β-d-Xylosidase from Selenomonas ruminantium: Thermodynamics of Enzyme-Catalyzed and Noncatalyzed Reactions

β-d-Xylosidase/α-l-arabinofuranosidase from Selenomonas ruminantium is the most active enzyme known for catalyzing hydrolysis of 1,4-β-d-xylooligosaccharides to d-xylose. Temperature dependence for hydrolysis of 4-nitrophenyl-β-d-xylopyranoside (4NPX), 4-nitrophenyl-α-l-arabinofuranoside (4NPA), and 1,4-β-d-xylobiose (X2) was determined on and off (k _non) the enzyme at pH 5.3, which lies in the pH-independent region for k _cat and k _non. Rate enhancements (k _cat/k _non) for 4NPX, 4NPA, and X2 are 4.3?×?10¹¹, 2.4?×?10⁹, and 3.7?×?10¹², respectively, at 25 °C and increase with decreasing temperature. Relative parameters k _cat ^4NPX/k _cat ^4NPA, k _cat ^4NPX/k _cat ^X2, and (k _cat/K _m)^4NPX/(k _cat/K _m)^X2 increase and (k _cat/K _m)^4NPX/(k _cat/K _m)^4NPA, (1/K _m)^4NPX/(1/K _m)^4NPA, and (1/K _m)^4NPX/(1/K _m)^X2 decrease with increasing temperature.     

Features of the kinetics of the liquid-phase oxidation of cyclohexanol

The kinetics of oxygen uptake in the cumyl peroxide-initiated oxidation of cyclohexanol (373 K, o-dichlorobenzene) is studied. The parameters of the oxidizability of k _p (2k _t )^?0.5 (which depend on [RH]) and the rate constants of the bi- and trimolecular reactions of chain initiation (k ₀ = 1.25 × 10^?8 L/(mol s) and k′₀ = 2.5 × 10^?9 L²/(mol² s), respectively) are determined by solving the inverse kinetic problem. It is demonstrated that the quadratic-law recombination of peroxyl radicals during cyclohexanol oxidation also occurs without chain termination. The recombination rates of peroxyl radicals with and without chain termination (k′/k _t ) are found to grow with increasing [RH], reaching their maxima at [RH] = 1.0 mol/L, and to diminish subsequently. We conclude that this can be attributed to changes in the ratio between the propagating peroxyl radicals (hydroperoxyl and 1-hydroxycyclohexylperoxyl) in the reaction medium.     

Nature of polarographic catalytic current in the indium-acetylsalicylic acid system

The dependence of the limiting catalytic current in the system of In(III)-acetylsalycilic acid on the concentration of indium ions and the ligand anions was described theoretically. Dissociation and protonation constants of acetylsalycilic acid molecular form and kinetic parameter connected with the rate constant of formation of polarographically active complex {K _a = 3.59×10^?3, mol dm^?3; K _H = 0.12 mol dm^?3; k = 4.0×10², A (mol/dm^?3)²} were calculated.     

Electrode kinetic studies of single electron charge transfer redox couples using the faradaic rectification method

Three single electron charge transfer redox reactions have been studied using the faradaic rectification method. The kinetic parameters obtained for the ferricyanide-ferrocyanide redox couple are α=0.49, k_a⁰=12×10^?2 cm s^?1; for the chromic-chromous system α=0.47, k_a⁰=2×10^?3 cm s^?1 and for the titanic-titanous reaction α=0.49 and k_a^o=6×10^?4 cm s^?1 at 27°C.     

