- The effect of the medium polarity on the mechanism of the reaction of hydroxybenzenes with hydrazyl radical in aprotic solvents
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Mechanisms of the reaction of di- and trihydroxybenzenes with 2,2′-diphenyl-1-picrylhydrazyl (stable radical) in aprotic media of different polarity have been elucidated by experimental and quantum-chemical methods. Kinetic, stoichiometric, and activation
- Belaya,Belyj,Zarechnaya,Scherbakov,Mikhalchuk,Doroshkevich
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p. 690 - 697
(2017/05/29)
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- Myricetin, rosmarinic and carnosic acids as superior natural antioxidant alternatives to α-tocopherol for the preservation of omega-3 oils
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22 natural polyphenols are compared to 7 synthetic antioxidants including BHT, BHA, TBHQ and PG with regard to their ability to protect omega-3 oils from autoxidation. The antioxidant efficiency of phenols is assessed using the DPPH test and the measurement of oxygen consumption during the autoxidation of oils rich in omega-3 fatty acids. Also, the bond dissociation enthalpies (BDE) of the Ar-OH bonds are calculated and excellent correlations between thermodynamic, kinetic and oxidation data are obtained. It is shown that kinetic rates of hydrogen transfer, number of radicals scavenged per antioxidant molecule, BDE and formation of antioxidant dimers from the primary radicals play an important role regarding the antioxidant activity of phenols. Based on this, it is finally shown that myricetin, rosmarinic and carnosic acids are more efficient than α-tocopherol and synthetic antioxidants for the preservation of omega-3 oils.
- Guitard, Romain,Paul, Jean-Fran?ois,Nardello-Rataj, Véronique,Aubry, Jean-Marie
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p. 284 - 295
(2016/07/12)
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- The chain-breaking antioxidant activity of phenolic compounds with different numbers of O-H groups as determined during the oxidation of styrene
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The technique based on monitoring oxygen, consumption was applied to test 18 polyphenols (PP) and model phenolics as a chain-breaking antioxidant during the oxidation of styrene initiated by 2,2′-azobis(2,4- dimethylvaleronitril) at 37°C. The chain-breaking capability of PP was characterized by two parameters: the rate constant k1 for the reaction of antioxidants with the peroxy radical produced from styrene and the stoichiometric coefficient of inhibition, f, which shows how many kinetic chains are terminated by one molecule of PP. Rate constants k1 × 105 (in M-1 s-1) were found to be 10 (catechol), 27 (pyrogallol), 34 (3,6-di-tert-Bucatechol), 4.3 (protocatechic acid), 12 (gallic acid), 15 (caffeic acid), 1 increases when going from one to two and three adjacent O-H groups in a benzene ring (catechol and pyrogallol derivatives, respectively). At the same time, two O-H. groups in metaposition in a A-ring of flavonoids actually do not participate in the inhibition. For the majority of PP, f is near to 2 independent of the number of OH groups. The correlation of k1 with the structure of PP and the O-H bond dissociation enthalpy has been discussed.
- Tikhonov, Ivan,Roginsky, Vitaly,Pliss, Evgeny
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experimental part
p. 92 - 100
(2009/06/28)
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- Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical
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The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction. The Royal Society of Chemistry 2006.
- Foti, Mario C.,Daquino, Carmelo
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p. 3252 - 3254
(2008/10/09)
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- Infrared spectrum of 2-hydroxyphenoxyl radical and photoisomerization between trans and cis 2-hydroxyphenyl nitrites
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Photoreaction of 2-nitrophenol has been investigated by low-temperature matrix-isolation IR spectroscopy and density-functional-theory (DFT) calculation. A reaction intermediate was identified as syn-trans 2-hydroxyphenyl nitrite, and the reaction product caused by dissociation of NO from the intermediate as 2-hydroxyphenoxyl radical. Both syn-trans and syn-cis 2-hydroxyphenyl nitrite isomers were produced by recombination of 2-hydroxyphenoxyl radical with NO in annealing at 27 K after the UV irradiation. The conformational change from cis to trans around the O-N axis occurred upon visible-light irradiation. The rate constants of photoreaction were determined by a least-squares fitting of the changes in the IR band intensities.
