Thermophilic laccase from xerophyte species Opuntia vulgaris

Two laccase temperature isoforms capable of oxidizing phenolic compounds to quinones were isolated and purified to homogeneity from the cladodes of the xerophyte species Opuntia vulgaris. These catalytically active proteins exhibit apparent molecular masses of 137 and 90 kDa. Under reducing conditions, both isoforms yielded a subunit molecular mass of 43 kDa, suggesting that the enzyme is a multimer of the 43 kDa subunit. The 137 kDa isoform when heated at 80°C for 3min generated three polypeptide bands on activity stained polyacrylamide gels exhibiting 137, 90 and 43 kDa molecular forms. All isoforms of the enzyme exhibited an optimum pH of 10 when 2,6‐dimethoxyphenol was used as a substrate. The optimum temperature of the 137 kDa enzyme form was noted to be 80°C and that of the 90 kDa enzyme form was 70°C. Denaturation kinetics of both the laccase isoforms carried out at their respective optimum temperatures for 30 min exhibited enzyme activity in excess of their t_1/2 values throughout the assay period. The K_m for the 137 kDa form was determined to be 2.2 ± 0.3 mm and the V_max was 2.8 ± 0.2 IU/mL. These high temperature stable laccase isoforms having alkaline pH optima can find significant industrial use. Copyright © 2010 John Wiley & Sons, Ltd.     

Phase separation in mixtures of poly(ethylene glycol monomethylether) and poly(propylene glycol) oligomers

Time-resolved light scattering was employed to investigate kinetics of phase separation in mixtures of poly (ethylene glycol monomethylether) (PEGE)/poly (propylene glycol) (PPG) oligomers. Phase diagrams for PEGE/PPG of varying molecular weights were established by means of cold point measurements. The oligomer mixtures reveal an upper critical solution temperature (UCST). Several temperature quench experiments were carried out with a 60/40 PEGE/PPG blend by rapidly quenching from a single phase (69°C) to two-phase temperatures (66–61°C) at 1°C intervals. As is typical for oligomer mixtures, the early stage of spinodal decomposition (SD) was not detected. The kinetics of phase decomposition was found to be dominated by the late stage of SD. Time-evolution of scattering intensity was analyzed in accordance with nonlinear and dynamical scaling theories. The time dependence of the peak intensity I_m and the corresponding peak wavenumber q_m was found to follow the power-law {I_m(t)? t^α, q_m(t)? t^-β} with the values of α = 3 ± 0.3 and β = 1 ± 0.2, which are very close to the values predicted by Siggia. This process has been attributed to a coarsening mechanism driven by surface tension. In the temporal scaling analysis, the structure function reveals university with time, suggesting self-similarity. Phase separation dynamics in 60/40 PEGE/PPG resembles the behavior predicted for off-critical mixtures.     

Rates and equilibria in the reaction system Br + i-C4H10 ⇌ HBr + t-C4H9. The heat of formation of the t-butyl radical

The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k₁ (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K₁ = (5.6 ± 1) × 10^?3 and hence log k_?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C₄H₉ → HBr + i? C₄H₈ is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K₁ gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol^?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies.     

Kinetics and equilibria in the system Br + t-BuO2H ⇆ HBr + t-BuO2. OH bond dissociation energy in t-BuO2-H

The kinetics and equilibria in the system Br + t-BuO₂H ? HBr + t-BuO₂· have been measured in the range of 300–350 K using the very low pressure reactor (VLPR) technique. Using an estimated entropy change in reaction (1) ΔS₁ = 3.0 ± 0.4 cal/mol·K together with the measured ΔG₁, we find ΔH₁ = 1.9 ± 0.2 kcal/mol and DHº (t-BuO₂-H) = 89.4 ± 0.2 kcal/mol ΔH_f·(tBuO₂·) = 20.7 kcal/mol and DH^º (t-Bu-O₂) = 29.1 kcal/mol. The latter values make use of recent values of ΔH_f·(t-Bu) = 8.4 ± 0.5 kcal/mol and the known thermochemistry of the other species. The activation energy E₁ is found to be 3.3 ± 0.6 kcal/mol, about 1 kcal lower than the value found for Br attack on H₂O₂. It suggests a bond 1 kcal stronger in H₂O₂ than in tBuO₂H.     

