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Kinetic Study of the Scavenging Reaction of the Aroxyl Radical by Seven Kinds of Rice Bran Extracts in Ethanol Solution. Development of an Aroxyl Radical Absorption Capacity (ARAC) Assay Method. 2.
J. Agric. Food Chem.
PUBLISHED: 11-14-2014
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Recently, a new assay method that can quantify the aroxyl radical (ArO•) absorption capacity (ARAC) of antioxidants (AOHs) was proposed. In the present work, the second-order rate constants (ksExtract) and ARAC values for the reaction of ArO• with seven kinds of rice bran extracts 1 - 7, which contain different concentrations of ?-, ?-, ?-, and ?-tocopherols and -tocotrienols (?-, ?-, ?-, and ?-Tocs and -Toc-3s) and ?-oryzanol, were measured in ethanol at 25oC using stopped-flow spectrophotometry. The ksExtract value (1.26×10-2 M-1s-1) of Nipponbare (extract 1) with the highest activity was 1.5 times larger than that (8.29×10-3) of Milyang-23 (extract 7) with the lowest activity. The concentrations (in mg/100 g) of ?-, ?-, ?-, and ?-Tocs and -Toc-3s and ?-oryzanol found in the seven extracts 1 - 7 were determined using HPLC-MS/MS and UV-vis absorption spectroscopy, respectively. From the results, it has been clarified that the ArO•-scavenging rates (ksExtract) (that is, the relative ARAC value) obtained for the seven extracts 1 - 7 may be approximately explained as the sum of the product {? ksAOH-i [AOH-i]/105} of the rate constant (ksAOH-i) and the concentration ([AOH-i]/105) of AOH-i (Tocs, Toc-3s, and ?-oryzanol) included in rice bran extracts. The contribution of ?-oryzanol to the ksExtract value was estimated to be between 3.0 - 4.7% for each extract. Taken together, these results suggest that the ARAC assay method is applicable to general food extracts.
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Development of singlet oxygen absorption capacity (SOAC) assay method. 4. Measurements of the SOAC values for vegetable and fruit extracts.
Biosci. Biotechnol. Biochem.
PUBLISHED: 11-01-2014
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Measurements of the second-order rate constants and the singlet oxygen absorption capacity (SOAC) values for the reaction of singlet oxygen ((1)O2) with 23 kinds of food extracts were performed in ethanol/chloroform/D2O (50:50:1, v/v/v) solution at 35 °C. It has been clarified that the SOAC method is useful to evaluate the (1)O2-quenching activity (i.e. the SOAC value) of food extracts having two orders of magnitude different rate constants from 3.18 × 10(4) L g(-1) s(-1) for tomato to 1.55 × 10(2) for green melon. Furthermore, comparison of the observed rate constants for the above food extracts with the calculated ones based on the concentrations of seven kinds of carotenoids included in the food extracts and the rate constants reported for each carotenoids was performed, in order to ascertain the validity of the SOAC assay method developed and to clarify the ratio of the contribution of principal carotenoids to the SOAC value.
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Kinetic study of the quenching reaction of singlet oxygen by ?-, ?-, ?-, ?-tocotrienols, and palm oil and soybean extracts in solution.
Biosci. Biotechnol. Biochem.
PUBLISHED: 08-06-2014
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Measurements of the singlet oxygen ((1)O2) quenching rates (kQ (S)) and the relative singlet oxygen absorption capacity (SOAC) values were performed for 11 antioxidants (AOs) (eight vitamin E homologues (?-, ?-, ?-, and ?-tocopherols and -tocotrienols (-Tocs and -Toc-3s)), two vitamin E metabolites (?- and ?-carboxyethyl-6-hydroxychroman), and trolox) in ethanol/chloroform/D2O (50:50:1, v/v/v) and ethanol solutions at 35?°C. Similar measurements were performed for five palm oil extracts 1-5 and one soybean extract 6, which included different concentrations of Tocs, Toc-3s, and carotenoids. Furthermore, the concentrations (wt%) of Tocs, Toc-3s, and carotenoids included in extracts 1-6 were determined. From the results, it has been clarified that the (1)O2-quenching rates (kQ (S)) (that is, the relative SOAC value) obtained for extracts 1-6 may be explained as the sum of the product {? kQ(AO-i) (S) [AO-i]/100} of the rate constant (kQ(AO-i) (S)) and the concentration ([AO-i]/100) of AO-i (Tocs, Toc-3s, and carotenoid) included.
