Kinetic Study of the Prooxidant Effect of α-Tocopherol. Hydrogen Abstraction from Lipids by α-Tocopheroxyl Radical

A kinetic study of the prooxidant effect of α-tocopherol was performed. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) by α-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (k _p) obtained are <1 × 10⁻² M⁻¹ s⁻¹ for 1, 1.90 × 10⁻² M⁻¹ s⁻¹ for 2, 8.33 × 10⁻² M⁻¹ s⁻¹ for 3, 1.92 × 10⁻¹ M⁻¹ s⁻¹ for 4, and 2.43 × 10⁻¹ M⁻¹ s⁻¹ for 5 at 25.0 °C. Fatty acid esters 3, 4, and 5 contain two, four, and six –CH₂– hydrogen atoms activated by two π-electron systems (–C=C–CH₂–C=C–). On the other hand, fatty acid ester 2 has four –CH₂– hydrogen atoms activated by a single π-electron system (–CH₂–C=C–CH₂–). Thus, the rate constants, k _abstr/H, given on an available hydrogen basis are k _p/4 = 4.75 × 10⁻³ M⁻¹ s⁻¹ for 2, k _p/2 = 4.16 × 10⁻² M⁻¹ s⁻¹ for 3, k _p/4 = 4.79 × 10⁻² M⁻¹ s⁻¹ for 4, and k _p/6 = 4.05 × 10⁻² M⁻¹ s⁻¹ for 5. The k _abstr/H values obtained for 3, 4, and 5 are similar to each other, and are by about one order of magnitude higher than that for 2. From these results, it is suggested that the prooxidant effect of α-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.     

Reaction of dipyridamole with the hydroxyl radical

Dipyridamole [2,6-bis-diethanolamino-4,8-dipiperidinopyrimido-(5,4-d) pyrimidine], a well known platelet aggregation inhibitor, shows powerful hydroxyl radical scavenging activity by inhibiting OH-dependent salicylate and deoxyribose degradation. Steady-state competition kinetics experiments with deoxyribose were carried out to evaluate the second-order rateconstant for the reaction between hydroxyl radical and dipyridamole. OH· radicals were generated either by a Fenton-type reaction or by X-ray irradiation of water solutions. A second-order rate constant k_{(Dipyridamole+OH·)} of 1.72±0.11×10¹⁰M⁻¹ s⁻¹ and of 1.54±0.15×10¹⁰ M⁻¹ s⁻¹ was measured by Fenton chemistry and by radiation chemistry, respectively. Mannitol was used as an internal standard for hydroxyl radicals in steady-state competition experiments with deoxyribose. A rate constant k_{(Mannitol+OH·)} of 1.58±0.13×10⁹ M⁻¹ s⁻¹ and 1.88±0.14×10⁹ M⁻¹ s⁻¹ was measured in the Fenton model and in the water radiolysis system, respectively. Both these rate constants are in good agreement with the published data obtained by the “deoxyribose assay” and by pulse radiolysis.     

Stoichiometric and kinetic studies on Ginkgo biloba extract and related antioxidants

Owing to increasing evidence showing the importance of lipid peroxidation in oxidative stress in vivo, the role and evaluation of antioxidants have received much attention. Ginkgo biloba extract (GBE), well-known as an efficient drug against diseases induced by free radicals, has been suggested to exert its effect by antioxidant action. A method was established to determine the activity of GBE as a hydrogen donor by stoichiometric and kinetic studies, and GBE was compared with several other antioxidants such as α-tocopherol, propyl gallate, and two kinds of flavonoids which are found in GBE, quercetin, and kaempferol. It was found that there were 6.62×10¹⁹ active hydrogens in 1g of GBE. Stoichiometric studies showed that one molecule of α-tocopherol reacted with one molecule of galvinoxyl radical. For quercetin, kaempferol and propyl gallate, the experimental stoichiometric numbers were 4.0, 1.9, and 3.1, respectively. The rates of reaction of antioxidants with galvinoxyl in ethanol were determined spectrophotometrically, using a stopped-flow technique. The second-order rate constant, k ₂, obtained at 25°C was 0.13 (g/l)⁻¹s⁻¹ for GBE and 5.9×10³, 2.1×10³, 1.2×10⁴, and 2.4×10³ M⁻¹s⁻¹ for quercetin, kaempferol, propyl gallate, and α-tocopherol, respectively. The second-order rate constant, k ₂′, on the molar basis of active hydroxyl groups in the tested substances obtained at 25°C decreased in the order of propyl gallate > α-tocopherol> quercetin>GBF∼kaempferol. This is the first study on GBE as an antioxidant which reports both stoichiometric and kinetic results.     

