- Quinone hemiacetal formation from protocatechuic acid during the DPPH radical scavenging reaction
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Protocatechuic acid was rapidly converted to protocatechuquinone 3-methyl hemiacetal and protocatechuquinone during the reaction with DPPH radical in methanol. The structure of the acetal was determined by comparing the NMR data with those of an authentic
- Saito, Shizuka,Okamoto, Yasuko,Kawabata, Jun,Kasai, Takanori
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- Electrochemical behaviour of piperine. Comparison with control antioxidants
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Piperine, as the most abundant alkaloid in pepper, gained a lot of attention for possible antioxidant and therapeutic properties. Electrochemical techniques were applied to widely evaluate the redox behavior of piperine by comparison to that of well-known antioxidants: ascorbic acid, protocatechuic acid, syringic acid, tyrosine and capsaicin used as controls. Also, electrochemistry was involved in an innovative way to investigate the potential antioxidant properties of piperine combined with different in vitro peroxidation and reducing assays: (i) 1,1-diphenyl-2-picryl-hydrazyl free radical (DPPH) scavenging; (ii) 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) scavenging; (iii) ferric ions (Fe3+) reducing power; (iv) hydrogen peroxide (H2O2) scavenging. Results show that piperine readily reacts with highly oxidizing radicals and bind redox-active metal ions in a similar manner as antioxidants used as model.
- Arvinte, Adina,Carp, Oana Elena,Moraru, Aurelian,Pinteala, Mariana
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- Study and application of a controlled-potential electrochemistry- electrospray emitter for electrospray mass spectrometry
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This paper discusses continued studies and new analytical applications of a recently developed three-electrode controlled-potential electrochemical cell incorporated into an electrospray ion source (Van Berkel, G. J.; Asano, K. G.; Granger, M. C. Anal. Chem. 2004, 76, 1493-1499.). This cell contains a porous flow-through working electrode (i.e., the emitter electrode) with high surface area and auxiliary electrodes with small total surface area that are incorporated into the emitter electrode circuit to control the electrochemical reactions of analytes in the electrospray emitter. The current at the working and auxiliary electrodes, and current at the grounding points upstream and downstream of the emitter in the electrospray circuit, were recorded in this study, along with the respective mass spectra of model compound reserpine, under various operating conditions to better understand the electrochemical and electrospray operation of this emitter cell. In addition to the ability to control analyte oxidation in positive ion mode (or reduction in negative ion mode) in the electrospray emitter, this emitter cell system was shown to provide the ability to efficiently reduce analytes in positive ion mode and oxidize analytes in negative ion mode. This was demonstrated by the reduction of methylene blue in positive ion mode and oxidation of 3,4-dihydroxybenzoic acid in negative ion mode. Also, the ability to control electrochemical reactions via potential control was used to selectively ionize (oxidize) analytes with different standard electrochemical potentials within mixtures to different charge states to overcome overlapping molecular ion isotopic clusters. The analytical benefit of this ability was illustrated using a mixture of nickel and cobalt octaethylporphyrin.
- Kertesz, Vilmos,Van Berkel, Gary J.,Granger, Michael C.
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- Reduction kinetics of the antiradical probe 2,2-diphenyl-1-picrylhydrazyl in methanol and acetonitrile by the antiradical activity of protocatechuic acid and protocatechuic acid methyl ester
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This work evaluates the reduction kinetics of the antiradical probe 2,2-diphenyl-1-picrylhydrazyl (DPPH?) in methanol and acetonitrile by the antiradical activity of protocatechuic acid (3,4-dihydroxybenzoic acid, 1) and protocatechuic acid methyl ester (2). The reduction kinetics of DPPH? in both solvents by the antiradical activity of the p-catechol group in 2 is regular, that is, coincide with the proposed standard kinetic model for the reduction kinetics of DPPH ? by the antiradical activity of an isolated p-catechol group. Therefore, the antiradical activity of 2 experimentally exhibits two rate-two stoichiometric constants in acetonitrile and three rate-three stoichiometric constants in methanol. In contrast, the reduction kinetics of DPPH ? in both solvents by the antiradical activity of the p-catechol group in 1 is perturbed, that is, deviate from the proposed standard kinetic model. The deviations arise from the presence of the reactive carboxylic acid function which, in methanol, induces an additional reversible side reaction and, in acetonitrile, turns an irreversible reaction reversible, thus modifying the otherwise regular reduction kinetics of DPPH? by the antiradical activity of the p-catechol group in 1. On the other hand, the approximated theoretical kinetic equation that applies for those p-catechol groups whose reduction kinetics is regular and that experimentally exhibit three rate-three stoichiometric constants has been derived and used for fitting.