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Finding of synergistic and cancel effects on the aroxyl radical-scavenging rate and suppression of prooxidant effect for coexistence of ?-tocopherol with ?-, ?-, and ?-tocopherols (or -tocotrienols).
J. Agric. Food Chem.
PUBLISHED: 07-30-2014
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Measurements of aroxyl radical (ArO(•))-scavenging rate constants (ks(AOH)) of antioxidants (AOHs) [?-, ?-, ?-, and ?-tocopherols (TocHs) and -tocotrienols (Toc-3Hs)] were performed in ethanol solution via stopped-flow spectrophotometry. ks(AOH) values of ?-, ?-, ?-, and ?-Toc-3Hs showed good agreement with those of the corresponding ?-, ?-, ?-, and ?- TocHs. ks(AOH) values were measured not only for each antioxidant but also for mixtures of two antioxidants: (i) ?-TocH with ?-, ?-, or ?-TocH and (ii) ?-TocH with ?-, ?-, ?-, or ?-Toc-3H. A synergistic effect in which the ks(AOH) value increases by 12% for ?-TocH (or by 12% for ?-Toc-3H) was observed for solutions including ?-TocH and ?-TocH (or ?-Toc-3H). On the other hand, a cancel effect in which the ks(AOH) value decreases (a) by 7% for ?-TocH (or 11% for ?-Toc-3H) and (b) by 24% for ?-TocH (or 25% for ?-Toc-3H) was observed for solutions including two kinds of antioxidants. However, only a synergistic effect may function in edible oils, because contents of ?- and ?-TocHs (and ?- and ?-Toc-3Hs) are much less than those of ?- and ?-TocHs (and ?- and ?-Toc-3Hs) in many edible oils. UV-vis absorption of ?-Toc(•), which was produced by reaction of ?-TocH with ArO(•), decreased remarkably for coexistence of ?-TocH with ?-, ?-, or ?-TocH (or ?-, ?-, or ?-Toc-3H), indicating that the prooxidant effect of ?-Toc(•) is suppressed by the coexistence of other TocHs and Toc-3Hs.
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Development of a new free radical absorption capacity assay method for antioxidants: aroxyl radical absorption capacity (ARAC).
J. Agric. Food Chem.
PUBLISHED: 10-10-2013
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A new free radical absorption capacity assay method is proposed with use of an aroxyl radical (2,6-di-tert-butyl-4-(4-methoxyphenyl)phenoxyl radical) and stopped-flow spectroscopy and is named the aroxyl radical absorption capacity (ARAC) assay method. The free radical absorption capacity (ARAC value) of each tocopherol was determined through measurement of the radical-scavenging rate constant in ethanol. The ARAC value could also be evaluated through measurement of the half-life of the aroxyl radical during the scavenging reaction. For the estimation of the free radical absorption capacity, the aroxyl radical was more suitable than the DPPH radical, galvinoxyl, and p-nitrophenyl nitronyl nitroxide. The ARAC value in tocopherols showed the same tendency as the free radical absorption capacities reported previously, and the tendency was independent of an oxygen radical participating in the scavenging reaction and of a medium surrounding the tocopherol and oxygen radical. The ARAC value can be directly connected to the free radical-scavenging rate constant, and the ARAC method has the advantage of treating a stable and isolable radical (aroxyl radical) in a user-friendly organic solvent (ethanol). The ARAC method was also successfully applied to a palm oil extract. Accordingly, the ARAC method would be useful in free radical absorption capacity assay of antioxidative reagents and foods.