Reactions of the iso-butyl radical during 1,1′-azoisobutane pyrolysis

Quantitative analysis of the products formed in 1,1′-azoisobutane pyrolyses in the temperature range of 553°–602°K has shown that the major reactions of the iso-butyl radical are Analysis of initial rate data gave log₁₀k₄/(k_c)^1/2(cm^?3/2.mol ^1/2.sec^?1/2) = 7.54±0.44 ? (136.5 + 4.8) kJ/mol/2.303RT, the Arrhenius parameters obtained being in good agreement with thermodynamic data for reaction (4). Measured values of k_a/(k_c)^1/2 where k_a is the rate constant of the reaction iC₄H₉ + AIB → iC₄H₁₀ +. AIB were consistent with published parameters determined by photolysis of 1,1′-azoisobutane. Combination of photolysis and pyrolysis data gave log₁₀ k_a/(k_c)^1/2(cm^3/2.mol^?1/22.sec^?1/2) = 3.68 ± 0.15 ? (27.2 ± 1.2) kJ/mol/2.303RT. The crosscombination ratio for methyl and iso-butyl radicals has been found to be 0.25, indicating that the geometric mean rule does not hold for methyl and iso-butyl radicals.     

Kinetics and mechanism of the oxidation of acrylic acid and methyl methacrylate

The kinetics of the initiated oxidation of acrylic acid and methyl methacrylate in the liquid phase were studied volumetrically by measuring oxygen uptake during the reaction. Both processes proceed via the chain mechanism with quadratic-law chain termination. The oxidation rate is described by the equation w = k ₂/(2k ₆)^1/2[monomer]w _i ^1/2 , where w _i is the initiation rate and k ₂ and k ₆ are the rate constants of chain propagation and termination. The parameter k ₂/(2k ₆)^1/2 is 7.58 × 10^?4 (l mol^?1 s^?1)^1/2 for acrylic acid oxidation and 2.09 × 10^?3 (l mol^?1 s^?1)^1/2 for the oxidation of methyl methacrylate (T = 333 K). For the oxidation of acrylic acid, k ₂ = 2.84 l mol^?1 s^?1 (T = 333 K) and the activation energy is E ₂ = 54.5 kJ/mol; for methyl methacrylate oxidation, k ₂ = 2.96 l mol^?1 s^?1 (T = 333 K) and E ₂ = 54.4 kJ/mol. The enthalpies of the reactions of RO ₂ ^? with acrylic acid and methyl methacrylate were calculated, and their activation energies were determined by the intersecting parabolas method. The contribution from the polar interaction to the activation energy was determined by comparing experimental and calculated E ₂ values: ΔE _μ = 5.7 kJ/mol for the reaction of RO ₂ ^? with acrylic acid and ΔE _μ = 0.9 kJ/mol for the reaction of RO ₂ ^? with methyl methacrylate. Experiments on the spontaneous oxidation of acrylic acid provided an estimate of the rate of chain initiation via the reaction of oxygen with the monomer: w _i,0 = (3.51 ± 0.85) × 10^?11 mol l^?1 s^?1 (T = 333 K).     

Investigations on adsorption potentiometry Part I. Derivative adsorption chronopotentiometry of the iron(III)-2-(5′-bromo-2′-pyridylazo)-5-diethylaminophenol system

An electroanalytical method, based on derivative chronopotentiometry of the iron complex with 2-(5′-bromo-2′- pyridylazo)-5-diethylaminophenol (5-Br-PADAP) accumulated adsorptively on the surface of a hanging mercury drop electrode, for determining trace iron in food has been developed. The dependences of the peak height on the dt/dE vs. E curve on the preconcentration time, preconcentration potential and electrode area are discussed. Optimum experimental conditions include 0.005 mol 1^?1 NH₃NH₄Cl, 2 × 10^?7 mol 1^?1 5-Br-PADAP and a preconcentration potential of ?0.40 V (vs. SCE). Under these conditions, the detection limit and the linear range are 2 × 10^?9 and 6.7 × 10^?9?1.7 × 10^?7 mol 1^?1, respectively. The relative standard error of the method is 1.5% for 6.7 × 10^?8 mol 1^?1 Fe(III). The method was applied to samples of microwave digested food.     

