Kinetics and mechanisms of the oxidation by permanganate of L-Phenylalanine

The oxidation of L-Phenylalanine by permanganate ion in aqueous phosphate buffers is autocatalized by the inorganic reaction product, which is stabilized in solution by adsorption of phosphate ions on its surface. This product is a soluble form of colloidal manganese dioxide. The rate of the noncatalytic reaction pathway is first-order in both the oxidizing and reducing agent. It is not affected by potassium chloride addition to the solution, but by phosphate addition. The rate increases with the pH of the medium. The autocatalytic pathway is first-order in both permanganate ion and colloidal manganese dioxide, (the permanganate ion according to the Langmuir isotherm). The autocatalytic rate increases with reductant concentration (follows the Langmuir adsorption isotherm). It is not affected by potassium chloride addition to the solution, whereas an increase in the phosphate concentration results in an increase in the rate with the same pH of the medium. Mechanisms consistent with the experimental data are proposed.     

Permanganate oxidation of glycine: Influence of amino acid on colloidal manganese dioxide

Permanganate ion oxidizes glycine in phosphate buffered solutions, and the product obtained was identified as a soluble form of colloidal manganese dioxide. The influence of glycine on the colloidal product shows glycine concentration changes extinction coefficients of the colloidal product, and only when the influence of glycine has been eliminated, the theoretical and experimental reaction rates coincide. The dependence of the rate of flocculation on several experimental variables was also considered.     

Effect of ionic and non-ionic surfactants on the reduction of water soluble colloidal MnO<Subscript>2</Subscript> by glycolic acid

Kinetics of the redox reaction between colloidal MnO₂ and glycolic acid have been studied spectrophotometrically by monitoring the decay in the absorbance of colloidal MnO₂ in absence and presence of surfactants. Anionic sodium dodecyl sulfate has no effect, non-ionic Triton X-100 catalyzed the reaction and experiments were not possible in presence of cationic cetyltrimethylammonium bromide due to the precipitation of MnO₂.The reaction followed the same type of kinetic behavior, i.e., fractional-, first- and fractional-order dependencies, respectively, in [glycolic acid], [MnO₂] and [H⁺ ] in both the media. Effects of gum arabic and manganese(II) have also been studied and discussed. Mechanisms in accordance with the experimental data are proposed for the reaction.     

Kinetics of colloidal MnO<Subscript>2</Subscript> reduction by L-arginine in absence and presence of surfactants

The kinetics of the oxidation of L-arginine by water-soluble form of colloidal manganese dioxide has been studied using visible spectrophotometry in aqueous as well as micellar media. To obtain the rate constants as functions of [L-arginine], [MnO₂] and [HClO₄], pseudo-first-order conditions are maintained in each kinetic run. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [L-arginine] and [HClO₄]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constant on manganese(II) concentration, which has a series of reactions with other reactants. The anionic surfactant SDS neither catalyzed nor inhibited the oxidation reaction, while in presence of cationic surfactant CTAB the reaction is not possible due to flocculation of reaction mixture. The reaction is catalyzed by the nonionic surfactant TX-100 which is explained in terms of the mathematical model proposed by Tuncay et al. Activation parameters have been evaluated using Arrhenius and Eyring equations. On the basis of observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂.     

Kinetics and mechanisms of the permanganate oxidation of L-valine in neutral aqueous solutions

The permanganate oxidation of L-valine has been studied by visible spectrophotometry in neutral aqueous solutions. Under these conditions, both the zwitterionic and anionic forms of the amino acid are oxidized, the reaction being autocatalyzed by soluble colloidal manganese dioxide. Kinetic data for both the uncatalyzed and autocatalytic reaction pathways have been obtained, and reaction sequences consistent with the experimental findings are proposed.     

Spectrophotometric determination of paraquat with BiI(-)(4) in the presence of gum arabic

A spectrophotometric method for determination of Paraquat, based on its reaction with BiI(-)(4) ion in the presence of gum arabic, is described. The apparent molar absorptivity of the product is 1.64 x 10(4) 1.mole(-1).cm(-1) at 515 nm. Results for the analysis of commercial herbicides are in good agreement with those obtained by the dithionite method.     