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Aroxyl-radical-scavenging rate increases remarkably under the coexistence of ?-tocopherol and ubiquinol-10 (or vitamin C): finding of synergistic effect on the reaction rate.
J Phys Chem B
PUBLISHED: 07-02-2013
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Measurements of aroxyl radical (ArO(•))-scavenging rate constants (ks(AOH)) of antioxidants (AOHs) (?-tocopherol (?-TocH), ubiquinol-10 (UQ10H2), and sodium ascorbate (Na(+)AsH(-))) were performed in 2-propanol/water (2-PrOH/H2O, 5/1, v/v) solution using stopped-flow spectrophotometry. ks(AOH) values were measured not only for each AOH but also for the mixtures of two AOHs ((i) ?-TocH and UQ10H2 and (ii) ?-TocH and Na(+)AsH(-)). A notable synergistic effect that the ks(AOH) values increase 1.6, 2.5, and 6.8 times for ?-TocH, UQ10H2, and Na(+)AsH(-), respectively, was observed for the solutions including two kinds of AOHs. Furthermore, measurements of the regeneration rates of ?-tocopheroxyl radical (?-Toc(•)) to ?-TocH by UQ10H2 and Na(+)AsH(-) were performed in 2-PrOH/H2O using double-mixing stopped-flow spectrophotometry. Second-order rate constants (kr) obtained for UQ10H2 and Na(+)AsH(-) were 2.01 × 10(5) and 1.19 × 10(6) M(-1) s(-1), respectively. In fact, UV-vis absorption of ?-Toc(•) (?max = 428 nm), which had been produced by reaction of ?-TocH with ArO(•), disappeared under the existence of UQ10H2 or Na(+)AsH(-) due to the above fast regeneration reaction. The result indicates that the prooxidant effect of ?-Toc(•) is suppressed by the coexistence of UQ10H2 or Na(+)AsH(-). As ?-TocH, UQ10H2, and ascorbate monoanion (AsH(-)) coexist in relatively high concentrations in plasma, blood, and various tissues, the above synergistic effect, that is, the increase of the free-radical-scavenging rate and suppression of the prooxidant reaction, may function in biological systems.
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Notable effects of the metal salts on the formation and decay reactions of ?-tocopheroxyl radical in acetonitrile solution. The complex formation between ?-tocopheroxyl and metal cations.
J Phys Chem B
PUBLISHED: 07-22-2011
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The measurement of the UV-vis absorption spectrum of ?-tocopheroxyl (?-Toc(•)) radical was performed by reacting aroxyl (ArO(•)) radical with ?-tocopherol (?-TocH) in acetonitrile solution including four kinds of alkali and alkaline earth metal salts (MX or MX(2)) (LiClO(4), LiI, NaClO(4), and Mg(ClO(4))(2)), using stopped-flow spectrophotometry. The maximum wavelength (?(max)) of the absorption spectrum of the ?-Toc(•) at 425.0 nm increased with increasing concentration of metal salts (0-0.500 M) in acetonitrile, and it approached constant values, suggesting an [?-Toc(•)-M(+) (or M(2+))] complex formation. The stability constants (K) were determined to be 9.2, 2.8, and 45 M(-1) for LiClO(4), NaClO(4), and Mg(ClO(4))(2), respectively. By reacting ArO(•) with ?-TocH in acetonitrile, the absorption of ArO(•) disappeared rapidly, while that of ?-Toc(•) appeared and then decreased gradually as a result of the bimolecular self-reaction of ?-Toc(•) after passing through the maximum. The second-order rate constants (k(s)) obtained for the reaction of ?-TocH with ArO(•) increased linearly with an increasing concentration of metal salts. The results indicate that the hydrogen transfer reaction of ?-TocH proceeds via an electron transfer intermediate from ?-TocH to ArO(•) radicals followed by proton transfer. Both the coordination of metal cations to the one-electron reduced anions of ArO(•) (ArO:(-)) and the coordination of counteranions to the one-electron oxidized cations of ?-TocH (?-TocH(•)(+)) may stabilize the intermediate, resulting in the acceleration of electron transfer. A remarkable effect of metal salts on the rate of bimolecular self-reaction (2k(d)) of the ?-Toc(•) radical was also observed. The rate constant (2k(d)) decreased rapidly with increasing concentrations of the metal salts. The 2k(d) value decreased at the same concentration of the metal salts in the following order: no metal salt > NaClO(4) > LiClO(4) > Mg(ClO(4))(2). The complex formation between ?-Toc(•) and metal cations may stabilize the energy level of the reactants (?-Toc(•) + ?-Toc(•)), resulting in the decrease of the rate constant (2k(d)). The alkali and alkaline earth metal salts having a smaller ionic radius of cation and a larger charge of cation gave larger K and k(s) values and a smaller 2k(d) value.
