Dependence Of Kobs Research Articles

Kinetics and mechanism of the one‐step, two‐electron oxidation of diaquadichloro(1,10‐phenanthroline)chromium(III) chloride dihydrate by periodate to chromium(V) in aqueous acidic solutions

AbstractThe kinetics of oxidation of [CrIIICl2(phen)(H2O)2]Cl⋅2H2O (phen = 1,10‐phenanthroline) by periodate {I(VII)} have been studied in aqueous acidic solutions. The oxidation product has been isolated and characterized by elemental analysis, IR, and ESR as [CrVCl2(phen)(O)]Cl⋅1.5H2O. Under pseudo–first‐order conditions, the reaction showed first‐order dependence on the initial Cr(III) complex concentration. At fixed reaction conditions, the dependence of kobs on [I(VII)] showed saturation kinetics. The rate constant, kobs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the most reactive species. A plot of kobs([H+] + K1) versus [H+] is linear with an intercept, indicating that the reaction proceeds via two parallel pathways. One pathway is independent of [H+] and the other on [H+]–1. Since water is known to be a very weak bridging ligand, the pathway involving the aqua form of the complex, almost certainly, is bridged to periodate via the chloro ligand of the chromium(III) complex. The thermodynamic parameters of the processes involved are reported. A one‐step two‐electron transfer via an inner‐sphere mechanism has been assigned to the two pathways based on the form of the rate law

Kinetics and mechanism of the two electron oxidation of cis-CrIII(dipy)2(H2O)2]3+ (dipy = 2,2′-dipyridyl) by periodate ion to cis-[CrV(dipy)2(O)2]+ in aqueous acidic solutions

The kinetics of oxidation of cis-[CrIII(dipy)2(H2O)2]3+ (dipy = 2,2′-dipyridyl) by IO4 − has been studied in aqueous acidic solutions. The initial oxidation product is certainly not Cr(VI). The isolated solid product is consistent with the formula cis-[CrV(dipy)2(O)2]IO4. The Cr(V) product has been isolated and characterized by elemental analysis, IR and ESR spectroscopy. In the presence of a vast excess of [IO4 −], the reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k obs, showed a very small change with increasing [IO4 −]. The dependence of k obs on [IO4 −] is consistent with Eq. (i). i $$ {\text{k}}_{\text{obs}} = {\text{a}}[{\text{IO}}_{4}^{\text{ - }} ]/({\text{b}} + {\text{c}}[{\text{IO}}_{4}^{\text{ - }} ]) $$ The pseudo-first-order rate constant, k obs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are also reported.

Kinetic studies on H+-catalyzed aquation and hydrogen peroxide oxidation of tris-asparaginatochromium(III)

Tris-asparaginatochromium(III), [Cr(Asn)3]0 (where Asn forms a 5-membered chelate ring via amine nitrogen and α-carboxylate oxygen atoms) and its mono- and diaqua-derivatives were obtained, and their acid-catalyzed aquation was studied. The first reaction for [Cr(Asn)3]0 and [Cr(Asn)2(H2O)2]+ is the chelate ring opening at the Cr-NH2 bond, leading to metastable intermediates. Kinetics of these processes were studied spectrophotometrically in 0.1–1.0 M HClO4 at 303 and 333 K, respectively. A linear dependence of kobs on [H+], kobs = a + b[H+] was determined for both the complexes. Additionally, oxidation of chromium(III) to chromate(VI) by hydrogen peroxide was studied. The process proceeds through a chromium(V) intermediate, which is next transformed, in faster parallel steps into CrO42− and [Cr(O2)2]3− anions. The latter species, a chromium(V)-peroxo complex, is metastable under a large excess of H2O2. Kinetics of oxidation of [Cr(Asn)3]0 were studied at 298 K, at constant [OH−], within 0.2–1.0 M H2O2 range. A linear dependence of kobs on H2O2 was established. A mechanism is proposed, where the rate-determining step is an inner sphere 2-electron transfer within a precursor chromium(III) complex with coordinated O2H− anion of the [Cr(Asn)2(OH)(HO2)]− formula. EPR results provided clear evidence for formation of a relatively stable tetrakis(η2-peroxo)chromate(V) complex, [Cr(O2)4]3−.