- Nagaya, Maki,Kudoh, Satoshi,Nakata, Munetaka
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p. 446 - 451
(2007/10/03)
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- Electron-Transfer Reaction of Cinnamic Acids and Their Methyl Esters with the DPPH. Radical in Alcoholic Solutions
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The kinetic behavior of cinnamic acids, their methyl esters, and two catechols 1-10 (ArOH) in the reaction with DPPH. in methanol and ethanol is not compatible with a reaction mechanism that involves hydrogen atom abstraction from the hydroxyl
- Foti, Mario C.,Daquino, Carmelo,Geraci, Corrada
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p. 2309 - 2314
(2007/10/03)
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- O-H Bond dissociation enthalpies in hydroxyphenols. A time-resolved photoacoustic calorimetry and quantum chemistry study
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Time-resolved photoacoustic calorimetry (TR-PAC) was used to investigate the energetics of O-H bonds of phenol, catechol, pyrogallol, and phloroglucinol. Values of -27.1 ± 3.9, -44.1 ± 4.4 and -1.6 ± 3.8 kJ mol-1, respectively, were obtained fo
- Correia, Catarina F.,Guedes, Rita C.,Borges Dos Santos, Rui M.,Costa Cabral, Benedito J.,Martinho Simoes, Jose A.
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p. 2109 - 2118
(2007/10/03)
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- Kinetics and mechanisms of reactions of the nitrate radical (NO 3) with substituted phenols in aqueous solution
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Second order rate constants were obtained for the reactions of the nitrate radical (NO3) with substituted phenols in aqueous solutions at 298 K and pH = 0.5. The following compounds were investigated and the corresponding rate constants are rep
- Barzaghi, Paolo,Herrmann, Hartmut
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p. 5379 - 5388
(2007/10/03)
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- Diphenol radical cations and semiquinone radicals as direct products of the free electron transfer from catechol, resorcinol and hydroquinone to parent solvent radical cations
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In the pulse radiolysis of solutions of catechol, resorcinol and hydroquinone in n-butylchloride, dihydroxybenzene radical cations (40%) as well as semiquinone radicals (60%) are observed as direct synchronously formed products of the electron transfer from the solvent parent ions to the solute. This is explained in terms of free electron transfer succeeding in nearly every encounter of the reactants, which in the case of the studied dihydroxybenzenes involves femtosecond molecular dynamics effects. The rotation of the C-OH bond causes cycling of the molecule through transient conformations also exhibiting different electron distributions. From the more chemical point of view, the diphenol radical cations represent the first and till now unknown intermediates of oxidative semiquinone radical formation.
- Brede, Ortwin,Kapoor, Sudhir,Mukherjee, Tulsi,Hermann, Ralf,Naumov, Sergej
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p. 5096 - 5104
(2007/10/03)
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- Naphthalene diols: a new class of antioxidants intramolecular hydrogen bonding in catechols, naphthalene diols, and their aryloxyl radicals.
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1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH*), k(ArOH/DOPPH)*, or as antioxidants during inhibited styrene autoxidation, k(ArOH/ROO)*, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k(ArOH/ROO)* = 6.0 x 10(6) M(-)(1) s(-1), of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (deltaBDEs), of 1-6 predict a HAT order of reactivity of 2 alpha(ROO)* = 0.32, gave the expected order, since the ROO* reaction is more exothermic than the DOPPH* reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log k(ArOH/ROO)* versus deltaBDE plot.
- Foti, Mario C,Johnson, Erin R,Vinqvist, Melinda R,Wright, James S,Barclay, L Ross C,Ingold
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p. 5190 - 5196
(2007/10/03)
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- Stopped-flow and spectrophotometric study on radical scavenging by tea catechins and the model compounds
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Radical scavenging of four tea catechins, (-)-epicatechin (EC), (-)- epigallocatechin (EGC), (-)-epicatechin gallate (ECg) and (-)- eppigallocatechin gallate (EGCg), and the model compounds of their partial structure was examined against the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical using stopped-flow and spectrophotometric methods. The number of DPPH radicals scavenged by a polyphenol molecule was larger than that of phenolic hydroxyl groups, suggesting that hydrogens which bond directly to the aromatic ring can also participate in radical scavenging. A model for the scavenging reaction was proposed in which the reaction proceeded with successive dehydrogenation from a polyphenol molecule. Analysis of the second order reaction rate constants and the activation parameters between DPPH and polyphenol at the early stage of the reaction showed that the values depended on the number of phenolic hydroxyl groups and their mutual position. Contribution of the A ring of catechins to the rate constants was estimated to be far smaller than that from the B ring. In the EGCg molecule, the B ring and the gallate group were not independent, but acted as a single group for DPPH radical scavenging.