Kinetic model of asymmetric reduction of 3-oxo-3-phenylpropionic acid ethyl ester using <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> CGMCC No.2266

The kinetic model of asymmetric reduction of 3-oxo-3-phenylpropionic acid ethyl ester using Saccharomyces cerevisiae CGMCC No.2266 with 10% glucose as co-substrate to realize the regeneration of NADPH was established. The effect factors on reduction, the type and the content of co-substrate and coenzyme, and the changes of the substrate and product content vs. time during the reaction process were investigated. The results indicate that 10% glucose can increase the reaction conversion from 23.0% to 98.4% and NADPH is reducer. The reduction process conforms with sequence mechanisms. The model parameters are as follows: v _m =5.0×10⁻⁴ mol·L⁻¹·h⁻¹, k₁=1.5×10⁻⁶ mol·L⁻¹·h⁻¹, k₂=3.0×10⁻³ mol·L⁻¹·h⁻¹. The kinetic model is in good agreement with the experimental data.     

Kinetics for free radical solution polymerization of heptadecafluorodecyl (meth)acrylate in supercritical carbon dioxide

Free radical solution polymerization of heptadecafluorodecyl acrylate (HDFDA) and heptadecafluorodecyl methacrylate (HDFDMA) was carried out by using 2,2′-azobisisobutyronitrile (AIBN) as the initiator in supercritical carbon dioxide (scCO₂). We performed solution polymerization with changing initiator concentration, temperature and polymerization time to study the polymerization kinetics. A nonlinear least square method and dead-end theory were used to determine the constant, K (K=(k _p √f)/√k _d k _d ) and initiator decomposition rate constant (k _d ) from experimental data. k _d was measured as 3.77 × 10⁻⁵ s⁻¹ at 62.7°C for poly(HDFDA) and 2.71 × 10⁻⁵ s⁻¹ at 62.5 °C for poly(HDFDMA), respectively, by nonlinear least square method.     

Rate constants for quenching singlet oxygen and activities for inhibiting lipid peroxidation of carotenoids and α-tocopherol in liposomes

The ¹O₂ quenching rate constants (k _Q ) of α-tocopherol (α-Toc) and carotenoids such as β-carotene, astaxanthin, canthaxanthin, and lycopene in liposomes were determined in light of the localization of their active sites in membranes and the micropolarity of the membrane regions, and compared with those in ethanol solution. The activities of α-Toc and carotenoids in inhibiting ¹O₂-dependent lipid peroxidation (reciprocal of the concentration required for 50% inhibition of lipid peroxidation: [IC₅₀]⁻¹) were also measured in liposomes and ethanol solution and compared with their k _Q values. The k _Q and [IC₅₀]⁻¹ values were also compared in two photosensitizing systems containing Rose bengal (RB) and pyrenedodecanoic acid (PDA), respectively, which generate ¹O₂ at different sites in membranes. The k _Q values of α-Toc were 2.9×10⁸M⁻¹s⁻¹ in ethanol solution and 1.4×10⁷ M⁻¹s⁻¹ (RB system) or 2.5×10⁶ M⁻¹s⁻¹ (PDA system) in liposomes. The relative [IC₅₀]⁻¹ value of α-Toc in liposomes was also five times higher in the RB system than in the PDA-system. In consideration of the local concentration of the OH-group of α-Toc in membranes, the k _Q value of α-Toc in liposomes was recalculated as 3.3×10⁶ M⁻¹s⁻¹ in both the RB and PDA systems. The k _Q values of all the carotenoids tested in two photosensitizing systems were almost the same. The k _Q value of α-Toc in liposomes was 88 times less than in ethanol solution, but those of carotenoids in liposomes were 600–1200 times less than those in ethanol solution. The [IC₅₀]⁻¹ value of α-Toc in liposomes was 19 times less than that in ethanol solution, whereas those of carotenoids in liposomes were 60–170 times less those in ethanol solution. There were no great differences (less than twice) in the k _q and [IC₅₀]⁻¹ values of any carotenoids. The k _Q values of all carotenoids were 40–80 times higher than that of α-Toc in ethanol solution but only six times higher that of α-Toc in liposomes. The [IC₅₀]⁻¹ values of carotenoid were also higher than that of α-Toc in ethanol solution than in liposomes, and these correlated well with the k _Q values.     

