Showing papers on "Reagent published in 2004"
TL;DR: The development of a simple, widely applicable antioxidant capacity index for dietary polyphenols and vitamins C and E is reported, utilizing the copper(II)-neocuproine [Cu(II-Nc] reagent as the chromogenic oxidizing agent.
Abstract: The chemical diversity of antioxidants makes it difficult to separate and quantify antioxidants from the vegetable matrix. Therefore, it is desirable to establish a method that can measure the total antioxidant activity level directly from vegetable extracts. The current literature clearly states that there is no "total antioxidant" as a nutritional index available for food labeling because of the lack of standard quantitation methods. Thus, this work reports the development of a simple, widely applicable antioxidant capacity index for dietary polyphenols and vitamins C and E, utilizing the copper(II)-neocuproine [Cu(II)-Nc] reagent as the chromogenic oxidizing agent. Because the copper(II) (or cupric) ion reducing ability of polyphenols is measured, the method is named by our research group "cupric reducing antioxidant capacity" abbreviated as the CUPRAC method. This method should be advantageous over the ferric reducing antioxidant power (FRAP) method because the redox chemistry of copper(II)-as opposed to that of ferric ion-involves faster kinetics. The method comprises mixing of the antioxidant solution (directly or after acid hydrolysis) with a copper(II) chloride solution, a neocuproine alcoholic solution, and an ammonium acetate aqueous buffer at pH 7 and subsequent measurement of the developed absorbance at 450 nm after 30 min. Because the color development is fast for compounds such as ascorbic acid, gallic acid, and quercetin but slow for naringin and naringenin, the latter compounds were assayed after incubation at 50 degrees C on a water bath for 20 min [after Cu(II)-Nc reagent addition] so as to force the oxidation reaction to reach completion. The flavonoid glycosides were hydrolyzed to their corresponding aglycons by refluxing in 1.2 M HCl-containing 50% MeOH so as to exert maximal reducing power toward Cu(II)-Nc. Certain compounds also needed incubation after acid hydrolysis to fully exhibit their reducing capability. The CUPRAC antioxidant capacities of synthetic mixtures of antioxidants were experimentally measured as Trolox equivalents and compared to those theoretically found by making use of the principle of additivity of absorbances assuming no chemical interaction between the mixture constituents. Because ascorbic acid is not resistant to elevated temperature incubation, it should be assayed initially by measuring the absorbance (at 450 nm) difference of original and ascorbate oxidase-added mixture solutions at the end of 1 min of Cu(II)-Nc reagent addition. Thus, the total CUPRAC antioxidant capacity of a mixture containing various antioxidants should be that finally measured after a suitable combination of hydrolysis and incubation procedures, added to the initially measured capacity due to ascorbate. The antioxidant polyphenolic compounds tested demonstrate that the highest capacities in the CUPRAC method were observed for epicatechin gallate, epigallocatechin gallate, quercetin, fisetin, epigallocatechin, catechin, and caffeic acid in this order, in accordance with theoretical expectations, because the number and position of the hydroxyl groups as well as the degree of conjugation of the whole molecule are important. The antioxidant potency of flavonoids is nearly proportional to the total number of -OH groups and is positively affected by the presence of an o-dihydroxy moiety in the B-ring. beta-Carotene, which did not react with the CUPRAC reagent in alcoholic aqueous medium, could be assayed in dichloromethane solvent. Linear calibration curves for ascorbic acid and flavonoids were redrawn in synthetic solutions containing a mixture of antioxidants, and also in real matrices such as grape and orange juices, green tea, and blackberry tea, showing an initial nonzero absorbance with the CUPRAC reagent. The parallellism of the linear calibration curves of pure compounds in a given complex matrix effectively demonstrated that there were no interferent chemical interactions among the solution constituents and that the antioxidant capacities of the tested antioxidants were additive. The CUPRAC reagent is reasonably selective, stable, easily accessible, and sensitive toward thiol-type oxidants, unlike the FRAP method. The reaction is carried out at nearly physiological pH as opposed to the unrealistic acidic pH of FRAP.
1,782 citations
TL;DR: An overview of glutaraldehyde as a crosslinking reagent is given by describing its structure and chemical properties in aqueous solution in an attempt to explain its high reactivity toward proteins, particularly as applied to the production of insoluble enzymes.