Palladium based heterogeneous catalyst by evaporation-induced self-assembly: use of biopolymer as a surface functionalizing and reducing agent

Palladium-based composites are widely used as a heterogeneous catalyst in carbon–carbon coupling reactions and in catalytic converters used in the car industry. In this work, we demonstrate a simple, green and scalable synthesis procedure to obtain palladium (Pd) based heterogeneous catalyst. Surface functionalized silica microparticles were obtained in one-step by spray-drying a colloidal suspension of silica nanoparticles and gum arabic, an environmental-friendly biopolymer. Subsequently, palladium nanoparticles were reduced and attached to the substrate by gum arabic. The as-synthesized composite was characterized by field-emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), small angle x-ray scattering (SAXS), and high-resolution transmission electron microscopy (HR-TEM). The Pd@SiO₂ composite was used as a catalyst in the reduction reaction of 4-nitrophenol by sodium borohydride. The catalyst showed good recycling properties. The present environmental-friendly approach for fabrication of Pd-based heterogeneous catalyst circumvents various complex chemical steps involved in conventional chemical methods and could be generalized for the production of ceramic or magnetite-based Pd composites.     

Palladium nanoparticles supported on gum arabic as a reusable catalyst for solvent-free Mizoroki-Heck reaction

Palladium nanoparticles (2–8 nm) supported on gum arabic has been prepared under green conditions in water. Gum arabic has been used as both support and reducing agent. These nanoparticles have been characterized by UV–Vis, XRD and EDX spectra as well as SEM and TEM images. The nanoparticles have been applied as the catalyst in Mizoroki-Heck reaction of different aryl halides (I, Br, Cl) with butyl acrylate in the presence of n-Pr₃N under solvent-free conditions at 140 °C. The reusability of the catalyst was also checked for five consecutive runs.     

Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum

Mesquite gum sourced from Prosopis velutina trees and gum arabic (Acacia spp.) were characterized using light scattering and Langmuir isotherms. Both gum materials were fractionated by hydrophobic interaction chromatography, yielding four fractions for both gums: FI, FIIa, FIIb and FIII in mesquite gum and FI, FII, FIIIa and FIIIb in gum arabic. In mesquite gum, the obtained fractions had different protein content (7.18-38.60 wt.-%) and macromolecular dimensions (M approximately 3.89 x 10(5)-8.06 x 10(5) g.mol(-1), RG approximately 48.83-71.11 nm, RH approximately 9.61-24.06 nm) and architecture given by the structure factor (RG/RH ratio approximately 2.96-5.27). The mechanical properties of Langmuir monolayers at the air-water interface were very different on each gum and their fractions. For mesquite gum, the most active species at the interface were those comprised in Fractions IIa and IIb and III, while Fraction I the pi/A isotherm lied below that of the whole gum. In gum arabic only Fraction III developed greater surface pressure at the same surface per milligram of material than whole gum. This is rationalized in terms of structural differences in both materials. Mesquite gum tertiary structure seems to fit best with an elongated polydisperse macrocoil in agreement with the "twisted hairy rope" proposal for arabinogalactan proteoglycans.     

Kinetics of the Degradation of l-Cysteine at Freshly Prepared Nano-sized MnO2 Surfaces in the Absence and Presence of TX-100

Nano-sized colloidal manganese dioxide was synthesized at room temperature by a chemical method in neutral medium without a stabilizing agent. The obtained MnO₂ nano-sized colloid was found to be stable for several months and was characterized by means of UV–Vis spectroscopy, energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy. The EDX analysis confirmed the presence of Mn and O in the sample. The paper reports on the use of nano-sized colloidal manganese dioxide as an oxidant in the oxidation of cysteine (Cyst) in the absence and presence of surfactant (TX-100) at 35 °C. The study was carried out as functions of [MnO₂], [Cyst], [HClO₄] and temperature. The results show that the reaction proceeds through fast adsorption of Cyst onto the surface of the colloidal MnO₂. Pseudo-first-order rate constants were found to increase with the increase in [TX-100]. This paper reports values of the reaction rates and activation parameters in the absence and presence of surfactant and proposes a plausible mechanism.     