Reaction Kinetics of Hydroxyl Radicals with Model Compounds of Fuel Cell Polymer Membranes

The chemical stability of perfluorinated and non‐perfluorinated low temperature fuel cell model compounds (MCs) against attack by hydroxyl radicals, HO^•, is compared using a competition kinetics approach in aqueous solutions at ambient temperature. HO^• radicals were generated in situ by UV photolysis of hydrogen peroxide in the electron spin resonance (ESR) resonator. Acetic acid (AA), trifluoroacetic acid (TFAA), methanesulfonic acid (MSA), trifluorosulfonic acid (TFSA), and perfluoro(2‐ethoxyethane)sulfonic acid (PFEESA) were chosen as MCs, while the rate constants of 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) and methanol (CH₃OH) served as reference for the determination of relative rate constants by means of steady state ESR signal amplitudes. In decreasing order the rate constants are: k_MSA = (4.8 ± 0.2) × 10⁷ M^–1 s^–1, k_AA = (4.2 ± 0.3) × 10⁷ M^–1 s^–1, k_PFEESA = (3.7 ± 0.1) × 10⁶ M^–1 s^–1, k_TFAA = (7.9 ± 0.2) × 10⁵ M^–1 s^–1, and k_TFSA < 1.0 × 10⁵ M^–1 s^–1. Applying these results to perfluorinated fuel cell membranes like Nafion®, the main points of attack by HO^• are concluded to be the ether groups of the side chains, followed by the remaining carboxyl groups from the manufacturing process of the polymers.     

Electrocatalytic oxidation of glutathione at carbon paste electrode modified with 2,7-<Emphasis Type="Italic">bis</Emphasis> (ferrocenyl ethyl) fluoren-9-one: application as a voltammetric sensor

Electrooxidation of glutathione (GSH) was studied at the surface of 2,7-bis (ferrocenyl ethyl) fluoren-9-one modified carbon paste electrode (2,7-BFEFMCPE). Cyclic voltammetry (CV), double potential-step chronoamperometry, and differential pulse voltammetry (DPV) were used to investigate the suitability of this ferrocene derivative as a mediator for the electrocatalytic oxidation of GSH in aqueous solutions with various pH. Results showed that pH 7.00 is the most suitable pH for this purpose. At the optimum pH, the oxidation of GSH at the surface of this modified electrode occurs at a potential of about 0.410 V versus Ag|AgCl|KCl_sat. The kinetic parameters such as electron transfer coefficient, α = 0.61, and rate constant for the chemical reaction between GSH and redox site in 2,7-BFEFMCPE, k _h = 1.73 × 10³ cm³ mol⁻¹ s⁻¹, were also determined using electrochemical approaches. Also, the apparent diffusion coefficient, D _app, for GSH was found to be 5.0 × 10⁻⁵ cm² s⁻¹ in aqueous buffered solution. The electrocatalytic oxidation peak current of GSH showed a linear dependence on the glutathione concentration, and linear calibration curves were obtained in the ranges of 5.2 × 10⁻⁵ M to 4.1 × 10⁻³ M and 9.2 × 10⁻⁷ M to 1.1 × 10⁻⁵ M with cyclic voltammetry and differential pulse voltammetry methods, respectively. The detection limits (3σ) were determined as 1.4 × 10⁻⁵ M and 5.1 × 10⁻⁷ M for the CV and DPV methods, respectively. This method was also examined as a selective, simple, and precise new method for voltammetric determination of GSH in real sample such as hemolysed erythrocyte.     

