Catalytic properties of recombinant class a TEM-1 β-lactamase and its inhibition by sulbactam,tazobactam, and clavulanic acid

A homogeneous preparation of recombinant class A TEM-1 β-lactamase was used in various expression constructs as a genetic marker to provide cell resistance to ampicillin; its kinetic parameters (K _M = 22 μM, V = 0.39 μM/s, k _cat = 31.2 s⁻¹, k _cat/K _M =1/4 μM/s⁻¹) were determined using the chromogenic substrate CENTA. Comparative analysis of the obtained K _M value and the literature data demonstrated that the recombinant enzyme is 3 times more specific against the CENTA substrate than the native enzyme (K _M = 70 μM). Competitive inhibition of recombinant β-lactamase by sulbactam, tazobactam, and clavulanic acid was demonstrated. The CENTA inhibition constants for sulbactam, tazobactam, and clavulanic acid (K _{I (sulbactam)} = 0.43 μM, K _{I (tazobactam)} = 0.041 μM, and K _{I (clavulanic acid)} = 0.046 μM) were determined for the first time. It was shown that tazobactam and clavulanic acids are the most efficient inhibitors of recombinant β-lactamase and produced the same inhibitory effect.     

A quartz crystal microbalance study of β-cyclodextrin self assembly on gold and complexation of immobilized β-cyclodextrin with adamantane derivatives

Quartz crystal microbalance (QCM) was used to study the self-assembly of per-6-thio-β-cyclodextrin (t₇-βCD) on gold surfaces, and the subsequent inclusion interactions of immobilized βCD with adamantane-poly(ethylene glycol) (5,000 MW, AD-PEG), 1-adamantanecarboxylic acid (AD-C) and 1-adamantylamine (AD-A). From a 50 μM solution of t₇-βCD in 60:40 DMSO:H₂O, a t₇-βCD layer was formed on gold with surface density of 71.7 ± 2.7 pmol/cm², corresponding to 80 ± 3% of close-packed monolayer coverage. Gold sensors with immobilized t₇-βCD were then exposed alternately to six different concentrations of AD-PEG, 500 μM AD-C or 500 μM AD-A aqueous solutions for association, and water for dissociation. Association of AD-PEG conformed to a Langmuir isotherm, with a best fit equilibrium constant K = 125,000 ± 18,000 M⁻¹. For AD-C and AD-A, association (k _a ) and dissociation (k _d ) rate constants were extracted from kinetic profiles by fitting to the Langmuir model, and equilibrium constants were calculated. The parameters for AD-C were found to be: k _a = 100 ± 5 M⁻¹ s⁻¹, k _d = 110 (±18) × 10⁻⁴ s⁻¹, and K = 9,400 ± 1,700 M⁻¹. For AD-A, k _a = 58 ± 6 M⁻¹ s⁻¹, k _d = 154 (±7) × 10⁻⁴ s⁻¹, and K = 3,800 ± 400 M⁻¹. The results demonstrate the utility of QCM as a tool for studying small molecule surface adsorption and guest–host interactions on surfaces. More specifically, the kinetic and thermodynamic data of AD-C, AD-A, and AD-PEG inclusion with immobilized t₇-βCD form a basis for further surface association studies of AD-X conjugates to advance surface sensory and coupling applications.     

Structure-function relationships of a catalytically efficient β-<Emphasis Type="SmallCaps">D</Emphasis>-xylosidase