- Senba, Yasushi,Nishishita, Tsukasa,Saito, Kieko,Yoshioka, Hiroe,Yoshioka, Hisashi
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p. 1369 - 1374
(2007/10/03)
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- EPR and spin-trapping investigation of free radicals from the reaction of 4-methoxybenzenediazonium tetrafluoroborate with melanin and melanin precursors
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The interaction of synthetic DOPA melanin (DM) and its precursors (catechols and phenols) with 4-methoxybenzenediazonium tetrafluoroborate (4-MeO-PhN2BF4) has been studied using EPR spectroscopy and the spin-trapping technique. We found that DM, catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyhydrocinnamic acid, 3,4-dihydroxyphenylalanine, and 6-hydroxydopamine all react with 4-MeO-PhN2+ through a one-electron-transfer process which gives rise to an aryl radical (4-MeO-Ph.) derived from the diazonium compound and to radicals from melanin and from the catechol(amine)s. The formation of aryl radicals is an autocatalytic process. To explain the autocatalysis, we postulate a mechanism in which the key step is the formation of o-quinones. In aerated solutions the aryl radicals react with oxygen, which leads to oxygen consumption. The reaction was found to be order 1, 0.5, and 0.35 with respect to 4-MeO-PhN2+, catechol, and oxygen concentration, respectively. Phenol, 4-hydroxyanisole, and tyrosine do not reduce 4-MeO-PhN2+ unless they are activated by the enzyme tyrosinase. In the presence of tyrosinase, tyrosine produces the most efficient reducing agent. This indicates that the conversion of phenols to o-dihydroxybenzene derivatives by tyrosinase is essential for aryl radical formation from 4-MeO-PhN2+. These observations substantiate the ability of hydroquinones and semiquinone radicals to promote the homolysis of diazonium salts to generate aryl radicals. Such reductive activation of diazonium compounds may be pertinent to their biological, mutagenic, and carcinogenic action.
- Reszka, Krzysztof J.,Chignell, Colin F.
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p. 7752 - 7760
(2007/10/02)
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- One-Electron Redox Reactions in Aqueous Solutions of Sulfite with Hidroquinone and Other Hydrophenols
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The one-electron oxidation of hydroquinone and other hydroxyphenols by the sulfite radical, SO3-, and the preoxysulfate radical, SO5(1-), has been investigated along with the oxidation of sulfite by several phenoxy radicals.Rate constants for t
- Huie, Robert E.,Neta, P.
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p. 3918 - 3921
(2007/10/02)
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- One-Electron Redox Potentials of Phenols. Hydroxy- and Aminophenols and Related Compounds of Biological Interest
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The rate constants for reversible electron transfer between a series of substituted phenolate ions and anilines and various substituted phenoxyl or anilino radicals in aqueous solution were measured by observing the formation or depletion of the radicals involved.Nonequilibrium concentrations of the radicals were produced in the presence of the corresponding phenols or anilines by using the pulse radiolysis technique.The relaxation of the system to equilibrium was monitored by optical detection methods.From the equilibrium constants for one-electron transfer, the one-electron redox potentials (E2) for 38 phenolic or anilino type compounds were determined, many of which are natural products.The redox potentials are strongly influenced by electron-donating or -withdrawing substituents at the aromatic system.
- Steenken, S.,Neta, P.
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p. 3661 - 3667
(2007/10/02)
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- Reaction of tert-Butoxy Radicals with Phenols. Comparison with the Reactions of Carbonyl Triplets
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Tert-butoxy radicals generated in the photodecomposition of di-tert-butyl peroxide react efficiently with phenols to yield the corresponding phenoxy radicals.Typical rate constants in benzene at 22 deg C are 3.3x108 and 1.6x109 Msup
- Das, P. K.,Encinas, M. V.,Steenken, S.,Scaiano, J. C.
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p. 4162 - 4166
(2007/10/02)
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