Kinetic study of the prooxidant effect of tocopherol. Hydrogen abstraction from lipid hydroperoxides by tocopheroxyls in solution

A kinetic study of the prooxidant effect of vitamin E (tocopherol, TocH) has been carried out. The rates of hydrogen abstraction (k₋₁) from methyl linoleate hydroperoxide (ML-OOH) by α-tocopheroxyl (α-Toc^.) (1) and eight types of alkyl substituted Toc^. radicals, (2–9) in benzene solution have been determined spectrophotometrically. The results show that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of Toc^. increases. The k₋₁ value (5.0×10⁻¹M⁻¹s⁻¹) obtained for α-Toc^. (1) was found to be about seven orders of magnitude lower than the k₁ value (3.2×10⁶M⁻¹s⁻¹) for the reaction of α-TocH with peroxyl radical, which is well known as the usual radical-scavenging reaction of α-TocH. The above reaction rates (k₋₁) obtained were compared with those (k₃) of methyl linoleate with Toc^. (1–9) in benzene solution. The rates (k₋₁) were found to be about six times larger than those (k₃) of the corresponding Toc^.. The results suggest that both reactions may relate, to the prooxidant effect of α-TocH at high concentrations in foods and oils. The effect of the phytyl side chain on the reaction rate, of Toc^. in micellar dispersions has also been studied. We have measured the rate constant, k₋₁, for the reaction of phosphatidylcholine hydroperoxide with a Toc^. radical in benzene,tert-butanol and in Triton X-100 micellar dispersions, and compared the observed k₋₁ values with the corresponding values for ML-OOH.     

Photo-initiated peroxidation of lipids in micelles by azaaromatics

The monoazaaromatics, pyridine (1), hexyl nicotinate (2), and quinoline (3) and diazaaromatics pyrimidine (4) and purine (5), readily act as photo-initiators for the peroxidation of methyl linoleate in 0.50 M SDS at 37°C giving free radical chain oxidations of linoleate. Quantitative kinetic runs on the order in substrate, RH, and in the rate of chain initiation, R_i, showed that the classical rate law for autoxidation,-d[O₂]/dt=(k _p/(2 k _t ^1/2))[RH]xR _i ^1/2, is applicable to these photo-initiated oxidations. The oxidizability of methyl linoleate under these conditions is 2.92×10⁻² M^−1/2 s^−1/2. These peroxidations were inhibited by chromanol phenolic antioxidants of the vitamin E class, such as lipid-soluble 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC) and water-soluble 2-carboxy-2,5,7,8-tetramethyl-6-hydroxychroman (Trolox) and derived rate constants for inhibition of peroxidation were k _inh (PMHC)=4.35×10⁴ M⁻¹ s⁻¹ and k _inh (Trolox)=2.81×10⁴ M⁻¹ s⁻¹ during inhibited oxidation of methyl linoleate photo-initiated by 4. The products from photo-initiated peroxidation of methyl linoleate by 1 through 5 were determined by reduction and high-performance liquid chromatography analyses to be the 9-and 13-positional hydroperoxides of the four geometrical isomers: cis-9, trans-11 (6), trans-10, cis-12 (7), trans-9, trans-11 (8), and trans-10, trans-12 (9)-octadecadienoates typical of the free radical chain mechanism of lipid peroxidation. Products from dye-sensitized oxidation by Methylene Blue or Rose Bengal of methyl linoleate gave a product distribution of six hydroperoxides typical of oxidation by singlet oxygen. Thermal or photo-initiated peroxidation of methyl linoleate in SDS gave some selectivity of oxidation at the 13-position of the linoleate chain. The ratio of 13-to 9-oxidation varied in the range 1.23 to 1.14 as the cis/trans to trans/trans ratio of geometric isomers varied from 0.44 to 1.25 during photooxidation of increased amounts of linoleate in SDS. This selectivity is attributed to loss of the pseudo symmetry around the pentadienyl system in the lipid chain in the SDS system during the peroxidation.     