β-d-Xylosidase from Selenomonas ruminantium is revealed as the best catalyst known (k _cat, k _cat/K _m) for promoting hydrolysis of 1,4-β-d-xylooligosaccharides. ¹H nuclear magnetic resonance experiments indicate the family 43 glycoside hydrolase acts through an inversion mechanism on substrates 4-nitrophenyl-β-d-xylopyranoside (4NPX) and 1,4-β-d-xylobiose (X2). Progress curves of 4-nitrophenyl-β-d-xylobioside, xylotetraose and xylohexaose reactions indicate that one residue from the nonreducing end of substrate is cleaved per catalytic cycle without processivity. Values of k _cat and k _cat/K _m decrease for xylooligosaccharides longer than X2, illustrating the importance to catalysis of subsites −1 and +1 and the lack there of subsite +2. Homology models of the enzyme active site with docked substrates show that subsites bey ond−1 are blocked by protein and subsites bey ond +1 are not formed; they suggest that D14 and E186 serve catalysis as general base and general acid, respectively. Individual mutations, D14A and E186A, erode k _cat and k _cat/K _m by <10³ and to asimilar extent for substrates 4NPX and 4-nitrophenyl-α-l-arabinofuranoside (4NPA), indicating that the two substrates share the same active site. With 4NPX and 4NPA, pH governs k _cat/K _m with pK _a values of 5.0 and 7.0 assigned to D14 and E186, respectively. k _cat (4NPX) has a pK _a value of 7.0 and k _cat (4NPA) is pH independent above pH 4.0, suggesting that the catalytically inactive, “dianionic” enzyme form (D14-E187-) binds 4NPX but not 4NPA. The mention of firm names or trade products does not imply that they are end orsed or recommended by the US Department of Agriculture over other firms or similar products not mentioned.     

A Laccase of <Emphasis Type="Italic">Fomes durissimus</Emphasis> MTCC-1173 and Its Role in the Conversion of Methylbenzene to Benzaldehyde

A laccase has been purified from the liquid culture growth medium containing bagasse particles of Fomes durissimus. The method involved concentration of the culture filtrate by ultrafiltration and anion exchange chromatography on diethyl aminoethyl cellulose. The sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS-PAGE) and native polyacrylamide gel electrophoresis both gave single protein band indicating that the enzyme preparation was pure. The molecular mass of the purified laccase determined from SDS-PAGE analysis was 75 kDa. Using 2,6-dimethoxyphenol as the substrate, the determined K _m and k _cat values of the laccase are 182 μM and 0.35 s⁻¹, respectively, giving a k _cat/K _m value of 1.92 × 10³ M⁻¹ s⁻¹. The pH and temperature optimum were 4.0 and 35 °C, respectively. The purified laccase has yellow colour and does not show absorption band around 610 nm found in blue laccases. Moreover, it transformed methylbenzene to benzaldehyde in the absence of mediator molecules, property exhibited by yellow laccases.     

Aspects of antibody-catalyzed primary amide hydrolysis

Because there are many known C-terminally amidated peptides of biological importance, there is great potential in medicine and organic synthesis for antibodies that catalyze primary amide bond hydrolysis or formation. We characterized a catalytic antibody, 13D11, raised to a phosphinate hapten, that hydrolyzed the primary amide of a dansyl-alkylated derivative of (R)-phenylalaninamide (DNS-(R)F-NH₂). At pH 9.0, 13D11 hydrolyzed DNS-(R)F-NH₂ with a k_cat of 1.65 × 10^-7 s^-1 (k_cat/k_uncat = 132) and a K_m of 432 μM, and was stereospecifically hapten-inhibited (K_i = 14.0 μM). Control experiments indicated that the catalytic activity was not the result of a contaminating protease. In accordance with the hapten being a transition-state analog of base hydrolysis, the rate of DNS-(R)F-NH₂ hydrolysis increased with hydroxide concentration to an optimum pH of 9.5. Above pH 9.5, activity declined rapidly suggesting the antibody was inactivated during the long incubation period. This work demonstrates the feasibility of generating catalytic antibodies to hydrolyze unactivated amide bonds without cofactor assistance.     

Protolytic properties and complexation with copper(II) ions of some azo derivatives of β-arylaminopropionic acids

The azo coupling reaction of N-(2-carboxyethyl)anthranilic acid and N,N,N′,N′-tetrabis(2-carboxyethyl)-1,3-phenylenediamine with diazosulfanilic acid yielded the complexones sodium 4-N-(2-carboxyethyl)amino-5-carboxyazobenzene-4′-sulfonate (I) and 2,4-N,N,N′,N′-tetrabis(2-carboxyethyl)diaminoazobenzene-4′-sulfonic acid (II), respectively. The acidity constants of I and II (20°C, μ = 0.1M KCl) were determined to be as follows: for I, pK ₀₀ = 1.29 ± 0.13, pK ₀ = 2.92 ± 0.07, pK ₁ = 3.92 ± 0.05, pK ₂ = 5.16 ± 0.03; for II, pK ₀₀ = 2.35 ± 0.06, pK ₀ = 2.81 ± 0.09, pK ₁ = 3.21 ± 0.11, pK ₂ = 3.81 ± 0.09, pK ₃ = 4.34 ± 0.04, pK ₄ = 5.03 ± 0.06, pK ₅ = 6.67 ± 0.07. The electronic absorption spectra of I and II were measured, and acid-base equilibrium scheme for I and II in aqueous solutions were suggested. The complexation constants of I and II with copper(II) ions were determined to be logK _CuQ^I= 5.47 ± 0.06 and logK _CuQ^II= 5.72 ± 0.13 (20°C, μ = 0.1 M KCl).     