Dilatometric constant for polymerization of α-methylstyrene

By measurement of the specific volume of solutions of poly-α-methylstyrene in α-methylstyrene monomer at 25°C, the dilatometric constant was found to be K_D = (0.002007 ± 0.000030)%^?1. Estimation of the temperature dependence resulted in the equation (K_D)_t = 1.81 × 10^?3 + 7.82 + 10^?6 t, where t denotes temperature in °C.     

Stressed Kinetics of Nanoemulsion Formulation Encapsulated Ropinirole with a Validated Ultra High Performance Liquid Chromatography–Synapt Mass Spectrometry (UPLC‐MS/MS ESI‐Q‐TOF)

A highly sensitive ultra high pressure liquid chromatography (UHPLC‐MSMS) method for estimation of ropinirole in rat brain homogenate and plasma has been validated. The method was successfully used for the degradation kinetics in different stress condition and regulated temperature. The chromatographic separation was achieved using isocratic mobile phase, consisting of acetonitrile–2mM ammoniumacetate (28:72 v/v; 0.25 mL min^?1). The mass spectrometer was operated in synapt mass spectrometry mode via positive electrospray ionization using the transitions m/z 260 → m/z 261 for ropinirole, and m/z 324.39 → m/z 262.161 as a parent ion of escitalopram (IS). The assay for ropinirole was linear over the range of 0.5–100 ng mL^?1 (r²; 0.999). The intra‐ and inter day precisions were less than 11.2% in terms of relative standard deviation (R.S.D.), and the accuracy was within ±6.4% in terms of relative error (RE). The mean extraction‐efficiency of QC samples (MQC, 8 ng/mL) was ≥80%. The lower limit of quantification (LLOQ) was 0.049 ng/mL where as lower limit of detection (LLOD) was 0.016 ng/mL. All the peaks of degradation were well resolved. The degradation kinetics of ropinirole, showed highest stability (t_1/2 256.66/h; t_0.9, 39.11/h) in acidic medium, lower stability in alkaline environment (t_1/2, 103.43/h; t_0.9, 15.76/h) and highly susceptible in oxidative environment (t_1/2, 21.58/h; t_0.9, 3.28/h). The applicability of this assay was demonstrated and successfully applied for pharmacokinetic profiling of ropinirole in Wister rat brain homogenate after intranasal administration.     

Analytical and kinetic study of the aqueous hydrolysis of four organophosphorus and two carbamate pesticides

The hydrolysis of six selected pesticides has been studied in aqueous solution. Four organophosphorus pesticides (disulfoton, isofenfos, isazofos and profenfos) and two N-methylcarbamate derivatives (oxamyl and ethiofencarb) were selected. Hydrolysis was performed in purified buffered water at different pH in the range 7.0–10.0 (ionic strength?=?2.5?mM, T?=?25°C). At pH?=?8.0, isofenfos and disulfoton (t _1/2?≈?4 years, t _1/2?≈?1 year, resp.) were found to be far more stable than isazofos (t _1/2?≈?5 months), ethiofencarb and profenofos (t _1/2<1 month), themselves more stable than oxamyl (t _1/2?≈?1 day). As expected, a strong dependence on pH was observed for all pesticides: the rate of degradation increased when the pH increased. Degradation products were identified by GC–MS and/or LC–MS. Possible structures are presented in the article.     

Antioxidant Activity Degradation,Formulation Optimization,Characterization, and Stability of Equisetum Arvense Extract Nanoemulsion

Equisetum arvense supercritical CO₂ extracts (EAE) contained an initial antioxidant activity of 10.3 mM TEAC that suffered a sharp first order kinetics decay characterized by half-life time (t_1/2) of 1.05 ± 0.03 and 0.86 ± 0.03 weeks at 25 and 4°C, respectively. The aim of the present work was to develop a nanoemulsion formulation for providing EAE protection against deleterious environmental factors and extending its shelf-life functionality. The nanoemulsion area was identified by constructing pseudoternary phase diagrams and response surface methodology was used to optimize the preparation conditions. The best formulation achieved an EAE encapsulation efficiency of 97.5 ± 0.5% and antioxidant activity half-life time (t_1/2) was extended to 12.32 ± 0.40 and 14.56 ± 0.60 weeks at 25 and 4°C, respectively.      