Kinetic characterization of Prussian Blue-modified graphite electrodes for amperometric detection of hydrogen peroxide

Prussian Blue-modified graphite electrodes (G/PB) with electrocatalytic activity toward H₂O₂ reduction were obtained by PB potentiostatic electrodeposition from a mixture containing 2.5 mm FeCl₃ + 2.5 mm K₃[Fe(CN)₆] + 0.1 m KCl + 0.1 m HCl. From cyclic voltammetric measurements, performed in KCl aqueous solutions of different concentrations (5 × 10⁻²–1 m), the rate constant for the heterogeneous electron transfer (k _s) was estimated by using the Laviron treatment. The highest k_s value (10.7 s⁻¹) was found for 1 m KCl solution. The differences between the electrochemical parameters of the voltammetric response, as well as the shift of the formal potential, observed in the presence of Cl⁻ and NO₃⁻ compared to those observed in the presence of SO₄²⁻ ions, points to the involvement of anions in the redox reactions of PB. The G/PB electrodes showed a good electrochemical stability proved by a low deactivation rate constant (0.8 × 10⁻¹² mol cm² s⁻¹). The electrocatalytic efficiency, estimated as the ratio , was found to be 3.6 (at an applied potential of 0 mV vs. SCE; Γ = 5 × 10⁻⁸ mol cm⁻²) for a H₂O₂ concentration of 5 mm, thus indicating G/PB electrodes as possible H₂O₂ sensors.     

Reactions of Photoexcited Triplet States of Zinc Porphyrin with Transient Radicals in Aqueous Solutions

The technique of simultaneous pulse radiolysis and photolysis, PRAP, has been utilized to study the reactions of various radicals with ground state ZnTPPS and the triplet state ZnTPPS^T in aqueous solutions. The radicals H and OH add to both states with k ∼ 1 × 10¹⁰ M⁻¹ s⁻¹. The CH₂C(CH₃)₂OH radical from t-BuOH is relatively inert toward ZnTPPS but reacts rapidly (k = 1.8 × 10⁹ M⁻¹ s⁻¹) with ZnTPPS^T to form an adduct. Electron transfer reactions are found to be about an order of magnitude faster with the triplet than with the ground state. The (CH₃)₂COH radical reduces both ZnTPPS (k = 1 × 10⁸ M⁻¹ s⁻¹) and ZnTPPS^T (k = 3 × 10⁹ M⁻¹ s⁻¹) to the anion radical (ZnTPPS)⁻. The radical Br⁻₂ oxidizes both states to the cation radical (ZnTPPS)⁺ with k = 8 × 10⁸ M⁻¹ s⁻¹ for the ground state and 5 × 10⁹ M⁻¹ s⁻¹ for the triplet. The transient cation Cd⁺ reduces both states with a diffusion-controlled rate (k = 1 × 10¹⁰ M⁻¹ s⁻¹) to produce the anion radical. The above mechanisms of radical addition and electron transfer are also supported by the product spectra.     

Chemistry and kinetics of LAS acid thermal decomposition

The emission of sulfur dioxide (SO₂) from linear alkylbenzene sulfonate (LAS) acid (LASH) at high temperatures has been studied. Rate constants and Arrhenius parameters have been determined, enabling estimation of the amount of SO₂ evolved under any time/temperature combination for risk assessment purposes. Further analysis of the kinetic data and comparison with earlier molecular modeling work on the mechanism of sulfonation of linear alkyl benzene (LAB) to make LASH provide insight into the reaction pathway of SO₂ formation by thermal decomposition of LASH. For risk assessment purposes, the calculation is as follows: Estimate k from k=3.9×10⁷·e^{−13,000/(273+T)}, where T is in degrees C and k is in s⁻¹. Estimate N(SO₂,t), the number of moles of SO₂ evolved when N(LASH₀) moles of LASH are heated for t s at T ^oC, from: N(SO₂,t)=N(LASH₀)×(1−e^−kt ).     