Water-soluble colloidal manganese dioxide as an oxidant for l-tyrosine in the absence and presence of non-ionic surfactant TX-100

Water-soluble colloidal manganese dioxide has been used to oxidize l-tyrosine in aqueous-acidic medium. The kinetics of the reaction was studied in the absence and presence of non-ionic surfactant (TX-100) using a spectrophotometric technique. As the reaction was fast under pseudo-first-order conditions ([l-tyrosine]  [MnO₂]), the rate constants as a function of [l-tyrosine], [MnO₂], [HClO₄] and temperature were obtained under second-order conditions. The rate of the reaction increased and decreased with the increase in [l-tyrosine] and [MnO₂], respectively. Perchloric acid, sodium pyrophosphate and sodium fluoride showed catalytic effect. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. The reaction is inhibited by the non-ionic surfactant TX-100. Activation parameters have been evaluated using Arrhenius and Eyring equations. Based on observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂ followed by one-step two-electron transfer rate limiting process.     

酶法动力学拆分制备光学活性γ-内酯

研究了脂肪酶对桃醛和γ-癸内酯的水解动力学拆分反应,考察了温度、pH值、酶的用量等因素对酶催化水解拆分过程的影响.选择氯化钙-阿拉伯胶水溶液作为反应介质,使γ-内酯拆分产物取得较高的ee值.回收拆分中产生的γ-羟基酸,外消旋化后再内酯化,投入下一批反应,提高了光学活性γ-内酯的总得率.     

Titanium Dioxide/Electrolyte Solution Interface: Electron Transfer Phenomena

Electron transfer between a titanium dioxide/electrolyte solution interface has been studied. As found by other researchers of similar interfaces (TiO(2)- and ZnO-electrolyte solution), a slow consumption of OH(-) ions takes place in this type of interface. A theoretical model has been developed for calculating the change in the Fermi energy of both electrolyte solution and semiconductor, showing that ion consumption from the solution is favoured by the decrease of the difference between their Fermi energies. A kinetic constant (upsilon) is found to characterise the consumption process, its value increasing with electrolyte and semiconductor mass concentrations. Furthermore, this process may be used to estimate the point of zero charge of a titanium dioxide colloidal dispersion. Copyright 2000 Academic Press.     

Oxidation of lactic acid by water soluble (colloidal) manganese dioxide

Spectrophotometric method has been used to characterize water‐soluble colloidal manganese dioxide obtained by the redox reaction between sodium thiosulphate and potassium permanganate in neutral aqueous medium which shows a single peak in the visible region with λ_max = 425 nm. The kinetics of the oxidation of lactic acid by colloidal manganese dioxide (oxidant) has been investigated spectrophotometrically under pseudo‐first‐order conditions of excess lactic acid. The rate of the noncatalytic reaction pathway was slow which increased with increasing lactic acid concentration. The reaction was first‐order with respect to [oxidant] as well as [lactic acid]. In presence of manganase(II) and fluoride ions, the noncatalytic path disappeared completely while the oxidation rate of autocatalytic path increased and decreased, respectively with increasing [Mn(II)] and [F^?]. A mechanistic scheme in conformity with the observed kinetics has been proposed with the rate‐law: © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 359–366 2004     

Oxidative degradation of levofloxacin by water-soluble manganese dioxide in aqueous acidic medium: a kinetic study