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

The Photocatalytic Degradation of Imazapyr

The degradation of imazapyr, an imidazolinone herbicide, in aqueous solution has been investigated with TiO₂ slurry as photocatalyst at 30°C under UV radiation. The depletion of imazapyr concentration in an aqueous suspension followed 1^st order kinetic behavior. The influence of pH and the charge densities of imazapyr geometries were calculated at the semi-empirical AM1 level, and the effect of temperature was investigated. The addition of electron acceptors such as potassium persulfate and hydrogen peroxide showed that the rate constant doubled at least. At higher persulfate concentrations the herbicide degradation was more efficient in direct photolysis than TiO₂-photocatalysis. The degradation rate constant increased by 38% upon variation of the temperature between 20.0 and 50.0°C and displayed non-Arrhenius behavior.     

The Photocatalytic Degradation of Imazapyr

Summary. The degradation of imazapyr, an imidazolinone herbicide, in aqueous solution has been investigated with TiO₂ slurry as photocatalyst at 30°C under UV radiation. The depletion of imazapyr concentration in an aqueous suspension followed 1^st order kinetic behavior. The influence of pH and the charge densities of imazapyr geometries were calculated at the semi-empirical AM1 level, and the effect of temperature was investigated. The addition of electron acceptors such as potassium persulfate and hydrogen peroxide showed that the rate constant doubled at least. At higher persulfate concentrations the herbicide degradation was more efficient in direct photolysis than TiO₂-photocatalysis. The degradation rate constant increased by 38% upon variation of the temperature between 20.0 and 50.0°C and displayed non-Arrhenius behavior.     

Degradation kinetics of fluvoxamine in buffer solutions: In silico ADMET profiling and identification of degradation products by LC-MS/ESI

The kinetics of hydrolysis of fluvoxamine maleate (FLV) has been investigated over the pH range 1.0–12.0 at 40, 60 and 80 °C. FLV degradation follows pseudo-first-order kinetics which is consistent with the kinetics of drugs that are not readily dissolved in aqueous medium. The hydrolytic degradation rate constant (k_obs) range from 0.92 (pH 6.0) to 13.8 × 10⁻⁴ min⁻¹ (pH 1.0). The k_obs represents the sum of six different degradation rate constants; the k_H has been found to be higher than k_OH. The FLV exhibits a typical rate- pH profile with a flat bottom over the pH range 3.0–6.0 which indicates its maximum stability at pH 6.0. Ten FLV degradants have been predicted by Zeneth software and among them four degradation products (D1, D2, D3 and D4) have been identified in degraded samples. The in-silico pharmacokinetics and toxicity of degradation products have been determined using Swiss ADME and admetSAR software. The toxicity profile reveals that D2 is both AMES toxic and carcinogenic while the rest of the products are non-AMES toxic and non-carcinogenic. All of the degradation products are high in causing fish toxicity thus their presence in pharmaceutical waste is alarming for environmental safety.     

Thermal decomposition of perfluoro-di-t-butyl peroxide

The thermal decomposition of perfluoro-di-t-butyl peroxide has been studied for the first time. The reaction was carried out in the gas phase between 5 and 600 torr in the 108-149°C temperature region. The products consisted solely of C₂F₆ and CF₃COCF₃. The decomposition was found to be first order and homogeneous. The rate constant is given by log k_decomp(sec^?1) = (16.2 ± 1.2) - (148.7 4.4)/2.3RT where R is 0.008314kJ/mol · °K. These Arrhenius parameters are consistent with those determined for the decomposition of di-t-butyl peroxide.     