Studies on Polarographic Adsorptive Wave of the System of the Rare Earth (III)-Copper(Ii)-M-Trifluomethyl Chlorophosphonazo

Abstract

A new heteronuclear complex, rare earth (III)-copper (II)-m-trifluomethyl chlorophosphonazo (CPA-mCF₃) system for determining trace rare earth ions is presented. In a medium of 0.02mol/L NH₄Cl,1. 0×10^?3mol/L Cu(II),1.0×10^?5 mol/L CPA-mCF₃, a very sensitive polarographic adsorptive wave is observed by using a single sweep oscillopolarograph at about –0.83V (vs. Ag/AgCl). The linear relationship between the peak current and the concentration of rare earth exists from 6. 0×10^?9 to 1. 0×10^?6 mol/L. The detection limit of rare earth is down to 2. 0×10^?9 mol/L for Tm³⁺. This method has been applied to determine trace RE in several samples of Chinese tea. The results are satisfactory. The composition of the complex is detected as RE (II): Cu (II): CPA-mCF₃ = 1: 1: 2.     

Rate constant of the gas-phase reaction between Fe atoms and CO2

The rate constant of the gas-phase reaction Fe(a ⁵ D ₄) + CO₂ at 1180–2380 K and a total gas density of (7.0–10.0) × 10^?6 mol/cm³ behind incident shock waves is k(Fe + CO₂) = 1.4 × 10^{14.0 ± 0.3}exp[?(14590 ± 1100)/T] cm³ mol^?1 s^?1, as determined by resonance atomic absorption photometry. Using thermochemical data available from the literature, the rate constant of the reverse reaction was calculated to be k(Fe + CO) = 9.2 × 10^{11.0 ± 0.3} (T/1000)^0.57exp[?(490 ± 1100)/T] cm³ mol^?1 s^?1. The results are compared with data reported earlier.     

Fourier transform infrared study of the self reaction of C2H5O2 radicals in air at 295 K

Fourier transform infrared spectroscopy was used to identify and quantify products of the self reaction of ethylperoxy radicals, C₂H₅O₂, formed in the photolysis of Cl₂/C₂H₆ mixtures in 700 torr total pressure of synthetic air at 295 K. From these measurements, branching ratios for the reaction channels (1) of k_1a/(k_1a + k_1b) = 0.68 and k_1c/(k_1a + k_1b + k_1c) ? 0.06 were established. Additionally, using the relative rate technique, the rate constant for the reaction of Cl atoms with C₂H₅OOH was determined to be (1.07 ± 0.07) × 10^?10 × cm³ molecule^?1 s^?1. Results are discussed with respect to the previous kinetic and mechanistic studies of C₂H₅O₂ radicals.     

Study on Dynamic Interfacial Tension of 3‐Dodecyloxy‐2‐hydroxypropyl Trimethyl Ammonium Chloride at n‐Decane/Water Interface

Dynamic interfacial tension (DIT) and interface adsorption kinetics at the n‐decane/water interface of 3‐dodecyloxy‐2‐hydroxypropyl trimethyl ammonium chloride (R₁₂TAC) were measured using spinning drop method. The effects of R_nTAC concentration and temperature on DIT have been investigated, the reason of the change of DIT with time has been discussed. The effective diffusion coefficient, D _a, and the adsorption barrier, ?_a, have been obtained with extended Word‐Tordai equation. The results show that the higher the concentration of surfactants is, and the smaller will be the DIT and the lower will be the curve of the DIT, and the R₁₂TAC solutions follow a mixed diffusion‐activation adsorption mechanism in this investigation. With increase of concentration in bulk solution of R₁₂TAC from 8×10^?4 mol · dm^?3 to 4×10^?3 mol · dm^?3, D _a decreases from 2.02×10^?10 m^?2 · s^?1 to 1.4×10^?11 m^?2 · s^?1 and ? _a increases from 2.60 kJ · mol^?1 to 9.32 kJ · mol^?1, while with increase of temperature from 30°C to 50°C, D _a increases from 2.02×10^?10 m^?2 · s^?1 to 5.86×10^?10 m^?2 · s^?1 and ε_a decreases from 2.60 kJ · mol^?1 to 0.73 kJ · mol^?1. This indicates that the diffusion tendency becomes weak with increase strength of the interaction between surfactant molecules and that the thermo‐motion of molecules favors interface adsorption.     