Abstract: Glutaraldehyde possesses unique characteristics that render it one of the most effective protein crosslinking reagents. It can be present in at least 13 different forms depending on solution conditions such as pH, concentration, temperature, etc. Substantial literature is found concerning the use of glutaraldehyde for protein immobilization, yet there is no agreement about the main reactive species that participates in the crosslinking process because monomeric and polymeric forms are in equilibrium. Glutaraldehyde may react with proteins by several means such as aldol condensation or Michael-type addition, and we show here 8 different reactions for various aqueous forms of this reagent. As a result of these discrepancies and the unique characteristics of each enzyme, crosslinking procedures using glutaraldehyde are largely developed through empirical observation. The choice of the enzyme-glutaraldehyde ratio, as well as their final concentration, is critical because insolubilization of the enzyme must result in minimal distortion of its structure in order to retain catalytic activity. The purpose of this paper is to give an overview of glutaraldehyde as a crosslinking reagent by describing its structure and chemical properties in aqueous solution in an attempt to explain its high reactivity toward proteins, particularly as applied to the production of insoluble enzymes.
1,515 citations
TL;DR: A novel, colorimetric and fully automated method for measuring total antioxidant response (TAR) against potent free radical reactions is described and can be used to measure TAR of samples against potentfree radical reactions.
1,423 citations
TL;DR: An iron-catalyzed cross-coupling reaction of a primary or secondary alkyl halide with an aryl Grignard reagent proceeds under mild conditions to give the corresponding coupling product in quantitative yield.
Abstract: An iron-catalyzed cross-coupling reaction of a primary or secondary alkyl halide with an aryl Grignard reagent proceeds under mild conditions to give the corresponding coupling product in quantitative yield.
431 citations
TL;DR: Characteristics and application of Marfey’s reagent (MR) including general protocols for synthesis of the reagent and diastereomers along with advantages, disadvantages and the required precautions are described.
Abstract: The present paper describes characteristics and application of Marfey's reagent (MR) including general protocols for synthesis of the reagent and diastereomers along with advantages, disadvantages and the required precautions. Applications, and comparison with other derivatizing agents, for the resolution of complex mixtures of DL-amino acids, amines and non-proteinogenic amino acids, peptides/amino acids from microorganisms, cysteine residues in peptides, and evaluation of racemizing characteristics have been discussed. Separation mechanisms of resolution of amino acid diastereomers and replacement of Ala-NH2 by suitable chiral moieties providing structural analogs and different chiral variants and their application as a derivatizing agent to examine the efficiency, and reactivity of the reagent have been focussed. Use of MR for preparing CSPs for direct enantiomeric resolution has also been included.
402 citations
TL;DR: The freeze-frame reaction offers both a strategically original approach for drug discovery and a means for kinetically controlled capture, as a high-affinity complex between the enzyme and its self-created inhibitor, of a highly reactive minor abundance conformer of a fluctuating protein template.
Abstract: The 1,3-dipolar cycloaddition reaction between unactivated azides and acetylenes proceeds exceedingly slowly at room temperature. However, considerable rate acceleration is observed when this reaction occurs inside the active center gorge of acetylcholinesterase (AChE) between certain azide and acetylene reactants, attached via methylene chains to specific inhibitor moieties selective for the active center and peripheral site of the enzyme. AChE catalyzes the formation of its own inhibitor in a highly selective fashion: only a single syn1-triazole regioisomer with defined substitution positions and linker distances is generated from a series of reagent combinations. Inhibition measurements revealed this syn1-triazole isomer to be the highest affinity reversible organic inhibitor of AChE with association rate constants near the diffusion limit. The corresponding anti1 isomer, not formed by the enzyme, proved to be a respectable but weaker inhibitor. The crystal structures of the syn1- and anti1-mouse AChE complexes at 2.45- to 2.65-A resolution reveal not only substantial binding contributions from the triazole moieties, but also that binding of the syn1 isomer induces large and unprecedented enzyme conformational changes not observed in the anti1 complex nor predicted from structures of the apoenzyme and complexes with the precursor reactants. Hence, the freeze-frame reaction offers both a strategically original approach for drug discovery and a means for kinetically controlled capture, as a high-affinity complex between the enzyme and its self-created inhibitor, of a highly reactive minor abundance conformer of a fluctuating protein template.
291 citations
TL;DR: A detailed analysis of the preparative results suggests that iron-catalyzed C-C bond formations can occur via different pathways and it is likely that reactions of methylmagnesium halides involve iron-ate complexes as the active components, whereas reactions of Grignard reagents with two or more carbon atoms are effected by highly reduced iron-clusters of the formal composition [Fe(MgX)(2)](n) generated in situ.