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Development of singlet oxygen absorption capacity (SOAC) assay method. 2. Measurements of the SOAC values for carotenoids and food extracts.
J. Agric. Food Chem.
PUBLISHED: 03-23-2011
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Recently a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants was proposed. In the present work, kinetic study of the reaction of singlet oxygen ((1)O(2)) with carotenoids and vegetable extracts has been performed in ethanol/chloroform/D(2)O (50:50:1, v/v/v) solution at 35 °C. Measurements of the second-order rate constants (k(Q)(S)) and the SOAC values were performed for eight kinds of carotenoids and three kinds of vegetable extracts (red paprika, carrot, and tomato). Furthermore, measurements of the concentrations of the carotenoids included in vegetable extracts were performed, using a HPLC technique. From the results, it has been clarified that the total (1)O(2)-quenching activity (that is, the SOAC value) for vegetable extracts may be explained as the sum of the product {? k(Q)(Car-i)(S) [Car-i](i)} of the rate constant (k(Q)(Car-i)(S)) and the concentration ([Car (i)]) of carotenoids included in vegetable extracts.
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Kinetic study of the quenching reaction of singlet oxygen by Pyrroloquinolinequinol (PQQH(2), a reduced form of Pyrroloquinolinequinone) in micellar solution.
J. Agric. Food Chem.
PUBLISHED: 02-10-2011
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A kinetic study of the quenching reaction of singlet oxygen ((1)O(2)) with pyrroloquinolinequinol (PQQH(2), a reduced form of pyrroloquinolinequinone (PQQ)), PQQNa(2) (disodium salt of PQQ), and seven kinds of natural antioxidants (vitamin C (Vit C), uric acid (UA), epicatechin (EC), epigallocatechin (EGC), ?-tocopherol (?-Toc), ubiquinol-10 (UQ(10)H(2)), and ?-carotene (?-Car)) has been performed. The second-order rate constants k(Q) (k(Q) = k(q) + k(r), physical quenching and chemical reaction) for the reaction of (1)O(2) with PQQH(2), PQQNa(2), and seven kinds of antioxidants were measured in 5.0 wt % Triton X-100 micellar solution (pH 7.4), using UV-visible spectrophotometry. The k(Q) values decreased in the order of ?-Car > PQQH(2) > ?-Toc > UA > UQ(10)H(2) > Vit C ? EGC > EC ? PQQNa(2). PQQH(2) is a water-soluble antioxidant. The singlet oxygen-quenching activity of PQQH(2) was found to be 6.3, 2.2, 6.1, and 22 times as large as the corresponding those of water-soluble antioxidants (Vit C, UA, EGC, and EC). Further, the activity of PQQH(2) was found to be 2.2 and 3.1 times as large as the corresponding activity of lipid-soluble antioxidants (?-Toc and UQ(10)H(2)). On the other hand, the activity of PQQH(2) is 6.4 times as small as that of ?-Car. It was observed that the chemical reaction (k(r)) is almost negligible in the quenching reaction of (1)O(2) by PQQH(2). The result suggests that PQQH(2) may contribute to the protection of oxidative damage in biological systems, by quenching (1)O(2).