Deuterium isotope effect in the T1 → S0 radiationless transition of propynal in the gas phase

The phosphorescence lifetimes of propynal-h₁ and propynal-d₁ have been measured at room temperature in the 40 mTorr-1 Torr pressure range The reciprocal of the zero-pressure lifetime (k₀) is (3.10 ± 0.05) × 10³ and (1.70 ± 0.04) × 10³ s^?1 for propynal-h₁ and propynal-d¹ For both compounds the rate constant for self-quenching between triplet and ground-state molecules is k_SQ = (1 2±007) × 10³ Torr^?1 s^?1 The deuterium isotope effect is attributed to the T₁ → S₀ radiationlcss decay, for which k^H_ISC/k^D_ISC = 2 4     

Enantioselective recognition of penicillamine enantiomers on bovine serum albumin-modified glassy carbon electrode

As a natural chiral selector, bovine serum albumin (BSA) has been used to recognize penicillamine (Pen) enantiomers through electrochemical methods. The recognition and assay rely on the stereoselectivity of BSA embedded in ultrathin Al₂O₃ sol–gel film coated on the surface of glassy carbon electrode (BSA/GCE). The enantioselective interaction between Pen enantiomers and BSA was monitored by cyclic voltammetry and electrochemical impedance spectroscopy measurements, from which larger response signals were obtained from d-Pen. The factors influencing the performance of the modified biosensor were also investigated. The association constant (K) was calculated to be 1.93?×?10⁴?L?mol^?1 for d-Pen and 1.20?×?10³?L?mol^?1 for l-Pen. A good linear response was exhibited with the concentration of Pen enantiomers by BSA/GCE over the range of 1?×?10^?8–1?×?10^?1?mol?L^?1 with a detection limit of 3.31?×?10^?9?mol?L^?1.     

Kinetic Study of Hydrogen–Deuterium Exchange of Glutamic Acid in Deuterated Hydrochloric Acid Solution

The kinetics of the hydrogen–deuterium (H–D) exchange at both the methine (alpha) and methylene (gamma) positions of glutamic acid in deuterated hydrochloric acid solution has been studied in the temperature range of 383–433 K by ¹H NMR detection. The reaction rates of H–D exchange at the two positions were described by applying multivariable linear regression (MLR) analysis and are determined as v = k[Glu]^3.3[D₃O⁺]^1.5 mol L^?1 h^?1 with k = 3.52 × 10¹⁶ × exp (–1.37 × 10⁵/RT) mol^?3.8 L h^?1 for the alpha position as well as v = k[Glu]^1.0[D₃O⁺]^0.45 mol L^?1 h^?1 with k = 1.77 × 10¹² × exp (–0.99 × 10⁵/RT) mol^?0.45 L h^?1 for the gamma position. The Arrhenius activation energy (E_a) at the gamma position is less than that at the alpha position, which implies that the deuteration reaction at the gamma position proceeded more easily.     

Study and Application of Polarographic Catalytic Wave of Chlordiazepoxide in the Presence of Persulfate

ＩｎｔｒｏｄｕｃｔｉｏｎＣｈｌｏｒｄｉａｚｅｐｏｘｉｄｅ (7 ｃｈｌｏｒｏ 2 ｍｅｔｈｙｌａｍｉｎｏ 5 ｐｈｅｎｙｌ 3Ｈ 1,4 ｂｅｎｚｏｄｉａｚｅｐｉｎｅ 4 ｏｘｉｄｅ)ｓｈｏｗｉｎｇｐｏｗｅｒｆｕｌａｎ ｔｉａｎｘｉｅｔｙｅｆｆｅｃｔｈａｓｂｅｅｎｗｉｄｅｌｙｕｓｅｄａｓａｐｓｙｃｈｏｔｈｅｒａｐｅｕ ｔｉｃｄｒｕｇ .Ｃｏｎｓｅｑｕｅｎｔｌｙ ,ｔｈｅｎｅｅｄａｒｏｓｅｆｏｒｓｅｎｓｉｔｉｖｅａｎｄｒａｐｉｄｄｅｔｅｒｍｉｎａｔｉｏｎｏｆｃｈｌｏｒｄｉａｚｅｐｏｘｉｄｅｉｎｂｌｏｏｄ ,ｕｒｉｎｅａｎ…     