Spectrometric Study of AOT-Hydrolysis Reaction in Water/AOT/Isooctane Microemulsions Using Phenolphthalein as a Chemical Probe

The kinetics of the alkaline hydrolysis of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in water/AOT/isooctane microemulsions has been studied by monitoring the absorbance change of the phenolphthalein in the system with time. The apparent first-order rate constant k(obs) has been obtained and found to be dependent on both the molar ratio of water to AOT ω and the temperature. The dependences of k(obs) on ω have been analyzed by a pseudophase model which gives the true rate constants k(i) of the AOT-hydrolysis reaction on the interface and the partition coefficients K(wi) for the distribution of OH(-) between aqueous and interface pseudophases at various temperatures; the latter is almost independent of the temperature and ω. The temperature dependences of the reaction rate constants k(obs) and k(i) have been analyzed to obtain enthalpy ΔH(≠), entropy ΔS(≠), and energy E(a) of activation, which indicate that the distribution of OH(-) between aqueous and interface pseudophases increases ΔS(≠) but makes no contribution to E(a) and ΔH(≠). The influence of the overall concentration of AOT in the system on the rate constant has been examined and found to be negligible. It contradicts with what was reported by García-Río et al. (1) but confirms that the first-order reaction of the AOT-hydrolysis takes place on the surfactant interface. The study of the influence of AOT-hydrolysis on the kinetics of the alkaline fading of crystal violet or phenolphthalein in the water/AOT/isooctane microemulsions suggests that corrections for the AOT-hydrolysis in these reactions are required.

One-step, two-electron oxidation of cis -diaquabis(1,10-phenanthroline)chromium(III) to cis -dioxobis(1,10-phenanthroline)chromium(V) by periodate in aqueous acidic solutions

The kinetics of oxidation of cis-[CrIII(phen)2(H2O)2]3+ (phen = 1,10-phenanthro- line) by IO4− has been studied in aqueous acidic solutions. In the presence of a vast excess of [IO4−], the reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, kobs, showed a very small change with increasing [IO4−]. The dependence of kobs on [IO4−] is consistent with Eq. (i). (i) The pseudo-first-order rate constant, kobs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are also reported. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 563–568, 2011

Potential biochromium sources; kinetic studies on acid- and base-catalyzed aquation of [Cr(ox)2(2-(aminomethyl)pyridine

Acid- and base-catalyzed hydrolysis of [Cr(ampy)(ox)2]−, where ampy = 2-(aminomethyl)pyridine, leads to successive dissociation of the ligands via concurrent reaction paths, whereas at pH 1–9 only ampy is liberated as a result of spontaneous processes. The first ligand dissociation proceeds via aqua intermediates with one-end bonded ampy (1) or ox ligands (2), respectively, which in alkaline media undergo rapid deprotonation to give the appropriate hydroxo-forms. The kinetics of two reaction stages, namely the chelate ring opening and the ligand liberation, were studied spectrophotometrically. In acidic media, the first stage is much faster than the second, whereas in alkaline solutions, both the stages are characterized by similar rate constants. The dependences of kobs on [H+] are as follows: kobs1,H = a1 + b1/[H+], kobs2,H = a2 + b2[H+]. At pH > 13, rate constants kobs1,OH and kobs2,OH are [OH−] independent. The effect of pH on the complex reactivity was rationalized based on proposed mechanisms.

Asymmetric Amino Acid Activation by Class II Histidyl-tRNA Synthetase from Escherichia coli

Aminoacyl-tRNA synthetases (ARSs) join amino acids to their cognate tRNAs to initiate protein synthesis. Class II ARS possess a unique catalytic domain fold, possess active site signature sequences, and are dimers or tetramers. The dimeric class I enzymes, notably TyrRS, exhibit half-of-sites reactivity, but its mechanistic basis is unclear. In class II histidyl-tRNA synthetase (HisRS), amino acid activation occurs at different rates in the two active sites when tRNA is absent, but half-of-sites reactivity has not been observed. To investigate the mechanistic basis of the asymmetry, and explore the relationship between adenylate formation and conformational events in HisRS, a fluorescently labeled version of the enzyme was developed by conjugating 7-diethylamino-3-((((2-maleimidyl)ethyl)amino)carbonyl)coumarin (MDCC) to a cysteine introduced at residue 212, located in the insertion domain. The binding of the substrates histidine, ATP, and 5'-O-[N-(l-histidyl)sulfamoyl]adenosine to MDCC-HisRS produced fluorescence quenches on the order of 6-15%, allowing equilibrium dissociation constants to be measured. The rates of adenylate formation measured by rapid quench and domain closure as measured by stopped-flow fluorescence were similar and asymmetric with respect to the two active sites of the dimer, indicating that conformational change may be rate-limiting for product formation. Fluorescence resonance energy transfer experiments employing differential labeling of the two monomers in the dimer suggested that rigid body rotation of the insertion domain accompanies adenylate formation. The results support an alternating site model for catalysis in HisRS that may prove to be common to other class II aminoacyl-tRNA synthetases.