The modes of complexation of benzimidazole with aqueous β-cyclodextrin explored by phase solubility, potentiometric titration, 1H-NMR and molecular modeling studies

The results of rigorous modeling of phase solubility diagrams, pH solubility profiles and potentiometric titrations revealed the following for benzimidazole (BZ) and BZ/β-CD complexation in aqueous solution: (a) the pK _a value of BZ estimated at 5.66 ± 0.08 was reduced to 5.33 ± 0.06 in the presence of 15 mM β-CD at 25 °C, thus indicating inclusion complex formation; (b) BZ forms soluble 1:1 and 2:1 BZ/β-CD complexes with complex formation constants K ₁₁ = 104 ± 8 M⁻¹ and K ₂₁ = 16 ± 6 M⁻¹; (c) protonated BZ forms only 1:1 complex with K ₁₁ = 42 ± 12 M⁻¹; (d) ¹H-NMR studies in D₂O showed significant upfield chemical shift displacements for inner cavity β-CD protons indicating inclusion complex formation, while (e) Molecular modeling of BZ-β-CD interactions in water clearly indicated complete inclusion of one BZ molecule into the β-CD cavity.     

Determination of gross α and β activities in Ankara airborne particulate samples in 2003–2004

The complex formation of curium(III) with L2-aminobutyric acid was characterized by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at trace Cm(III) concentrations (3·10⁻⁷ M). The various curium(III) species, M_pH_qL_r, identified are characterized by their individual luminescence spectra and luminescence lifetimes. The following formation constants were determined log β₁₀₁ = 5.17±0.07, log β₁₀₂ = 9.00±0.07, and log β₁₀₃ = 11.30±0.09 at ionic strength I = 0.5M. Possible structures of the curium aminobutyrate species will be discussed on the basis of the luminescence lifetime measurements and the magnitude of the formation constants.     

Characterization of a Thermostable Family 1 Glycosyl Hydrolase Enzyme from <Emphasis Type="Italic">Putranjiva roxburghii</Emphasis> Seeds

A 66-kDa thermostable family 1 Glycosyl Hydrolase (GH1) enzyme with β-glucosidase and β-galactosidase activities was purified to homogeneity from the seeds of Putranjiva roxburghii belonging to Euphorbiaceae family. N-terminal and partial internal amino acid sequences showed significant resemblance to plant GH1 enzymes. Kinetic studies showed that enzyme hydrolyzed p-nitrophenyl β-d-glucopyranoside (pNP-Glc) with higher efficiency (K _cat/K _m = 2.27 × 10⁴ M⁻¹ s⁻¹) as compared to p-nitrophenyl β-d-galactopyranoside (pNP-Gal; K _cat/K _m = 1.15 × 10⁴ M⁻¹ s⁻¹). The optimum pH for β-galactosidase activity was 4.8 and 4.4 in citrate phosphate and acetate buffers respectively, while for β-glucosidase it was 4.6 in both buffers. The activation energy was found to be 10.6 kcal/mol in the temperature range 30–65 °C. The enzyme showed maximum activity at 65 °C with half life of ~40 min and first-order rate constant of 0.0172 min⁻¹. Far-UV CD spectra of enzyme exhibited α, β pattern at room temperature at pH 8.0. This thermostable enzyme with dual specificity and higher catalytic efficiency can be utilized for different commercial applications.     

