plasmin was inhibited by 200 nM AT + 0–5000 nM UFH or by 200 nM ATH in the presence of 2.5 mM Ca2+. Reactions were neutralized by the simultaneous addition of a solution containing polybrene and plasmin substrate S-2366™ in buffer. Residual plasmin activity was measured and the final k2 values calculated. For experiments involving tPA, wells containing 40nM tPA and increasing concentrations of AT, UFH or ATH, at mole ratios ranging from 0 to 20:1, were incubated for 15 min. Reactions with tPA were neutralized by simultaneous addition of a solution containing either polybrene and tPA substrate, S-2288™ in buffer, (ATH and UFH) or only the substrate S-2288™ in buffer (AT). Enzyme activity was then determined by measuring rate of substrate cleavage (Vmax). Results: When plasmin was inhibited by AT in the absence of UFH, k2 values of 2.82×105 +/− 4.46×104 M−1 min−1 were observed. The k2 values increased with addition of successively higher concentrations of UFH up to a plateau with maximal k2 of 5.74×106 +/− 2.78×105 M−1 min−1 at a UFH concentration of 3000nM. For inhibition of plasmin by ATH, k2 values of 6.39 × 106 +/− 5.88 × 105 M−1 min−1 were observed. Inhibition of plasmin by ATH was not significantly different when compared to the highest k2 values obtained with UFH. (p=0.36) No statistically significant difference in tPA enzyme activity was observed when Vmax values for tPA alone were compared with those in the presence of AT, UFH or ATH. (p=0.932, p=0.085, p=0.31 respectively) Significance: The characteristic shape of the curve obtained from the k2vs. UFH plot suggests that the mechanism responsible for inhibition of plasmin by AT+UFH involves conformational activation of the serpin. The k2 values in this study for inhibition of plasmin by both AT+UFH and ATH were three orders of magnitude lower than k2 values for inhibition of thrombin or factor Xa. Furthermore these results suggest that tPA is not inhibited by AT + UFH or ATH, and is not influenced by the presence of UFH alone. Cumulatively, this indicates that the fibrinolytic pathway is minimally impacted by AT + UFH or ATH, allowing maximal antithrombotic potential to be achieved during anticoagulation. Overall, the favourable anticoagulant properties of ATH combined with the findings of this study strengthens the utility of the covalent conjugate over conventional UFH for the treatment of thromboembolic disorders. Disclosures: No relevant conflicts of interest to declare.

Ambident nucleophiles. II. Nitrogen-bonded and carbon-bonded σ-complex formation in the reaction of pyrrolide anions with 1,3,5-trinitrobenzene

1982 ◽  
Vol 60 (15) ◽  
pp. 1988-1995 ◽  
Author(s):  
J. C. Halle ◽  
M. J. Pouet ◽  
M. P. Simonnin ◽  
F. Debleds ◽  
F. Terrier
Keyword(s):  
Aqueous Solution ◽  
Complex Formation ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Reactive Species ◽  
Second Order ◽  
Potassium Salts ◽  
Strong Base ◽  
Order Rate

Reaction of 1,3,5-trinitrobenzene (TNB) with pyrrole, 2,5-dimethyl pyrrole, and 2,4-dimethyl-3-ethyl pyrrole in the presence of a strong base (CH3O−) yields nitrogen- and/or carbon-bonded 1:1 σ-complexes in dimethylsulphoxide (DMSO). Depending on the stoichiometry of the reagents, 1:2 and 2:1 pyrrole–TNB diadducts are also formed. Identification of all complexes was effected by nmr. The reactive species are shown to be the pyrrolide ions and the results emphasize the ambident character of these anions towards an aromatic electrophile. Some of the complexes have been isolated as crystalline potassium salts when experiments are performed in acetonitrile. Among the isolable complexes, the kineticallybutnotthermodynamicallyfavored nitrogen adduct of pyrrole (5a) is remarkably unreactive. The second-order rate constant kH+ for is H+-catalyzed decomposition in aqueous solution is only 1 L mol−1 s−1 (t = 25 °C).