- Sentandreu, Enrique,Navarro, Jose L.,Sendra, Jose M.
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- Kinetic characterisation of o-aminophenols and aromatic o-diamines as suicide substrates of tyrosinase
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We study the suicide inactivation of tyrosinase acting on o-aminophenols and aromatic o-diamines and compare the results with those obtained for the corresponding o-diphenols. The catalytic constants follow the order aromatic o-diamines max/KmS, follows this order: o-diphenols > o-aminophenols > aromatic o-diamines.
- Munoz-Munoz, Jose Luis,Garcia-Molina, Francisco,Berna, Jose,Garcia-Ruiz, Pedro Antonio,Varon, Ramon,Tudela, Jose,Rodriguez-Lopez, Jose N.,Garcia-Canovas, Francisco
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experimental part
p. 647 - 655
(2012/08/08)
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- Dirhodium-catalyzed phenol and aniline oxidations with T-HYDRO. Substrate scope and mechanism of oxidation
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Dirhodium caprolactamate, Rh2(cap)4, is a very efficient catalyst for the generation of the tert-butylperoxy radical from tert-butyl hydroperoxide, and the tert-butylperoxy radical is a highly effective oxidant for phenols and anilines. These reactions are performed with 70% aqueous tert-butyl hydroperoxide using dirhodium caprolactamate in amounts as low as 0.01 mol % to oxidize para-substituted phenols to 4-(tert-butyldioxy) cyclohexadienones. Although these transformations have normally been performed in halocarbon solvents, there is a significant rate enhancement when Rh 2(cap)4-catalyzed phenol oxidations are performed in toluene or chlorobenzene. Electron-rich and electron-poor phenolic substrates undergo selective oxidation in good to excellent yields, but steric influences from bulky para substituents force oxidation onto the ortho position resulting in ortho-quinones. Comparative results with RuCl2(PPh 3)3 and CuI are provided, and mechanistic comparisons are made between these catalysts that are based on diastereoselectivity (reactions with estrone), regioselectivity (reactions with p-tert-butylphenol), and chemoselectivity in the formation of 4-(tert-butyldioxy)cyclohexadienones. The data obtained are consistent with hydrogen atom abstraction by the tert-butylperoxy radical followed by radical combination between the phenoxy radical and the tert-butylperoxy radical. Under similar reaction conditions, para-substituted anilines are oxidized to nitroarenes in good yield, presumably through the corresponding nitrosoarene, and primary amines are oxidized to carbonyl compounds by TBHP in the presence of catalytic amounts of Rh 2(cap)4.
- Ratnikov, Maxim O.,Farkas, Linda E.,McLaughlin, Emily C.,Chiou, Grace,Choi, Hojae,El-Khalafy, Sahar H.,Doyle, Michael P.
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scheme or table
p. 2585 - 2593
(2011/06/19)
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- DPPH (=2,2-diphenyl-1-picrylhydrazyl) radical-scavenging reaction of protocatechuic acid (=3,4-dihydroxybenzoic acid): Difference in reactivity between acids and their esters
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Prolocatechuic acid (=3,4-dihydroxybenzoic acid; 1) exhibits a significantly slow DPPH (=2,2-diphenyl-1-picrylhydrazyl) radical-scavenging reaction compared to its esters in alcoholic solvents. The present study is aimed at the elucidation of the difference between the radical-scavenging mechanisms of protocatechuic acid and its esters in alcohol. Both protocatechuic acid (1) and its methyl ester 2 rapidly scavenged 2 equiv. of radical and were converted to the corresponding o-quinone structures 1a and 2a, respectively (Scheme). Then, a regeneration of catechol (=benzene-1,2-diol) structures occurred via a nucleophilic addition of a MeOH molecule to the o-quinones to yield alcohol adducts 1f and 2c, respectively, which can scavenge additional 2 equiv. of radical. However, the reaction of protocatechuic acid (1) beyond the formation of the o-quinone was much slower than that of its methyl ester 2. The results suggest that the slower radical-scavenging reaction of 1 compared to its esters is due to a dissociation of the electron-withdrawing carboxylic acid function to the electron-donating carboxylate ion, which decreases the electrophilicity of the o-quinone, leading to a lower susceptibility towards a nucleophilic attack by an alcohol molecule.