Laccase of Coriolus zonatus

Laccase is one of the lignolytic enzymes found in liquid cultures of the fungus Coriolus zonatus in defined medium. The enzyme was isolated from culture liquid and characterized. Laccase from C. zonatus is a single-chain protein with a molecular mass of 60 kDa. Carbohydrate moiety of enzyme consisted of mannose, galactose and N-acetyl-glucosamine in a ratio of 6:2:0,6 respectively, and comprised 10% of the entiremolecule lsoelectric point was detected at pH 4.6. Laccase was found to have a pH optimum of 4.9 and temperature optimum of 55°C. Substrate specificity studies were conducted with catechol, K-ferrocyanide, hydroquinone, and sinapinic acid as substrates. The highest efficiency of catalysis was observed with sinapic acid as the substrate. The kinetic constants k _cat and K₂₈ of this reaction were 624 s⁻¹ and 7 μM, respectively.     

Kinetics of peroxidase-catalyzed oxidation of quercetin in the presence of β-cyclodextrin

It is established that the rate of peroxidase-catalyzed oxidation of flavonoid quercetin is increased by 20% in the presence of macrocyclic complexing agent β-cyclodextrin. The comparison of the kinetic parameters of the indicated reaction in the presence and in the absence of β-cyclodextrin shows that its introduction does not significantly influence the specificity of the enzyme with respect to the reducing substrate (characterized as k _cat/K _M ratio), while the increase in the reaction rate does not depend on the duration of the incubation of quercetin with β-cyclodextrin. It is assumed that the increase of the reaction rate is associated with nonspecific interaction between β-cyclodextrin and quercetin oxidation product.     

Recombination System Based on Cre α Complementation and Leucine Zipper Fusions

A gene encoding β-1,3-1,4-glucanase was cloned by polymerase chain reaction (PCR) from Bacillus subtilis MA139. Sequencing result showed 97% homology to the corresponding gene from Bacillus licheniformis. The open reading frame (ORF) of the gene contained 690 bp coding for a 226 amino-acid matured protein with the estimated molecular weight of 24.44 kDa. The β-1,3-1,4-glucanase gene was subcloned into an expression vector of pET28a and expressed in Escherichia coli BL21 and then purified by metal affinity chromatography using a nickel–nitrilotriacetic acid (Ni–NTA) column. The purified β-1,3-1,4-glucanase demonstrated 24.05 and 12.52 U ml^-1 activities for the substrates of barley β-glucan and lichenan, respectively, and the specific activities were 728.79 and 379.1 U mg^-1 for them, respectively. The optimal temperature and pH of the purified enzyme were 40°C and 6.4, respectively. When barley β-glucan was used as the substrate, K _m was 5.34 mg ml^-1, and K _cat showed 7,206.71 S^-1, thus the ratio of K _cat and K _m was 1,349.67 ml s^-1 mg^-1. The activity of β-1,3-1,4-glucanase was affected by a range of metal ions or ethylenediaminetetraacetic acid (EDTA).     

Simulaaneous ethanol and cellobiose inhibition of cellulose hydrolysis studied with integrated equations assuming constant or variable substrate concentration

The integrated forms of the Michaelis-Menten equation assuming variable substrate (depletion) or constant substrate concentration were used to study the effect of the simultaneous presence of two exoglucanase Cel7A inhibitors (cellobiose and ethanol) on the kinetics of cellulose hydrolysis. The kinetic parameters obtained, assuming constant substrate (K _m =21 mM, K _ic =0.035 mM; K _icl =1.5×10¹⁵mM; k _cat=12 h⁻¹) or assuming variable substrate (K _m =16 mM, K _ic =0.037 mM; K _icl =5.8×10¹⁴ mM; k _cat=9 h⁻¹), showed a good similarity between these two alternative methodologies and pointed out that bothethanol and cellobiose are competitive inhibitors. Nevertheless, ethanol is a very weak inhibitor, as shown by the large value estimated for the kinetic constant K _icl . In addition, assuming different concentrations of initial accessible substrate present in the reaction, both inhibition and velocity constants are at the same order of magnitude, which is consistent with the obtained values. The possibility of using this kind of methodology to determine kinetic constants in general kinetic studies is discussed, and several integrated equations of different Michaelis-Menten kinetic models are presented. Also examined is the possibility of determining inhibition constants without knowledge of the true accessible substrate concentration.     