Covalently Bound Substrate at the Regulatory Site of Yeast Pyruvate Decarboxylases Triggers Allosteric Enzyme Activation

The mechanism by which the enzyme pyruvate decarboxylase from two yeast species is activated allosterically has been elucidated. A total of seven three-dimensional structures of the enzyme, of enzyme variants, or of enzyme complexes from two yeast species, three of them reported here for the first time, provide detailed atomic resolution snapshots along the activation coordinate. The prime event is the covalent binding of the substrate pyruvate to the side chain of cysteine 221, thus forming a thiohemiketal. This reaction causes the shift of a neighboring amino acid, which eventually leads to the rigidification of two otherwise flexible loops, one of which provides two histidine residues necessary to complete the enzymatically competent active site architecture. The structural data are complemented and supported by kinetic investigations and binding studies, providing a consistent picture of the structural changes occurring upon enzyme activation.

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol by cerium(IV) in sulfuric acid media

The kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol (THFA) by cerium(IV) in sulfuric acid media have been investigated spectrophotometrically in the temperature range 298–313 K. It is found that the reaction is first-order with respect to CeIV, and exhibits a positive fractional order with respect to THFA and RuIII. The pseudo first-order ([THFA]≫[CeIV]≫[RuIII]) rate constant kobs decreases with the increase of [HSO4−]. Under the protection of nitrogen, the reaction system can initiate polymerization of acrylonitrile, indicating the generation of free radicals. On the basis of the experimental results, a reasonable mechanism has been proposed and the rate equations derived from the mechanism can explain all the experimental results. From the dependence of kobs on the concentration of HSO4−, \({{\rm Ce}({\rm SO}_{4})_{2}}\) has been found as the kinetically active species. Furthermore, the rate constants of the rate determining step together with the activation parameters were evaluated.

Effects of Micellar Aggregates on the Kinetics and Mechanism of the Reaction between 4‐Nitrobenzenediazonium Ions and Some Amino Acids

AbstractWe have explored the kinetics and mechanism of the reaction between 4‐nitrobenzenediazonium ions (4NBD), and the hydrophilic amino acids (AA) glycine and serine in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by means of UV/VIS spectroscopy. The observed rate constants kobs were obtained by monitoring the disappearance of 4NBD with time at a suitable wavelength under pseudo‐first‐order conditions. In aqueous acid (buffer‐controlled) solution, in the absence of SDS, the dependence of kobs on [AA] was obtained from the linear relationship found between the experimental rate constant and [AA]. At a fixed amino acid concentration, kobs values show an inverse dependence on acidity in the range of pH 5–6, suggesting that the reaction takes place through the nonprotonated amino group of the amino acid. All kinetic evidence is consistent with an irreversible bimolecular reaction with k=2390±16 and 376±7 M−1 s−1 for glycine and serine, respectively. Addition of SDS inhibits the reaction because of the micellar‐induced separation of reactants originated by the electrical barrier imposed by the SDS micelles; kobs values are depressed by factors of 10 (glycine) and 6 (serine) on going from [SDS]=0 up to [SDS]=0.05M. The hypothesis of a micellar‐induced separation of the reactants was confirmed by 1H‐NMR spectroscopy, which was employed to investigate the location of 4NBD in the micellar aggregate: the results showed that the aromatic ring of the arenediazonium ion is predominantly located in the vicinity of the C(β) atom of the surfactant chain, and hence the reactive N$\rm{ _2^ + }$ group is located in the Stern layer of the micellar aggregate. The kinetic results can be quantitatively interpreted in terms of the pseudophase kinetic model, allowing estimations of the association constant of 4NBD to the SDS micelles.