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Kinetic study of aroxyl radical-scavenging action of vitamin E in membranes of egg yolk phosphatidylcholine vesicles.
Chem. Phys. Lipids
PUBLISHED: 01-03-2011
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Vitamin E is localized in membranes and functions as an efficient inhibitor of lipid peroxidation in biological systems. In this study, we measured the reaction rates of vitamin E (?-, ?-, ?-, ?-tocopherols, TocH) and tocol with aroxyl radical (ArO·) as model lipid peroxyl radicals in membranes by stopped-flow spectrophotometry. Egg yolk phosphatidylcholine (EYPC) vesicles were used as a membrane model. EYPC vesicles were prepared in the aqueous methanol solution (MeOH:H(2)O=7:3, v/v) that gave the lowest turbidity in samples. The second-order rate constants (k(s)) for ?-TocH in MeOH/H(2)O solution with EYPC vesicles were apparently 3.45×10(5)M(-1)s(-1), which was about 8 times higher than that (4.50×10(4)M(-1)s(-1)) in MeOH/H(2)O solution without EYPC vesicles. The corrected k(s) of ?-TocH in vesicles, which was calculated assuming that the concentration of ?-TocH was 133 times higher in membranes of 10mM EYPC vesicles than in the bulk MeOH/H(2)O solution, was 2.60×10(3)M(-1)s(-1), which was one-seventeenth that in MeOH/H(2)O solution because of the lower mobility of ?-TocH in membranes. Similar analyses were performed for other vitamin E analogues. The k(s) of vitamin E in membranes increased in the order of tocol
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Kinetic study of the quenching reaction of singlet oxygen by carotenoids and food extracts in solution. Development of a singlet oxygen absorption capacity (SOAC) assay method.
J. Agric. Food Chem.
PUBLISHED: 08-24-2010
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A kinetic study of the quenching reaction of singlet oxygen (1O2) with eight kinds of carotenoids and ?-tocopherol was performed in ethanol/chloroform/D2O (50:50:1, v/v/v) solution at 35 °C. The overall rate constants, kQ (=kq+kr, physical quenching+chemical reaction), for the reaction of carotenoids with 1O2 were measured, using the competition reaction method, where endoperoxide was used as a singlet oxygen generator, 2,5-diphenyl-3,4-benzofuran (DPBF) as an UV-vis absorption prove, and ?-tocopherol as a standard compound. The rate constants, kQ (S) and kQ (t1/2), were determined by analyzing the first-order rate constant (S) and the half-life (t1/2) of the decay curve of DPBF with carotenoids, respectively, showing good accordance with each other. Similar measurements were performed for tomato and carrot extracts. From the results, a new assay method that can quantify the singlet oxygen absorption capacity (SOAC) of antioxidants, including carotenoids, ?-tocopherol, and vegetable extracts, has been proposed.
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Tunneling effect in regeneration reaction of vitamin E by ubiquinol.
J Phys Chem B
PUBLISHED: 04-27-2010
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A kinetic study of the regeneration reaction of vitamin E by ubiquinol was carried out by means of double-mixing stopped-flow spectroscopy. A substantial deuterium kinetic-isotope effect was observed on the second-order rate constant and the activation energy. In the regeneration reaction of alpha-tocopherol, deuteration of ubiquinol increased and decreased the activation energy and the second-order rate constant by 6.1 kJ/mol and a factor of 18.3, respectively. From this result, it is considered that proton tunneling plays an important role in the regeneration reaction of vitamin E by ubiquinol. The conditions under which the tunneling effect becomes an important factor were discussed in conjunction of our experimental results.
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Kinetic study of the aroxyl radical-scavenging reaction of alpha-tocopherol in methanol solution: notable effect of the alkali and alkaline earth metal salts on the reaction rates.