Effect of Pb2＋ on the Kinetic and Spectral Characterization of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase

本文利用各种光谱手段在体外研究了各种浓度的Pb2+对菠菜Rubisco活性影响的机制。 结果表明,Rubisco活性随着Pb2+处理浓度的增加而逐渐下降,低浓度Pb2+下Rubisco的动力学常数和最大反应速率分别为1.74 µM 和 0.42 µmol CO2/mg protein∙min,高浓度Pb2+下Rubisco的动力学常数和最大反应速率分别为11.82 µM and 0.28 µmol CO2/mg protein∙min。光谱学分析证实Pb2+可直接结合到Rubisco上, 其结合位点数为1.1个,结合常数分别为8.63×104 和 2.18×105 L/mol。ICP-MS和圆二色谱分析证实Pb2+取代了酶活性中心的Mg2+ 并改变了酶的构象。     

Polymerization of Styrene Initiated by 1,4-Dimethyl-1,4-bis(p-anisyl)-2-tetrazene

Abstract

The polymerization of styrene (St) initiated by 1,4-dimethyl-1,4-bis(p-anisyl)-2-tetrazene (1a) was studied kinetically in benzene. The polymerization proceeds through a radical mechanism. The rate of polymerization is proportional to [1a]^0.5 and [St]^1.0. The overall activation energy for the polymerization is found to be 81.2 kJ/mol within the temperature range of 65 to 80°C. The activation parameters for the decomposition of 1a at 70°C are k_d = 1.88 × 10^?5s^?1, δH? = 133.1 kJ/mol, and δS? = 29.9 J/mol·deg.     

Effects of alkyltrimethylammonium bromide surfactants on the kinetics of the oxidation of tris(1,10-phenanthroline)iron(II) by azido-pentacyanocobaltate(III) complex

The redox reaction between tris(1,10-phenanthroline)iron(II), [Fe(phen)₃]²⁺, and azido-pentacyanocobaltate(III), [Co(CN)₅N₃]^3? was investigated in three cationic surfactants: dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB) in the presence of 0.1?M NaCl at 35°C. Second-order rate constant in the absence and presence of surfactant, k_w and k_ψ, respectively, were obtained in the concentration ranges DTAB?=?0???4.667?×?10^?4?mol?dm^?3, TTAB?=?0–9.364?×?10^?5?mol?dm^?3, CTAB?=?0???6.220?×?10^?5?mol?dm^?3. Electron transfer rate was inhibited by the surfactants with premicelllar activity. Inhibition factors, k_w/k_ψ followed the trend CTAB?>?TTAB?>?DTAB with respect to the surfactant concentrations used. The magnitudes of the binding constants obtained suggest significant electrostatic and hydrophobic interactions. Activation parameters ΔH^≠, ΔS^≠, and E_a have larger positive values in the presence of surfactants than in surfactant-free medium. The electron transfer is proposed to proceed via outer-sphere mechanism in the presence of the surfactants.     

Germane decomposition: Kinetic and thermochemical data

Rate constants for the two stages of germane dissociation (GeH₄ → GeH₂ + H₂(I) and GeH₂ → Ge + H₂(II) have been derived from the studies of the chemiluminescence kinetics during germane dissociation in the presence of nitrous oxide behind shock waves at 1060–1300 K and the full density equal to ～10^?5 mol/cm³. Analysis in terms of the RRKM model gave the following expressions for the rate constants of these reactions in the high and low pressure limits: k _{1, ∞} = 2.0 × 10¹⁴exp(?208.0/RT) s^?1; k _{1, 0} = 1.7 × 10¹⁸(1000/T)^3.85exp(?208.0/RT) cm³/(mol s); and k _{2, 0} = 2.8 × 10¹⁵(1000/T)^1.32exp(?135.0/RT) cm³/(mol s). The results, in combination with the available enthalpies of formation of radical GeH₂, show that the back reaction for stage (I) has an energy barrier of about 66 kJ/mol.