Pharmaceuticals, especially fluoroquinolone antibiotics, have received increasing global concern, due to their intensive use in the environment and potential harm to ecological system as well as human health. Degradation of antibiotics, such as oxidative degradation by metal oxides, often plays an important role in the elimination of antibiotics from the environment. The kinetics of oxidation of levofloxacin by water-soluble manganese dioxide has been studied in aqueous acidic medium at 25 °C temperature. The stoichiometry for the reaction indicates that the oxidation of 1 mol of levofloxacin requires 1 mol of manganese dioxide. The reaction is second order, that is first order with respect to manganese dioxide and levofloxacin. The rate of reaction increases with the increasing [H⁺] ion concentration. A probable reaction mechanism, in agreement with the observed kinetic results, has been proposed and discussed. The energy and enthalpy of activation have been calculated to be 30.54 and 28.07 kJ mol^?1, respectively.     

In situ stabilization of Pd(0) nanoparticles into a mixture of natural carbohydrate beads: A novel and highly efficient heterogeneous catalyst system for Heck coupling reactions

A convenient, mild and cost‐effective synthesis of palladium nanoparticles stabilized by a mixture of natural carbohydrate beads (gum arabic and pectin) as a new bio‐organometallic catalyst is reported. Powder X‐ray diffraction, transmission and scanning electron microscopies and energy‐dispersive X‐ray and UV–visible spectroscopies were employed to characterize this supported Pd nanoparticles/gum arabic/pectin catalyst. The nanocatalyst exhibited efficient activity in Mizoroki–Heck cross‐coupling reactions between various aryl halides and n ‐butyl acrylate under solvent‐free conditions. The catalyst can easily be recovered from the reaction system and reused several times with high yields. The products were obtained in short reaction times with excellent yields.     

Dissection of the mechanism of manganese porphyrin-catalyzed chlorine dioxide generation

Chlorine dioxide, an industrially important biocide and bleach, is produced rapidly and efficiently from chlorite ion in the presence of water-soluble, manganese porphyrins and porphyrazines at neutral pH under mild conditions. The electron-deficient manganese(III) tetra-(N,N-dimethyl)imidazolium porphyrin (MnTDMImP), tetra-(N,N-dimethyl)benzimidazolium (MnTDMBImP) porphyrin, and manganese(III) tetra-N-methyl-2,3-pyridinoporphyrazine (MnTM23PyPz) were found to be the most efficient catalysts for this process. The more typical manganese tetra-4-N-methylpyridiumporphyrin (Mn-4-TMPyP) was much less effective. Rates for the best catalysts were in the range of 0.24-32 TO/s with MnTM23PyPz being the fastest. The kinetics of reactions of the various ClO(x) species (e.g., chlorite ion, hypochlorous acid, and chlorine dioxide) with authentic oxomanganese(IV) and dioxomanganese(V)MnTDMImP intermediates were studied by stopped-flow spectroscopy. Rate-limiting oxidation of the manganese(III) catalyst by chlorite ion via oxygen atom transfer is proposed to afford a trans-dioxomanganese(V) intermediate. Both trans-dioxomanganese(V)TDMImP and oxoaqua-manganese(IV)TDMImP oxidize chlorite ion by 1-electron, generating the product chlorine dioxide with bimolecular rate constants of 6.30 × 10(3) M(-1) s(-1) and 3.13 × 10(3) M(-1) s(-1), respectively, at pH 6.8. Chlorine dioxide was able to oxidize manganese(III)TDMImP to oxomanganese(IV) at a similar rate, establishing a redox steady-state equilibrium under turnover conditions. Hypochlorous acid (HOCl) produced during turnover was found to rapidly and reversibly react with manganese(III)TDMImP to give dioxoMn(V)TDMImP and chloride ion. The measured equilibrium constant for this reaction (K(eq) = 2.2 at pH 5.1) afforded a value for the oxoMn(V)/Mn(III) redox couple under catalytic conditions (E' = 1.35 V vs NHE). In subsequent processes, chlorine dioxide reacts with both oxomanganese(V) and oxomanganese(IV)TDMImP to afford chlorate ion. Kinetic simulations of the proposed mechanism using experimentally measured rate constants were in agreement with observed chlorine dioxide growth and decay curves, measured chlorate yields, and the oxoMn(IV)/Mn(III) redox potential (1.03 V vs NHE). This acid-free catalysis could form the basis for a new process to make ClO(2).     