Molecular Determinants for the Selective Inhibition of Cyclooxygenase-2 by Lumiracoxib

Lumiracoxib is the first example of a marketed COX-2 inhibitor of the arylacetic acid class, and it is reported to be the most selective COXIB in vivo. However, the molecular basis of its COX-2 inhibition has not been completely defined. Using standard assays, lumiracoxib was found to be a poor inhibitor of purified ovine COX-1 and a relatively weak inhibitor of purified human COX-2. The extent of COX-2 inhibition plateaued at around 50% and suggested that the inhibitor may be reversibly bound to the enzyme. Kinetic studies with lumiracoxib demonstrated that it was a time-dependent and slowly reversible inhibitor of human COX-2 that exhibited at least two binding steps during inhibition. Derivatives of lumiracoxib were synthesized with or without the methyl group on the phenylacetic acid ring and with various substitutions on the lower aniline ring. Inhibition studies demonstrated that the methyl group on the phenylacetic acid ring is required for COX-2 selectivity. The chemical identity and position of the substituents on the lower aniline ring were important in determining the potency and extent of COX inhibition as well as COX-2 selectivity. Mutation of Ser-530 to Ala or Val-349 to Ala or Leu abolished the potent inhibition observed with wild-type human COX-2 and key lumiracoxib analogs. Interestingly, a Val-349 to Ile mutant was inhibited with equal potency to human COX-2 with 2,6-dichloro-, 2,6-dimethyl-, or 2-chloro-6-methyl-substituted inhibitors and, in the case of lumiracoxib, actually showed an increase in potency. Taken together with a recent crystal structure of a lumiracoxib-COX-2 complex, the kinetic analyses presented herein of the inhibition of mutant COX-2s by lumiracoxib allows the definition of the molecular basis of COX-2 inhibition.

Rapid Kinetics of Na+ Binding to Thrombin

The kinetic mechanism of Na(+) binding to thrombin was resolved by stopped-flow measurements of intrinsic fluorescence. Na(+) binds to thrombin in a two-step mechanism with a rapid phase occurring within the dead time of the spectrometer (<0.5 ms) followed by a single-exponential slow phase whose k(obs) decreases hyperbolically with increasing [Na(+)]. The rapid phase is due to Na(+) binding to the enzyme E to generate the E:Na(+) form. The slow phase is due to the interconversion between E(*) and E, where E(*) is a form that cannot bind Na(+). Temperature studies in the range from 5 to 35 degrees C show significant enthalpy, entropy, and heat capacity changes associated with both Na(+) binding and the E to E(*) transition. As a result, under conditions of physiologic temperature and salt concentrations, the E(*) form is negligibly populated (<1%) and thrombin is almost equally partitioned between the E (40%) and E:Na(+) (60%) forms. Single-site Phe mutations of all nine Trp residues of thrombin enabled assignment of the fluorescence changes induced by Na(+) binding mainly to Trp-141 and Trp-215, and to a lesser extent to Trp-148, Trp-207, and Trp-237. However, the fast phase of fluorescence increase is influenced to different extents by all Trp residues. The distribution of these residues over the entire thrombin surface demonstrates that Na(+) binding induces long-range effects on the structure of the enzyme as a whole, contrary to the conclusions drawn from recent structural studies. These findings elucidate the mechanism of Na(+) binding to thrombin and are relevant to other clotting factors and enzymes allosterically activated by monovalent cations.

Vertebrate Myosin VIIb Is a High Duty Ratio Motor Adapted for Generating and Maintaining Tension

Kinetic adaptation of muscle and non-muscle myosins plays a central role in defining the unique cellular functions of these molecular motor enzymes. The unconventional vertebrate class VII myosin, myosin VIIb, is highly expressed in polarized cells and localizes to highly ordered actin filament bundles such as those found in the microvilli of the intestinal brush border and kidney. We have cloned mouse myosin VIIb from a cDNA library, expressed and purified the catalytic motor domain, and characterized its actin-activated ATPase cycle using quantitative equilibrium and kinetic methods. The myosin VIIb steady-state ATPase activity is slow (approximately 1 s(-1)), activated by very low actin filament concentrations (K(ATPase) approximately 0.7 microm), and limited by ADP release from actomyosin. The slow ADP dissociation rate constant generates a long lifetime of the strong binding actomyosin.ADP states. ADP and actin binding is uncoupled, which enables myosin VIIb to remain strongly bound to actin and ADP at very low actin concentrations. In the presence of 2 mm ATP and 2 microm actin, the duty ratio of myosin VIIb is approximately 0.8. The enzymatic properties of actomyosin VIIb are suited for generating and maintaining tension and favor a role for myosin VIIb in anchoring membrane surface receptors to the actin cytoskeleton. Given the high conservation of vertebrate class VII myosins, deafness phenotypes arising from disruption of normal myosin VIIa function are likely to reflect a loss of tension in the stereocilia of inner ear hair cells.