Substituenteneffekte auf die C–C-Bindungsstärke, 8. Standardbildungsenthalpien und Spannungsenthalpien von 1,1,2,2-Tetraethylethylenglykol-dimethylether und D,L-1,2-Dimethyl-1,2-diphenylethylenglykol-dimethylether

Effects of Substituents on the Strength of C - C Bonds, 8¹. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol ^-1, s. d. in parentheses) ΔH°_c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔH^v = 14.3 (± 0.3) and the heat of sublimation of 2 ΔH^sub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°_t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (_s) of 15.9 and 8.0 kcal mol^-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group.     

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S₂O₈^2? or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 10⁸ m ^?1 s^?1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash‐photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2‐chlorobenzylalcohol and 2‐chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O₂(a¹Δ_g) generation (λ = 400 nm) for the VCE in D₂O, Φ_Δ = (1.3 ± 0.1) × 10^?3, was determined by measuring the O₂(a¹Δ_g) phosphorescence at 1270 nm. The value of the overall quenching constant of O₂(a¹Δ_g) by clomazone was found to be (5.7 ± 0.3) × 10⁷ m ^?1 s^?1 in D₂O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was k_r = (5.4 ± 0.1) × 10⁷ m ^?1 s^?1, which means that the quenching process is mainly reactive.     

Lamellar thickening in nascent poly(acrylonitrile) upon annealing

No systematic study has been reported on the lamellar thickening in atactic poly(acrylonitrile) (PAN) upon annealing because PAN, in the form of solution‐cast films or their drawn products, generally shows no small‐angle X‐ray scattering (SAXS) maximum corresponding to the lamellar thickness. In this work, PAN crystals were precipitated during the thermal polymerization of acrylonitrile in solution. The nascent PAN film, obtained by the filtration of the crystal suspension, exhibited a clear SAXS maximum revealing the lamellar structure. The lamellar thickening upon annealing of the nascent PAN films was studied in the temperature range 100–180 °C, where the degradation was minimal, as confirmed by the absence of an IR absorption band at 1605 cm⁻¹ ascribed to the cyclized nitrile groups. Above 190 °C, the degradation of the samples was significant, and the SAXS became too broad to determine the scattering maximum. The long period was significantly affected by the annealing time (t_a) and the temperature (T_a). Depending on t_a, three stages were observed for the lamellar thickening behavior. The lamellar thickness stayed constant in stage I (t_a = 0.5–3 min, depending on T_a), rapidly increased in stage II (t_a = 0.5–8 min), and stayed at a constant value characteristic for each T_a at yet longer t_a's in stage III. The lamellar thickness characteristic for T_a increased rapidly with increasing T_a at 165 °C (or higher), which was 152 °C lower than the estimated melting temperature of PAN (T_m = 317 °C). A possible mechanism for such lamellar thickening in PAN far below the T_m is discussed on the basis of the enhanced chain mobility in the crystalline phase above the crystal/crystal reversible transition at 165–170 °C detected by differential scanning calorimetry and wide‐angle X‐ray diffraction. The structural changes associated with annealing are also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2571–2579, 2000     

Kinetics of the mercury(II)‐catalyzed substitution of coordinated cyanide ion in hexacyanoruthenate(II) by nitroso‐R‐salt

The kinetics and mechanism of Hg²⁺‐catalyzed substitution of cyanide ion in an octahedral hexacyanoruthenate(II) complex by nitroso‐R‐salt have been studied spectrophotometrically at 525 nm (λ_max of the purple‐red–colored complex). The reaction conditions were: temperature = 45.0 ± 0.1°C, pH = 7.00 ± 0.02, and ionic strength (I) = 0.1 M (KCl). The reaction exhibited a first‐order dependence on [nitroso‐R‐salt] and a variable order dependence on [Ru(CN)₆^4?]. The initial rates were obtained from slopes of absorbance versus time plots. The rate of reaction was found to initially increase linearly with [nitroso‐R‐salt], and finally decrease at [nitroso‐R‐salt] = 3.50 × 10^?4 M. The effects of variation of pH, ionic strength, concentration of catalyst, and temperature on the reaction rate were also studied and explained in detail. The values of k₂ and activation parameters for catalyzed reaction were found to be 7.68 × 10^?4 s^?1 and E_a = 49.56 ± 0.091 kJ mol^?1, ΔH^≠ = 46.91 ± 0.036 kJ mol^?1, ΔS^≠ = ?234.13 ± 1.12 J K^?1 mol^?1, respectively. These activation parameters along with other experimental observations supported the solvent assisted interchange dissociative (I_d) mechanism for the reaction. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 215–226, 2009     