J Phys Chem B
PUBLISHED: 09-17-2009
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A kinetic study of the aroxyl (ArO*) radical-scavenging reaction of alpha-tocopherol (alpha-TocH) has been performed in the presence of six kinds of alkali and alkaline earth metal salts (LiI, LiClO(4), NaI, NaClO(4), KI, and Mg(ClO(4))(2)) in methanol solution, using stopped-flow spectrophotometry. The decay rate of the ArO* for the reaction of alpha-TocH with ArO* increased linearly with increasing concentration of metal salts. The second-order rate constants (k(s)) for the reaction of alpha-TocH with ArO* increased in the order of no metal salt < KI approximately NaClO(4) approximately NaI
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Kinetic study of the prooxidant effect of alpha-tocopherol. Hydrogen abstraction from lipids by alpha-tocopheroxyl radical.
Lipids
PUBLISHED: 06-26-2009
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A kinetic study of the prooxidant effect of alpha-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 alpha-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (k (p)) obtained are <1 x 10(-2) M(-1 )s(-1) for 1, 1.90 x 10(-2) M(-1 )s(-1) for 2, 8.33 x 10(-2 )M(-1 )s(-1) for 3, 1.92 x 10(-1) M(-1 )s(-1) for 4, and 2.43 x 10(-1 )M(-1 )s(-1) for 5 at 25.0 degrees C. Fatty acid esters 3, 4, and 5 contain two, four, and six -CH(2)- hydrogen atoms activated by two pi-electron systems (-C=C-CH(2)-C=C-). On the other hand, fatty acid ester 2 has four -CH(2)- hydrogen atoms activated by a single pi-electron system (-CH(2)-C=C-CH(2)-). Thus, the rate constants, k (abstr)/H, given on an available hydrogen basis are k (p)/4 = 4.75 x 10(-3 )M(-1 )s(-1) for 2, k (p)/2 = 4.16 x 10(-2) M(-1 )s(-1) for 3, k (p)/4 = 4.79 x 10(-2 )M(-1 )s(-1) for 4, and k (p)/6 = 4.05 x 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 alpha-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.
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Kinetic study of the antioxidant activity of pyrroloquinolinequinol (PQQH(2), a reduced form of pyrroloquinolinequinone) in micellar solution.
J. Agric. Food Chem.
PUBLISHED: 02-14-2009
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Kinetic study of the aroxyl radical-scavenging action of pyrroloquinolinequinol [PQQH(2), a reduced form of pyrroloquinolinequinone (PQQ)] and water-soluble antioxidants (vitamin C, cysteine, glutathione, and uric acid) has been performed. The second-order rate constants (k(s)) for the reaction of aroxyl radical with PQQH(2) and water-soluble antioxidants were measured in Triton X-100 micellar solution (5.0 wt %) (pH 7.4), using stopped-flow and UV-visible spectrophotometers. The k(s) values decreased in the order PQQH(2) > vitamin C > cysteine > uric acid > glutathione. The aroxyl radical-scavenging activity of PQQH(2) was 7.4 times higher than that of vitamin C, which is well-known as the most active water-soluble antioxidant. Furthermore, PQQNa(2) (disodium salt of PQQ) was easily reduced to PQQH(2) by reaction of PQQNa(2) with glutathione and cysteine in buffer solution (pH 7.4) under nitrogen atmosphere. The result suggests that PQQ exists as a reduced form throughout the cell and plays a role as antioxidant.
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Development of singlet oxygen absorption capacity (SOAC) assay method. 3. Measurements of the SOAC values for phenolic antioxidants.