Loop 1 of Transducer Region in Mammalian Class I Myosin, Myo1b, Modulates Actin Affinity, ATPase Activity, and Nucleotide Access

Loop 1, a flexible surface loop in the myosin motor domain, comprises in part the transducer region that lies near the nucleotide-binding site and is proposed from structural studies to be responsible for the kinetic tuning of product release following ATP hydrolysis (1). Biochemical studies have shown that loop 1 affects the affinity of actin-myosin-II for ADP, motility and the V(max) of the actin-activated Mg2+-ATPase activity, possibly through P(i) release (2-8). To test the influence of loop 1 on the mammalian class I myosin, Myo1b, chimeric molecules in which (i) loop 1 of a truncated form of Myo1b, Myo1b1IQ, was replaced with either loop 1 from other myosins; (ii) loop 1 was replaced with glycine; or (iii) some amino acids in the loop were substituted with alanine and were expressed in baculovirus, and their interactions with actin and nucleotide were evaluated. The steady-state actin-activated ATPase activity; rate of ATP-induced dissociation of actin from Myo1b1IQ; rate of ADP release from actin-Myo1b1IQ; and the affinity of actin for Myo1b1IQ and Myo1b1IQ.ADP differed in the chimeras versus wild type, indicating that loop 1 has a much wider range of effects on the coupling between actin and nucleotide binding events than previously thought. In particular, the biphasic ATP-induced dissociation of actin from actin-Myo1b1IQ was significantly altered in the chimeras. This provided evidence that loop 1 contributes to the accessibility of the nucleotide pocket and is involved in the integration of information from the actin-, nucleotide-, gamma-P(i)-, and calmodulin-binding sites and predicts that loop 1 modulates the load dependence of the motor.

Kinetics of base‐catalysed ring closure of methyl 2,6‐dinitrophenylsulfanylethanoate

AbstractThe base‐catalysed ring closure of methyl 2,6‐dinitrophenylsulfanylethanoate gives 2‐methoxycarbonyl‐7‐nitrobenzo[d]thiazole‐3‐oxide. The kinetics of this ring closure were studied by means of UV–visible spectrophotometry in methanolic buffers of N‐methylmorpholine–N‐methylmorpholinium chloride and N‐methylpiperidine–N‐methylpiperidinium chloride at 25°C at ionic strength I = 0.1 mol l−1. The dependences of kobs on the base concentration are not linear, their slopes decreasing with increasing base concentration at constant ratio of the buffer components. The reaction mechanism was suggested on the basis of comparison with the ring closure kinetics of methyl 2,4,6‐trinitrophenylsulfanylethanoate. The rate‐limiting step of the reaction sequence consists in formation of the conjugated base of substrate 1. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Additivity in Effects of Vitronectin and Monoclonal Antibodies against α-Helix F of Plasminogen Activator Inhibitor-1 on Its Reactions with Target Proteinases

The serpin plasminogen activator inhibitor-1 (PAI-1) is a potential therapeutic target in cardiovascular and cancerous diseases. PAI-1 circulates in blood as a complex with vitronectin. A PAI-1 variant (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-3-diazole (NBD) P9 PAI-1) with a fluorescent tag at the reactive center loop (RCL) was used to study the effects of vitronectin and monoclonal antibodies (mAbs) directed against alpha-helix F (Mab-2 and MA-55F4C12) on the reactions of PAI-1 with tissue-type and urokinase-type plasminogen activators. Both mAbs delay the RCL insertion and induce an increase in the stoichiometry of inhibition (SI) to 1.4-9.5. Binding of vitronectin to NBD P9 PAI-1 does not affect SI but results in a 2.0-6.5-fold decrease in the limiting rate constant (klim) of RCL insertion for urokinase-type plasminogen activator at pH 6.2-8.0 and for tissue-type plasminogen activator at pH 6.2. Binding of vitronectin to the complexes of NBD P9 PAI-1 with mAbs results in a decrease in klim and in a 1.5-22-fold increase in SI. Thus, vitronectin and mAbs demonstrated additivity in the effects on the reaction with target proteinases. The same step in the reaction mechanism remains limiting for the rate of RCL insertion in the absence and presence of Vn and mAbs. We hypothesize that vitronectin, bound to alpha-helix F on the side opposite to the epitopes of the mAbs, potentiates the mAb-induced delay in RCL insertion and the associated substrate behavior by selectively decreasing the rate constant for the inhibitory branch of PAI-1 reaction (ki). These results demonstrate that mAbs represent a valid approach for inactivation of vitronectin-bound PAI-1 in vivo.