Abstract: Cheap, readily available, air stable, nontoxic, and environmentally benign iron salts such as Fe(acac)3 are excellent precatalysts for the cross-coupling of Grignard reagents with alkenyl triflates and acid chlorides. Moreover, it is shown that dichloroarene and -heteroarene derivatives as the substrates can be selectively monoalkylated by this method. All cross-coupling reactions proceed very rapidly under notably mild conditions and turned out to be compatible with a variety of functional groups in both reaction partners. A detailed analysis of the preparative results suggests that iron-catalyzed C−C bond formations can occur via different pathways. Thus, it is likely that reactions of methylmagnesium halides involve iron−ate complexes as the active components, whereas reactions of Grignard reagents with two or more carbon atoms are effected by highly reduced iron-clusters of the formal composition [Fe(MgX)2]n generated in situ. Control experiments using the ate-complex [Me4Fe]Li2 corroborate this inter...
286 citations
TL;DR: Through the development of relevant IBX-based protocols, a plethora of useful synthetic intermediates, including imines, oximes, ketones, and aromatic N-heterocycles, were found to be readily accessible under notably mild conditions.
Abstract: o-Iodoxybenzoic acid (IBX), a highly versatile hypervalent iodine(V) reagent, was found to efficiently mediate the dehydrogenation of amines in addition to facilitating the oxidative cleavage of dithioacetals and dithioketals. Through the development of relevant IBX-based protocols, a plethora of useful synthetic intermediates, including imines, oximes, ketones, and aromatic N-heterocycles, were found to be readily accessible under notably mild conditions. Further investigation of these transformations led to the elucidation of valuable mechanistic details, resulting in the conclusion that they proceed via ionic rather than single electron transfer (SET) pathways.
266 citations
TL;DR: In this paper, a series of starch esters with different side-chain lengths and moderate DS-values were prepared and studied by acylation of the gelatinised starch with the appropriate acid chloride in 2.5 M aqueous sodium hydroxide (NaOH) solution.
235 citations
TL;DR: In this article, the authors describe the oxidation of several model S-containing molecules with hydrogen peroxide in a two liquid-liquid (L-L) phase system with a phase transfer catalyst under atmospheric pressure in the 333-353 K temperature range.
210 citations
TL;DR: The results indicate that even dilute concentrations of solvents, including H202, can increase the reactivity of O2- in water, probably by changing its solvation sphere.
Abstract: The reactive oxygen species responsible for the transformation of carbon tetrachloride (tetrachloromethane, CT) by modified Fenton's reagent using hydrogen peroxide (H2O2) concentrations >0.1 M was investigated. Addition of the hydroxyl radical scavenger 2-propanol to modified Fenton's reactions did not significantly lower CT transformation rates. Scavenging by 2-propanol not only confirmed that hydroxyl radicals are not responsible for CT destruction, but also suggested that a major product of an iron (III)-driven initiation reaction, superoxide radical anion (O2•-), is the species responsible for CT transformation. To investigate this hypothesis, CT degradation was studied in aqueous KO2 reactions. Minimal CT degradation was found in CT−KO2 reactions; however, when H2O2 was added to the KO2 reactions at concentrations similar to those in the modified Fenton's reactions (0.1, 0.5, and 1 M), CT degradation increased significantly. Similar results were obtained when 1 M concentrations of other solvents wer...
TL;DR: In this article, a review of manganese-, iron-, chromium-, and cobalt-catalyzed carbon−carbon bond formation reactions using Grignard reagents is presented.
TL;DR: A method for nickel-catalyzed asymmetric carbon dioxide incorporation via carbon-carbon bond formation was developed to afford cyclic carboxylic acids in good yields and with high enantioselectivities.
Abstract: A method for nickel-catalyzed asymmetric carbon dioxide (CO2) incorporation via carbon−carbon bond formation was developed In the presence of a catalytic amount of Ni(acac)2 and MeO-MOP, various bis-1,3-dienes reacted with CO2 (1 atm) and a diorganozinc reagent (Me2Zn or Ph2Zn) to afford cyclic carboxylic acids in good yields (71−100%) and with high enantioselectivities (90−96% ee)
TL;DR: In this article, the anti-bacterial activity of the catalysts were determined qualitatively by testing the presence of coliforms in water after contacting with the catalyst, using a Readycult reagent.