- Saito, Shizuka,Kawabata, Jun
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p. 1395 - 1407
(2007/10/03)
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- Formation of a blue adduct between 4-tert-butyl-1,2-benzoquinone and 4- amino-N,N-diethylaniline
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The reaction of 4-tert-butyl-1,2-benzoquinone (BQ) with 4-amino-N,N- diethylaniline (ADA) produces a stable blue adduct with λ(max) 625 nm (ε 11,120 M-1 cm-1). This makes the reaction a sensitive analytical tool, suitable for the detection and spectrophotometric determination of this quinone even in the presence of biological compounds. The adduct was identified as 3-tert-butyl-4-(4'-diethylamino)phenylimino-6-hydroxycyclohexa- 2,5-dien-1-one. (C) 2000 Elsevier Science Ltd.
- Valgimigli, Luca,Pedulli, Gian Franco,Cabiddu, Salvatore,Sanjust, Enrico,Rescigno, Antonio
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p. 659 - 662
(2007/10/03)
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- Oxidation of benzenediols by hexabromoiridate(IV): Kinetics at ambient and elevated pressures
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The kinetics of oxidation of several benzenediols by the hexabromoiridate(IV) ion have been studied spectrophotometrically by the stopped-flow method. In 0.010 mol dm-3 HClO4 and an ionic strength of 0.10 mol dm-3 (NaClO4) at 25.0°C the second-order rate constants (the reaction is first order in each reactant concentration), vary from 1.26 × 102 to 9.3 × 104 dm3 mol-1 s-1. The enthalpies of activation range from about 44 kJ mol-1 for the slowest reacting substrate to about 20 kJ mol-1 for the faster reactions. The ΔS? values do not vary over a wide range; the reaction rates are governed more by the enthalpy barrier. Application of pressure (up to 125 MPa) causes significant rate accelerations, giving rise to ΔV? values in the -17 to -26 cm3 mol-1 range, consistent with the large, negative ΔS? values. This indicates that the rate limiting step is largely characterised by an increase in species ordering and electrostriction, and in the present case slightly less than for the corresponding reactions with the less bulky hexachloroiridate(IV) ion.
- Ciosto, Cornelia,Bajaj, Hari C.,Van Eldik, Rudi,Hubbard, Colin D.
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p. 1503 - 1507
(2007/10/03)
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- Kinetics of the Oxidation of Ascorbic Acid and Substituted 1,2- and 1,4-Dihydroxybenzenes by the Hexacyanoruthenate(III) Ion in Acidic Perchlorate Media
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The kinetics of oxidation of ascorbic acid and a series of substituted 1,2- and 1,4-dihydroxybenzene compounds (H2Q) by 3- have been investigated in acidic perchlorate media.The inverse dependences of the rate constants on acid concentrations for 2,3-dicyano-1,4-dihydroxybenzene, 4,5-dihydroxybenzene-1,3-disulphonate, and ascorbic acid have been attributed to concurrent rate-determining pathways involving the one-electron oxidations of H2Q or HQ(-) by 3- to the corresponding semiquinone or ascorbate radical intermediate.The cross-reaction rate constants have been correlated with the semiquinone or ascorbate reduction potentials in terms of the Marcus relationship to yield a 3--4- electron self-exchange rate constant of (1.0 +/- 0.8) . 105 dm3mol-1s-1.This is compared with those for other low-spin d5-d6 transition-metal complex couples and discussed in terms of the inner-sphere and solvent reorganization energies.
- Hoddenbagh, J. Mark A.,Macartney, Donal H.
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p. 615 - 620
(2007/10/02)
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- Mechanisms of Reduction of trans-Cyclohexane-1,2-diamine-NNN'N'-tetra-acetatomanganate(III) by Hydrazine, Hydroxylamine, and Substituted Benzene-1,2-diols
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The kinetics and mechanism of reduction of trans-cyclohexane-1,2-diamine-NNN'N'-tetra-acetatomanganate(III), III(cdta)(H2O)>-, by hydrazine hydroxylamine, and a series of substituted benzene-1,2-diols C6H3(OH)2R (R=H, 4-Me, 4-CO2H, or 3-CO2H) have been investigated by the stopped-flow technique.Simple first-order dependence on MnIII complex and reductant concentrations has been observed for hydrazine, which shows a rate with +>-1 dependence.For hydroxylamine, complex kinetic dependences on reductant and hydrogen-ion concentration suggest a composite inner-sphere mechanism.Also, in the case of the diols an inner-sphere mechanism, through intermediate association, is proposed; for these substrates experimental rates, higher than those computed with the Marcus cross relation, and limiting kinetics are in agreement with the lability of the oxidizing complex.
- Arselli, Patrizia,Mentasti, Edoardo
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p. 689 - 696
(2007/10/02)
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