A kinetic study of the oxidation of 2-pyridinemethanol, 2-pyridinecarboxaldehyde and 2,6-pyridinedimethanol by chromium(VI) in acidic aqueous media

Oxidation of 2-pyridinemethanol (2-pyol), 2,6-pyridinedimethanol (2,6-pydol) and 2-pyridinecarboxaldehyde (2-pyal) by CrVI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H+ aq concentrations; [CrVI] = 8 × 10−4 M, [reductant] = 0.025–0.20 M, [HClO4] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A parabolic dependence of the pseudo-first-order rate constant (k obs) versus [H+] was observed for all the reductants. A linear dependence of k obs on [2,6-pydol] and, unusually, higher than first-order dependence on [2-pyol] and [pyal] was established. The apparent activation parameters for reactions studied at constant [H+] at I = 1.2 and 2.1 M were determined. The presence of chromium species at the intermediate oxidation states: CrV, CrIV and CrII, was deduced based on e.s.r. measurements and the kinetic effects of MnII or O2 (Ar), respectively. Comparison of the available second-order rate constants for aromatic alcohols and aldehydes demonstrated that chelating abilities of the reductant facilitates the redox process, whereas the electron-withdrawing effect caused by protonating the pyridine nitrogen atom acts in the opposite direction. The unusual low reactivity of 2-pyol was ascribed to intramolecular hydrogen bond formation.

Kinetics of Alkaline Hydrolysis of a Polyimide Surface

The alkaline hydrolysis of a polyimide (PMDA-ODA) surface was studied as a function of time, temperature and hydroxide ion concentration. Quantification of the number of carboxylic acid groups formed on the modified polyimide surface was accomplished by analysis of data from contact angle titration experiments. Using a large excess of base, pseudo-first-order kinetics were found, yielding kobs ≈ 0.1−0.9 min-1 for conversion of polyimide to poly(amic acid) depending on [OH-]. From the dependence of kobs on [OH-], a rate equation is proposed. Conversion of the polyimide surface to one of poly(amic acid) was found to reach a limiting value with a formation constant, K, in the range 2−10 L·mol-1.

Kinetics of the pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous micellar solutions: effects of the charge and structure of the surfactant

The pH-independent hydrolysis of 4-nitrophenyl chloroformate in the presence of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlorides (alkyl group = cetyl and dodecyl) and polyoxyethylene (9) nonylphenyl ether was studied spectrophotometrically. The observed rate constants, kobs, decrease in the following order: cationic micelles > bulk water > non-ionic micelles > anionic micelles. Surfactant–substrate association constants, Ks, were determined from the dependence of kobs on surfactant concentration, and were found to be only slightly dependent on the charge of the surfactant and, for similarly charged micelles, on the length of their hydrophobic tail. A 1H NMR study of the solubilization of a model compound, 4-nitrophenyl chloroacetate, showed that all surfactant segments are affected by the solubilizate and the effect is more pronounced toward the middle of the hydrocarbon chain. The average solubilization site of the acetate ester does not depend on the charge of the micelle or the length of the surfactant hydrophobic tail. Micellar effects on observed rate constants are analyzed in terms of a ‘medium’ effect and an ‘electrostatic’ effect. The lower microscopic polarity at the reaction site retards the reaction, whereas electrostatic interactions of the polar transition state with the charged interface result in a rate decrease by anionic micelles and a rate enhancement by cationic micelles. Copyright © 1999 John Wiley & Sons, Ltd.

Kinetics of Base-Catalysed Hydrolysis and Cyclisation of Substituted Acetamide and Benzamide O-(Phenoxycarbonyl)oximes

The reaction kinetics of acetamide O-(4-nitrophenoxycarbonyl)oxime have been studied in aqueous buffers at pH 2-11. At pH &gt; 9, the pH dependence of kobs is linear with slope 1, the cyclisation to 3-methyl-1,2,4-oxadiazol-5(4H)-one and 4-nitrophenol being the only reaction. At pH &lt; 7.5, the only reaction is the hydrolysis giving 4-nitrophenol and acetamidoxime. The dependence of kobs on pH has been used to determine the rate equation and to propose the reaction mechanism. The cyclisation kinetics of substituted benzamide O-(phenoxycarbonyl)oximes have been studied in the pH range from 9.25 to 11. The reaction mechanism has been proposed based on the ρ constants found. In the first reaction step, the proton is split off from the NH2 group; the subsequent, rate-limiting step involves simultaneous N-C bond formation and C-O bond splitting.