Synthesis,pH‐ and temperature‐induced micellization and gelation of doubly hydrophilic triblock copolymer of poly(N,N‐dimethylamino‐2‐ethylmethacrylate)‐b‐poly(ethylene glycol)‐b‐poly(N,N‐dimethylamino‐2‐ethylmethacrylate) in aqueous solutions

A doubly hydrophilic triblock copolymer of poly(N,N‐dimethylamino‐2‐ethyl methacrylate)‐b‐Poly(ethylene glycol)‐b‐poly(N,N‐dimethylamino‐2‐ethylmethacrylate) (PDMAEMA‐b‐PEG‐b‐PDMAEMA) with well‐defined structure and narrow molecular weight distribution (M_w/M_n = 1.21) was synthesized in aqueous medium via atom transfer radical polymerization (ATRP) of N,N‐dimethylamino‐2‐ethylmethacrylate (DMAEMA) initiated by the PEG macroinitiator. The macroinitiator and triblock copolymer were characterized with ¹H NMR and gel permeation chromatography (GPC). Fluorescence spectroscopy, dynamic light scattering (DSL), transmittance measurement, and rheological characterization were applied to investigate pH‐ and temperature‐induced micellization in the dilute solution of 1 mg/mL when pH > 13 and gelation in the concentrated solution of 25 wt % at pH = 14 and temperatures beyond 80 °C. The unimer of R_h = 3.7 ± 0.8 nm coexisted with micelle of R_h = 45.6 ± 6.5 nm at pH 14. Phase separation occurred in dilute aqueous solution of the triblock copolymer of 1 mg/mL at about 50 °C. Large aggregates with R_h = 300–450 nm were formed after phase separation, which became even larger as R_h = 750–1000 nm with increasing temperature. The gelation temperature determined by rheology measurement was about 80 °C at pH 14 for the 25 wt % aqueous solution of the triblock copolymer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5869–5878, 2008     

Autoxidation of NO in aqueous solution

The reaction of NO with O₂ has been investigated in aqueous solution. As demonstrated by ion chromatography, the sole product is NO₂^?. Kinetic studies of the reaction by stopped-flow methods with absorbance and conductivity detection are in agreement that the rate law is -d[O₂]/dt=k[NO]²[O₂] with k = 2.1 × 10⁶ M^?2 s^?1 at 25°C. This rate law is unaffected by pH over the range from pH 1 to 13, and it holds with either NO or O₂ in excess. By studying the reaction over the temperature range from 10 to 40°C, the following activation parameters were obtained: ΔH^≠ = 4.6 ± 2.1 kJ mol^?1 and ΔS^≠=?96 plusmn; 4 J K^?1 mol^?1. © 1993 John Wiley & Sons, Inc.     

Kinetics of reaction between acetic acid and Ag2+ in nitric acid medium

The reaction kinetics between acetic acid and Ag²⁺ in nitric acid medium is studied by spectrophotometry. The effects of concentrations of acetic acid (HAc), H⁺, NO^?₃, and temperature on the reaction are investigated. The rate equation has been determined to be –dc(Ag²⁺)/dt = kc(Ag²⁺)c(HAc)c^?1(H⁺), where k = (610 ± 15) (mol/L)^?1 min^?1 with an activation energy of about (48. 8 ± 3.5) kJ mol^?1 when the reaction temperature is 25°C and the ionic strength is 4.0 mol L^?1. The reduction rate of Ag²⁺ increases with the increase in HAc concentration and/or temperature and the decrease in HNO₃ concentration. However, the effect of NO^?₃ concentrations within 0.5–2.5 mol L^?1 on the reaction rate is negligible. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 47–51, 2013