Heterologous Expression of Aspergillus niger β-d-Xylosidase (XlnD): Characterization on Lignocellulosic Substrates

The gene encoding a glycosyl hydrolase family 3 xylan 1,4-beta-xylosidase, xlnD, was successfully cloned from Aspergillus niger strain ATCC 10864. The recombinant product was expressed in Aspergillus awamori, purified by column chromatography, and verified by matrix-assisted laser desorption ionization, tandem time of flight (MALDI-TOF/TOF) mass spectroscopy of tryptic digests. The T _max was determined using differential scanning microcalorimetry (DSC) to be 78.2 °C; the K _m and k _cat were found to be 255 μM and 13.7 s⁻¹, respectively, using pNP-β-d-xylopyranoside as substrate. End-product inhibition by d-xylose was also verified and shown to be competitive; the K _i for this inhibition was estimated to be 3.3 mM. XlnD was shown to efficiently hydrolyze small xylo-oligomers to monomeric xylose, making it a critical hydrolytic activity in cases where xylose is to be recovered from biomass conversion processes. In addition, the presence of the XlnD was shown to synergistically enhance the ability of an endoxylanase, XynA from Thermomyces lanuginosus, to convert xylan present in selected pretreated lignocellulosic substrates. Furthermore, the addition of the XynA/XlnD complex was effective in enhancing the ability of a simplified cellulase complex to convert glucan present in the substrates.     

Inclusion complexation of diclofenac with natural and modified cyclodextrins explored through phase solubility, 1H-NMR and molecular modeling studies

Guest–host interactions were examined for neutral diclofenac (Diclo) and Diclofenac sodium (Diclo sodium) with each of the cyclodextrin (CD) derivatives: α-CD, β-CD, γ-CD and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), all in 0.05 M aqueous phosphate buffer solution adjusted to 0.2 M ionic strength with NaCl at 20 °C, and with β-CD at different pHs and temperatures. The pH solubility profiles were measured to obtain the acid–base ionization constants (pK _as) for Diclo in the presence and absence of β-CD. Phase solubility diagrams (PSDs) were also measured and analyzed through rigorous procedures to obtain estimates of the complex formation constants for Diclo/CD and Diclo sodium/CD complexation in aqueous solutions. The results indicate that both Diclo and Diclo sodium form soluble 1:1 complexes with α-, β-, and HP-β-CD. In contrast, Diclo forms soluble 1:1 Diclo/γ-CD complexes, while Diclo sodium forms 1:1 and 2:1 Diclo/γ-CD, but the 1:1 complex saturates at 5.8 mM γ-CD with a solubility product constant (pK _sp = 5.5). Therefore, though overall complex stabilities were found to follow the decreasing order: γ-CD > HP-β-CD > β-CD > α-CD, some complex precipitation problems may be faced with aqueous formulations of Diclo sodium with γ-CD, where the overall concentration of the latter exceeds 5.8 mM γ-CD. Both ¹H-NMR spectroscopic and molecular mechanical modeling (MM⁺) studies of Diclo/β-CD indicate the possible formation of soluble isomeric 1:1 complexes in water.     

Cisapride/β-cyclodextrin complexation: stability constants, thermodynamics, and guest–host interactions probed by 1H-NMR and molecular modeling studies

Phase solubility techniques were used to obtain the complexation parameters of cisapride (Cisp) with β-cyclodextrin (β-CD) in aqueous 0.05 M citrate buffer solutions. From the UV absorption spectra and the pH solubility profile, two basic pK _as were estimated: pK _a(1+) = 8.7 and pK _a(2+) < 2. The inherent solubility (S _o) of Cisp was found to increase as pH decreases, but is limited by the solubility product of the CispH⁺·citrate¹⁻ salt at low pH (pK _sp = 3.0). Cisp forms soluble 1:1 and 1:2 Cisp/β-CD complexes. A quantitative measure of the hydrophobic effect (desolvation) contribution to 1:1 complex formation was obtained from the linear variation of free energy of 1:1 Cisp/β-CD complex formation (ΔG ₁₁ = −RT ln K ₁₁ < 0) with that of the inherent solubility of Cisp . The results show that the hydrophobic character of Cisp contributes about 35% of the total driving force to 1:1 complex formation (slope = −0.35), while other factors, including specific interactions, contribute −10.6 kJ/mol (intercept). Protonated 1:1 Cisp/β-CD complex formation at pH 6.0 is driven by favorable enthalpy (ΔH° = −9 kJ/mol) and entropy (ΔS° = 51 J/mol K) changes. In contrast, inherent Cisp solubility is impeded by unfavorable enthalpy (ΔH° = 12 kJ/mol) and entropy (ΔS° = 90 J/mol K) changes. ¹H-NMR spectra in D₂O and molecular mechanical studies indicate the formation of inclusion complexes. The dominant driving force for neutral Cisp/β-CD complexation in vacuo was predominantly van der Waals with very little electrostatic contribution.     