Abstract: The present work highlights the high efficiency of silver nano-particles deposited over activated carbon, by electro-chemical deposition method, in controlling microorganisms in water. The anti-bacterial activity of the catalysts were determined qualitatively by testing the presence of coliforms in water after contacting with the catalyst, using a Readycult reagent. The catalytic characteristics of these materials are obtained by SEM (scanning electron microscopy), XRD (X-ray diffraction) and TPR (temperature programmed reduction). TPR results clearly indicate the presence of metallic silver in the dried catalyst prepared by electro-chemical deposition method and the presence of silver oxide and/or nitrate precursors in the catalysts prepared by impregnation method. SEM results indicate the presence of Ag particles in nanometer size. Comparison of the anti-bacterial activity of the Ag/C catalyst prepared by electro-chemical deposition method with that of the Ag/C catalyst prepared by conventional impregnation technique indicate that lower amount of former is sufficient in controlling the microorganism which is not the case with the latter. The main advantage of Ag/C catalyst prepared by electro-chemical deposition is that no pretreatment conditions like reduction are required for deactivation of microorganism in water, which is not the case with the catalysts prepared by impregnation technique.
TL;DR: An overview of the current knowledge of the reagent SmI2 as a reducing agent, in particular with additives that increase its reactivity is given in this article, where the use of various proton sources is covered, as well as the effect of co-solvents.
Abstract: Samarium(II) iodide is a one-electron transfer reagent that has become highly appreciated as a mild and selective reducing agent in recent years. It has been found experimentally that various additives and co-solvents largely control the reactivity of SmI2. This microreview provides an overview of the current knowledge of the reagent SmI2 as a reducing agent, in particular with additives that increase its reactivity. The use of various proton sources is covered, as well as the effect of co-solvents. Furthermore, the very powerful reagent mixture SmI2/H2O/amine is also described. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
TL;DR: These methods provide a fundamentally new approach to making small molecules containing the sulfonamide functional group.
Abstract: A general new method for the preparation of sulfonamides and activated sulfonate esters by the direct coupling of sulfonic acid salts with amines and alcohols using the reagent triphenylphosphine ditriflate is described. A new reusable polymer-supported reagent for these transformations under heterogeneous conditions is also described. These methods provide a fundamentally new approach to making small molecules containing the sulfonamide functional group.
TL;DR: The removal of pollutants in saline medium by the Fenton's reagent needs a more detailed investigation, since the presence of chloride may inhibit or retard degradation.
TL;DR: Alcohol oxidase plus Purpald is shown to be a simple and sensitive way to measure the methanol released from plant material following the thermal activation of endogenous pectin methylesterase.
Abstract: Three colorimetric reagents for the determination of formaldehyde, the Nash reagent (ammonia plus acetylacetone), Purpald (4-amino-3-hydrazino-5-mercapto-1,2,4-triazole), and N-methylbenzothiazolinone-2-hydrazone (MBTH), were compared for the determination of methanol when used in conjunction with alcohol oxidase. The combination of alcohol oxidase plus the commonly used Nash reagent was specific for methanol versus ethanol, but had the lowest sensitivity of the three reagents tested. Substituting Purpald for the Nash reagent increased the sensitivity 3-fold while still maintaining a high (59-fold) selectivity for methanol versus ethanol. Using MBTH increased the sensitivity still further, but with a loss of the selectivity toward methanol. Since MBTH reacted with aldehydes under neutral conditions, it could be included along with the alcohol oxidase to act as an aldehyde trap. This prevented further oxidation reactions by alcohol oxidase and allowed for extended incubations. A procedure for assaying low levels of pectin methylesterase activity that relies on this trapping ability is described. In addition, alcohol oxidase plus Purpald is shown to be a simple and sensitive way to measure the methanol released from plant material following the thermal activation of endogenous pectin methylesterase.
Patent•
18 Jun 2004TL;DR: The inventive test strip as discussed by the authors includes a reagent layer that extends across the width of the test strip and also extends to the sample-receiving end, such that the edges of the Reagent layer are aligned with the side and dose receiving edges of a test strip.
Abstract: A test strip having a small sample-receiving chamber on the order of less than 1 microliter. The inventive test strip includes a reagent layer that extends across the width of the test strip and also extends to the sample-receiving end, such that the edges of the reagent layer are aligned with the side and dose receiving edges of the test strip. The hydrophilic reagent extending to the dosing edge of the strip promotes wicking of the sample into the test strip. The end and side edges of the reagent layer are preferably formed as part of a cutting process that forms individual test strips from a larger web, which results in a smooth and thin reagent layer with a uniform thickness, substantially covering the entire floor of the sample-receiving chamber. The inventive mass production process helps improve the reproducibility of the quantity, location, thickness and other properties of the reagent layer, which in turn improves the accuracy of the test result.