Characterisation of natural polysaccharides (plant gums) used as binding media for artistic and historic works by capillary zone electrophoresis

The monosaccharide constituents of plant gums were separated by capillary electrophoresis at pH 12.1 and detected with indirect UV absorbance. The plant gums investigated were gum arabic, gum acacia, gum tragacanth, cherry gum and locust bean gum (carob gum). The monosaccharides obtained after hydrolysis with 2M trifluoroacetic acid and lyophilisation of the hydrolysate were arabinose, galactose, mannose, rhamnose, xylose, fucose, and glucose, and the two sugar acids galacturonic and glucuronic acid, in accordance with the literature. They were separated in a background electrolyte consisting of NaOH to adjust the pH, 20 mM 2,6-pyridinedicarboxylic acid as chromophore for detection and 0.5 mM cetyltrimethylammonium bromide as additive to reverse the electroosmotic flow. Based on their electropherograms, the plant gums could be identified by their typical composition (depicted in a decision scheme) as follows: a peak of glucuronic acid, together with that of rhamnose, is indicative for gum arabic. Peaks of galacturonic acid and fucose point to gum tragacanth. Locust bean gum shows a major peak for mannose (with the concomitant galactose peak in ratio 4-1), whereas a glucuronic acid and a mannose peak together with a prominent arabinose peak indicates cherry gum. The method was applied to identify the plant gums in samples like watercolours and in several paint layers like gum tempera or those with egg white or drying oils as additives. Artificial aging experiments of thin layers of gum arabic on paper or glass carried out with UV-A radiation (366 nm) did not result in changes of the saccharide patterns, in contrast to the simultaneously conducted aging of a drying oil layer.     

Molecular association and function of arabinogalactan protein complexes from tree exudates

Gum arabic is an exudate from the Acacia trees found in the arid conditions of the Sahelian belts of Africa. The particular gum discussed here is from the species Acacia senegal is a complex polydisperse molecule and is widely used in the food industry, mainly as a natural emulsifier. It is the high molecular weight arabinogalactan protein (AGP) component which is responsible for its emulsification functionality. A process is described which can increase the content and molecular weight of this component and as a result a new series of commercial gum arabic products have been produced, designated Acacia senegal(sey)SUPERGUM. The maturation process used to prepare these products is similar to that which occurs in the natural state of the tree. The paper describes how aggregation of the AGP can be controlled whereby low molecular weight components can be associated to form stable products. The mechanism of this association is discussed and explained in terms of inter-molecular equilibria. The results satisfactorily account for the large variation found in natural gum arabic and in commercial samples. Since this molecular variation can adversely affect the effectiveness of the gum arabic as an emulsifier, it is important that it can now be standardized and enhanced. There are indications that the basic building block of Acacia senegal is ca 400,000 Da but that aggregates of this unit can form depending on local conditions and commercial processing methods.     

Effect of phosphate complexation on Cd2+ sorption by manganese dioxide (beta-MnO2)

Sorption of metal ions on oxide/hydroxide surfaces mediates the fate and transport of these ions in many natural systems. These metallic ions often exist in bulk in the aqueous phase as complexes with inorganic and organic ligands. In the present study, we investigated the sorption properties of manganese dioxide in the presence of phosphate which is thought to be one of the most important complex forming species. The surface area, point of zero charge and structural morphology of the solid manganese dioxide were determined. Cd(2+) sorption studies were carried out on manganese dioxide as a function of pH, temperature and phosphate concentration. Cd(2+) sorption increased with increasing pH, temperature and phosphate concentration. It was found that phosphate formed both outer and inner sphere complexes via metal and ligand-like adsorption. The Langmuir equation was applied to describe the data and from the constants of this equation different thermodynamic parameters such as DeltaH(0), DeltaS(0) and DeltaG(0) were evaluated.