Biomimetic sequestration of carbon dioxide using an enzyme extracted from oyster shell

Carbon dioxide sequestration activity was compared and evaluated using bovine carbonic anhydrase (BCA), and a water soluble protein extract derived from hemocytes from diseased shell (HDS). Para-nitrophenyl acetate (p-NPA) was used to measure the reaction rate. The k _cat /K _m values obtained from the Lineweaver-Burk and Michaelis-Menten equations were 230.7M⁻1s⁻¹ for BCA and 194.1M⁻s⁻ for HDS. Without a biocatalyst, CaCO₃ production took 15 seconds on average, while it took 5 seconds on average when BCA or HDS were present, indicating an approximately 3-fold enhancement of CaCO₃ production rate by the biocatalysts. The biocatalytic hydration of CO₂ and its precipitation as CaCO₃ in the presence of biocatalysts were investigated.     

Heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated titania suspensions: effect of ammonia

Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO₂ suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k _Fe(VI)[Fe(VI)]^1.25 where k _Fe(VI) = a[Ammonia]+b), a = 6.0 × 10³ μm ^0.25 s and b = 4.1 × 10⁶ μm ^−1.25s⁻¹. The rate constant, k _Fe(VI), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).     

Electrocatalytic oxidation of hydroxylamine at Ni(II)-morin complex modified carbon nanotube paste electrode

A modified electrode, nickel(II)-morin complex modified multi-wall carbon nanotube paste electrode (Ni(II)-MR-MWCNT-PE), has been fabricated by electrodepositing Ni(II)-MR complex on the surface of MWCNT-PE in alkaline solution. The Ni(II)-MR-MWCNT-PE exhibits the characteristic of improved reversibility and enhanced current responses of the Ni(III)/Ni(II) couple compared with Ni(II)-MR complex modified carbon paste electrode (CPE). It also shows better electrocatalytic activity toward the oxidation of hydroxylamine than the Ni(II) modified MWCNT-PE (Ni(II)-MWCNT-PE) and Ni(II)-MR-CPE. Kinetic parameters such as the electron transfer coefficient α, rate constant k _s of the electrode reaction and the catalytic rate constant k _cat of the catalytic reaction are determined. Moreover, the catalytic currents present linear dependence on the concentration of hydroxylamine from 2.5 × 10⁻⁶ to 4.0 × 10⁻⁴ mol L⁻¹ by amperometry. The detection limit and sensitivity are 8.0 × 10⁻⁷ mol L⁻¹ and 56.2 mA L mol⁻¹, respectively. The modified electrode for hydroxylamine determination is of the property of simple preparation, good stability, fast response and high sensitivity.     

Synthesis,crystal structure and dehydration kinetics of NaB(OH)<Subscript>4</Subscript>·2H<Subscript>2</Subscript>O

Sodium metaborate tetrahydrate (NaB(OH)₄·2H₂O) was synthesized by reaction of anhydrous borax (Na₂O·2B₂O₃) with sodium hydroxide (NaOH) under conditions at 90 °C for 150 min. The structure was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM) and Thermogravimetric (TG) analyses. Moreover, dehydration kinetics of NaB(OH)₄·2H₂O was carried out under non-isothermal conditions and the Coats-Redfern method was applied to analyze the TG data for calculation of activation energies (E _a ) and pre-exponential factors (k _o ) for different heating rates. It was determined that dehydration of sodium metaborate tetrahydrate occurred in five steps. According to the Coats-Redfern non-isothermal model, E _a and k _o were calculated as 50.89 kJ/mol and 26×10⁴ min⁻¹ for region I, 18.51 kJ/mol and 0.87×10³ min⁻¹ for region II, 15.72 kJ/mol and 0.52×10³ min⁻¹ for region III, 4.37 kJ/mol and 0.04×10³ min⁻¹ for region IV and 37.42 kJ/mol and 8.56×10³ min⁻¹ for region V, respectively.     