Equilibria in the system of aquachlorohydroxogold(III) complexes in aqueous solution

AuCl₄⁻ + jOH⁻ + kH₂O = AuCl_{4 − j − k} OH _j (H₂O) _k ^{k − 1} + (j + k)Cl⁻ equilibria at 20°C were studied spectrophotometrically, and the constants β _jk in acid aqueous solutions were determined for I = 2.0 mol/L (HClO₄).     

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO<Subscript>4</Subscript> and NaOH solutions

Chromium(III)-lutidinato complexes of general formula [Cr(lutH) _n (H₂O)_6−2n ]³⁻ⁿ (where lutH⁻ is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)₃]⁰ leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO₄ and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H⁺] are as follows: k _obs1 = k ₁ + k ₋₁/K ₁[H⁺] and k _obs2 = k ₂ K ₂[H⁺]/(1 + K ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k ₋₁ is the rate constant for the chelate-ring closure, and K ₁ and K ₂ are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH⁻]: k _obs1 = k _OH(1) + k _O[OH⁻], where k _OH(1) and k _O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K ₂ is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)₄]³⁻ were also studied and the values of the pseudo-first-order rate constants are [OH⁻] independent.     

Kinetic Study of the Oxidation of N,N-Dimethylethanolamine by Bis(Hydrogenperiodato)Argentate(III) in Aqueous Solution

The oxidation of N,N-dimethylethanolamine (DMEA) by bis(hydrogenperiodato) argentate(III) ([Ag(HIO₆)₂]⁵⁻) was studied in aqueous alkaline medium. Formaldehyde and dimethylamine were identified as the major oxidation products after the oxidation of DMEA. The oxidation kinetics was followed spectrophotometrically in the temperature range of 25.0 °C–40.0 °C. It was found that the reaction was first order in [Ag(III)]; the oberved first-order rate constants k _obsd as functions of [DMEA], [OH⁻] and total concentration of periodate ([IO₄^-]_tot[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}) were analyzed and were revealed to follow a rate expression: k_obsd = (k₁ +k₂[OH^-])K₁K₂[DMEA]/{f([OH^-])[IO₄^-]_tot+ K₁ + K₁K₂[DMEA]}k_{\mathrm{obsd}} = (k_{1} +k_{2}[\mathrm{OH}^{-}])K_{1}K_{2}[\mathrm{DMEA}]/\{f([\mathrm{OH}^{-}])[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}+ K_{1} + K_{1}K_{2}[\mathrm{DMEA}]\}. Rate constants k ₁ and k ₂ and equilibrium constant K ₂ were derived; activation parameters corresponding to k ₁ and k ₂ were computed. In the proposed reaction mechanism, a peridato-Ag(III)-DMEA ternary complex is formed indirectly through a reactive intermediate species [Ag(HIO₆)(OH)(H₂O)]²⁻. In subsequent rate-determining steps as described by k ₁ and k ₂, the ternary complex decays to Ag(I) through two reaction pathways: one of which is spontaneous and the other is prompted by an OH⁻.     

Electron transfer at tetrahedral cobalt(II). Part V: kinetics of permanganate ion reduction

Summary The kinetics and mechanism of the reduction of MnO₄ ⁻ by CoW₁₂O₄O₆₋ in aqueous HC1O₄ were studied. The reaction follows the rate law:-d[MnO _inf4 ^sup− ]/dt = 5K _a k[H⁺][MnO _inf4 ^sup− ][CoW₁₂O₄O⁶⁻] with K _a = 2.99 × 10⁻³mol⁻¹ dm³ and k = 2.00 ± 0.02 × 10³dm⁶mol⁻²s⁻¹ at 25°C. Close agreement between k _obs and k _calc on the basis of Marcus theory suggest an outersphere mechanism operates. Alkali metal ions catalyse the reaction in the order K⁺ > Na⁺ > Li⁺ and this result has been rationalized.