scholarly journals Kinetic studies on the nitrite reductase of Wolinella succinogenes

1986 ◽  
Vol 233 (2) ◽  
pp. 553-557 ◽  
Author(s):  
R Blackmore ◽  
T Brittain
Keyword(s):  
Rate Constant ◽  
Nitrite Reductase ◽  
Flash Photolysis ◽  
Spectral Characteristics ◽  
Order Rate Constant ◽  
Second Order ◽  
Stopped Flow ◽  
Dependent Manner ◽  
Wolinella Succinogenes ◽  
Order Rate

The six haem groups of the nitrite reductase enzyme isolated from Wolinella succinogenes are rapidly reduced by the addition of dithionite (S2O4(2-)). The reduction, however, is not homogeneous. Two of the haem groups, namely those that show spectral characteristics typical of five-co-ordinated haem groups, are reduced in a dithionite-concentration-dependent fashion with a rate limit of 1.5 S-1. The other four haem groups, which show spectral characteristics very similar to those of normal six-co-ordinate c-haem groups, reduce in a linear dithionite-concentration-dependent manner with a second-order rate constant of 150 M-1/2 X S-1. The ratio of the amplitudes of the two reduction phases observed in stopped-flow studies is found to be dependent on the concentration of dithionite used. A model is proposed to account for these observations, and computer simulations show that the model represents a good fit to the experimental data. The two haem groups with five-co-ordinate spectral characteristics bind CO. Flash photolysis of the CO complex exhibits one major recombination process with a linear dependence in rate on CO concentration with a second-order rate constant of 2 × 106 M-1 × S-1. By contrast, stopped-flow mixing of the reduced protein with CO shows a very complex pattern of combination, with most of the observed absorbance change associated with a concentration-independent step. These findings are rationalized in terms of structural changes in the protein consequent to ligand binding.

Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

1984 ◽  
Vol 62 (3) ◽  
pp. 580-585 ◽  
Author(s):  
Parminder S. Surdhar ◽  
Rizwan Ahmad ◽  
David A. Armstrong
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Order Rate Constant ◽  
Second Order ◽  
Radical Species ◽  
Spectral Changes ◽  
Order Rate ◽  
Visible Spectra ◽  
Uv Visible ◽  
Presence And Absence

Spectral changes and rates of reaction of flavins and several radical species have been investigated at pH 7, 9, and 10 in the presence and absence of β-mercaptoethanol. The radicals •CO2−, eaq−, and [Formula: see text] reacted with FAD at pH 10 to give a spectrum of FAD •Fl− with rate constants of 7 ± 1 × 108 and 4 ± 1 × 108 M−1 s−1 for •CO2− and [Formula: see text] respectively. At pH 7 only •FlH was observed and at pH 9 a mixture of •FIH and •Fl−.Interactions between flavin radicals and sulphydryl at 10−4 M concentration did not cause perturbations in the uv–visible spectra until either the radical and/or the sulphydryl were ionized. With FAD at pH 9 or 10 and LFl at pH 10 the 370 nm peak of •Fl− was enhanced by about 15% and a second larger growth occurred near 450 nm in the presence of 10−4 to 10−2 M sulphydryl. We attribute this to the formation of labile intermediate RSHFl•−, which must also be involved in the reduction of Fl by [Formula: see text] at pH 9 or 10.The second order rate constant k13 for reaction of [Formula: see text] with FAD at pH 9 and 10 was found to be 4.2 ± 0.5 × 108 M−1 s−1 and 2.0 ± 0.4 × 108 M−1 s−1 respectively. The rate constant for the reaction between [Formula: see text] and LFl at pH 10 was slightly faster, 7 ± 1 × 108 M−1 s−1, probably reflecting the fact that LFl lacks the bulky negatively charged adenine dinucleotide group of FAD.

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