TL;DR: In this article, the authors developed efficient and selective processes for oxidation reactions, in particular with H 2 O 2, oxirane ring-opening, and aza-Diels-Alder reactions without any catalyst.
Abstract: Owing to their unique properties (high hydrogen bonding donor ability, low nucleophilicity, high ionizing power and ability to solvate water), fluorinated alcohols, hexafluoroisopropanol (HFIP) and trifluoroethanol (TFE),modify the course of reactions when they are used as solvents, allowing reactions, which usually require the use of added reagents or metal catalysts to be carried out under neutral and mild conditions. New efficient and selective processes were developed for oxidation reactions, in particular with H 2 O 2 , oxirane ring-opening, and aza-Diels-Alder reactions without any catalyst. Products were easily isolated in high yields, and the fluoroalcohols could be directly recovered from the reaction medium and reused. These processes bring an improvement from an environmental point of view, by suppression of effluents, in particular heavy metals and are particularly simple, since most of the reactions do not require any work-up.
TL;DR: The procedure for measurement of different forms of four plasma thiols cysteine, cysteinylglycine, glutathione and homocysteine is proposed and is applied for plasma samples donated by apparently healthy volunteers.
TL;DR: The synthesis of aromatic fluoromonomer compounds functionalized by halogen atoms, organic functions or acid groups prepared and characterized by different methods, are described in this paper, which deals with the synthesis and the polymerization of fluorostyrene bearing the fluorine atom(s) in α and/or β position, mainly.
TL;DR: After esterification, the thioglycuronic acids proved to be useful as both donor and acceptor in sulfonium-mediated condensations toward acidic di- and trisaccharides.
TL;DR: Degradation of methyl tert-butyl ether in aqueous solution by Fenton's reagent (Fe2+ and H2O2) showed that MTBE was decomposed in a two-stage reaction, and a reaction mechanism involving two different pathways for the decomposition of MTBE by Fetus' reagent was proposed.
TL;DR: The conversion of an alkenyl iodide bearing a keto group to the corresponding silylated cyanohydrin allows preparation of the corresponding Grignard reagent affording, after acylation and deprotection, unsaturated 1,4-diketones.
TL;DR: In this paper, the authors investigated the mechanism of combustion synthesis of lanthanum chromite by carrying out simultaneous differential thermal analysis (DTA), thermal-gravimetric analysis (TGA), and quadrupole mass spectrometry measurements (MS).
Abstract: The mechanism of combustion synthesis of lanthanum chromite was investigated by carrying out simultaneous differential thermal analysis (DTA), thermal-gravimetric analysis (TGA) and quadrupole mass spectrometry measurements (MS). The whole process was found to involve several phenomena: urea and nitrates thermal decomposition, exothermal reactions occurring directly between nitrates and urea as well as between their decomposition products, final reaction between solid oxides. In order to better understand the complex calorimetric, thermal-gravimetric and mass spectrometric curves, the behaviour on heating of each single reagent as well as that of binary mixtures, made by combining urea with each single nitrate, were also studied. Combustion synthesis was performed either under an oxidizing atmosphere or under an inert one. Chemical reactions possibly occurring during this combustion synthesis have been proposed on the basis of MS analysis of gaseous reaction products. The effect of using an excess of sacrificial fuel jointly with the addition of ammonium nitrate was also investigated.
TL;DR: Examples of some common processes (Fischer esterification, lithiation, butyllithium/THF compatibility, olefin metathesis, and a quantification assay), each interrogated in its native solvent, are presented.
TL;DR: In this paper, a dianion mechanism through a transition state [RX2−] was proposed for Grignard reactions of certain vinyl and aryl halides, even when intermediate radicals R would isomerize very rapidly.
TL;DR: Perchloric acid supported on silica gel acts as an excellent reagent system in converting glucals into 2,3-unsaturated-O-glucosides in good to excellent yields in short reaction time with good alpha selectivity.
Abstract: Perchloric acid supported on silica gel acts as an excellent reagent system in converting glucals into 2,3-unsaturated-O-glucosides in good to excellent yields in short reaction time with good a selectivity. Primary, secondary, and allylic alcohols, phenols, and thiols react with 3,4,6-tri-O-acetyl glucal with equal ease. In addition to this, a chiral furan diol is obtained from unprotected D-glucal or D-galactal in good yields.