Structural changes of sinapic acid during alkali-induced air oxidation and the development of colored substances

Structural changes of sinapic acid were induced by air oxidation in aqueous solutions at pH 7–10 and followed by spectral and high-performance liquid chromatographic (HPLC) analysis. Color properties of the sinapic acid solutions were determined by taking the transmittance spectra, calculating the Commission Internationale de l’Eclairage (CIE) 1931 tristimulus values, and converting to Hunter L a b values. Reaction rate constants for sinapic acid were determined by a kinetic study based on the quantitative results from HPLC analysis. These reactions were first order with respect to sinapic acid and fit the appropriate equation with a coefficient of R ² >0.97. Sinapic acid was converted to thomasidioic acid with reaction rate constants (k) of 8.54×10⁻⁶, 2.51×10⁻⁵, and 4.87×10⁻⁵ s⁻¹ in phosphate-boric acid buffers of pH 7, 8.5, and 10, respectively. Similar reactions in ammonium bicarbonate buffers were more than 10 times faster. With time, thomasidioic acid further converted to 2,6-dimethoxy-p-benzoquinone and 6-hydroxy-5,7-dimethoxy-2-naphthoic acid. Air oxidation of sinapic acid aqueous solutions caused darkening of the color for the system, with the 2,6-dimethoxy-p-benzoquinone as a major color contributor.     

Free Radical Polymerisation of Methylmethacrylate Using p-nitrobenzyltriphenyl Phosphonium Ylide as Novel Initiator

p-nitrobenzyl triphenyl phosphonium ylide initiated radical polymerisation of MMA in 1-4 dioxane at 65 ± 1°C for 2 h under a nitrogen blanket, follows ideal kinetics with bimolecular termination. The overall activation energy and average value of k _p ²/k _t are 75.7 kJ mol and 1.14 × 10² l mol⁻¹ s⁻¹. FTIR Spectroscopy confirms a band of 1,729 cm⁻¹ of the ester group. ¹H NMR and ¹³C NMR confirms methoxy protons at 3.8 δ ppm and 52 δ ppm, respectively. E.S.R studies confirm a free radical mode of polymerisation. TGDTA analysis confirms the atactic nature of polymer and its thermal stability up to 120°C. Ylide dissociates to give a phenyl radical which is responsible for polymerisation.     

Reaction between Hydrosulfide and Iron/cerium (hydr)oxide: Hydrosulfide Oxidation and Iron Dissolution Kinetics

Hydrosulfide oxidation and iron dissolution kinetics were studied at normal pressure, under inert (N₂) atmosphere, in a liquid–solid mechanically-stirred slurry reactor. The kinetic variables undergoing variations were: hydrosulfide initial concentration (0.90–3.30 mmol/L), oxide initial surface area (16–143 m²/L) and pH (8.0–11.0). The hydrosulfide consumption and products (thiosulfate and polysulfide) formation were quantified by means of capillary electrophoresis, while iron dissolution was monitored through atomic absorption spectroscopy. Most of Fe(II) produced at pH = 9.5 remained associated with the oxide surface in the time-scale of the experiments. The hydrosulfide oxidation by the iron/cerium (hydr)oxide was found to be surface-controlled, with rates (R_i) of both sulfide oxidation and Fe(II) dissolution expressed in terms of an empirical rate equation: R_i = k_i[HS⁻]_t=0^−0.5[A]_t=0[H⁺]_t=0^−0.5 , where k_i represents the apparent rate constants for the oxidation of HS⁻ (k_HS) or the dissolution of Fe(II) (k_Fe), [HS⁻]_{t = 0} is the initial hydrosulfide concentration, [A]_{t = 0} is the initial Fe/Ce (hydr)oxide surface area and [H⁺]_{t = 0} is the initial proton concentration. The rate constant, k_HS, for the oxidation of hydrosulfide at pH = 9.5 was (3.4219 ± 0.65) × 10⁻⁴ mol² L⁻¹ m⁻² min⁻¹, with the rate of hydrosulfide oxidation being ca. 10 times faster than the rate of Fe(II) dissolution (assuming a 1:2 stoichiometric ratio between HS⁻ oxidized and Fe(II) produced; k_Fe = (3.9116 ± 0.41) × 10⁻⁵ mol² L⁻¹ m⁻² min⁻¹).