J. Agric. Food Chem.
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Measurements of the singlet oxygen ((1)O(2)) quenching rates (k(Q) (S)) and the relative singlet oxygen absortpion capacity (SOAC) values were performed for 16 phenolic antioxidants (tocopherol derivatives, ubiquinol-10, caffeic acids, and catechins) and vitamin C in ethanol/chloroform/D(2)O (50:50:1, v/v/v) solution at 35 °C. It has been clarified that the SOAC method is useful to evaluate the (1)O(2)-quenching activity of lipophilic and hydrophilic antioxidants having 5 orders of magnitude different rate constants from 1.38 × 10(10) M(-1) s(-1) for lycopene to 2.71 × 10(5) for ferulic acid. The logarithms of the k(Q) (S) and the SOAC values for phenolic antioxidants were found to correlate well with their peak oxidation potentials (E(p)); the antioxidants that have smaller E(p) values show higher reactivities. In previous works, measurements of the k(Q) (S) values for many phenolic antioxidants were performed in ethanol. Consequently, measurements of the k(Q) (S) and relative SOAC values were performed for eight carotenoids in ethanol to investigate the effect of solvent on the (1)O(2)-quenching rate. The k(Q) (S) values for phenolic antioxidants and carotenoids in ethanol were found to correlate linearly with the k(Q) (S) values in ethanol/chloroform/D(2)O solution with a gradient of 1.79, except for two catechins. As the relative rate constants (k(Q)(AO) (S)/k(Q)(?-Toc) (S)) of antioxidants (AO) are equal to the relative SOAC values, the SOAC values do not depend on the kinds of solvent used, if ?-tocopherol is used as a standard compound. In fact, the SOAC values obtained for carotenoids in mixed solvent agreed well with the corresponding ones in ethanol.
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Notable effects of metal salts on UV-vis absorption spectra of ?-, ?-, ?-, and ?-tocopheroxyl radicals in acetonitrile solution. The complex formation between tocopheroxyls and metal cations.
J Phys Chem B
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The measurements of the UV-vis absorption spectra of ?-, ?-, ?-, and ?-tocopheroxyl (?-, ?-, ?-, and ?-Toc(•)) radicals were performed by reacting aroxyl (ArO(•)) radical with ?-, ?-, ?-, and ?-tocopherol (?-, ?-, ?-, and ?-TocH), respectively, in acetonitrile solution including three kinds of alkali and alkaline earth metal salts (LiClO(4), NaClO(4), and Mg(ClO(4))(2)) (MX or MX(2)), using stopped-flow spectrophotometry. The maximum wavelengths (?(max)) of the absorption spectra of the ?-, ?-, ?-, and ?-Toc(•) located at 425-428 nm without metal salts increased with increasing concentrations of metal salts (0-0.500 M) in acetonitrile and approached some constant values, suggesting (Toc(•)···M(+) (or M(2+))) complex formations. Similarly, the values of the apparent molar extinction coefficient (?(max)) increased drastically with increasing concentrations of metal salts in acetonitrile and approached some constant values. The result suggests that the formations of Toc(•) dimers were suppressed by the metal ion complex formations of Toc(•) radicals. The stability constants (K) were determined for Li(+), Na(+), and Mg(2+) complexes of ?-, ?-, ?-, and ?-Toc(•). The K values increased in the order of NaClO(4) < LiClO(4) < Mg(ClO(4))(2), being independent of the kinds of Toc(•) radicals. Furthermore, the K values increased in the order of ?- < ?- < ?- < ?-Toc(•) radicals for each metal salt. The alkali and alkaline earth metal salts having a smaller ionic radius of the cation and a larger charge of the cation gave a larger shift of the ?(max) value, a larger ?(max) value, and a larger K value. The result of the DFT molecular orbital calculations indicated that the ?-, ?-, ?-, and ?-Toc(•) radicals were stabilized by the (1:1) complex formation with metal cations (Li(+), Na(+), and Mg(2+)). Stabilization energy (E(S)) due to the complex formation increased in the order of Na(+) < Li(+) < Mg(2+) complexes, being independent of the kinds of Toc(•) radicals. The calculated result also indicated that the metal cations coordinate to the O atom at the sixth position of ?-, ?-, ?-, and ?-Toc(•) radicals.
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Kinetic study of the ?-tocopherol-regeneration reaction of ubiquinol-10 in methanol and acetonitrile solutions: notable effect of the alkali and alkaline earth metal salts on the reaction rates.
J Phys Chem B
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A kinetic study of regeneration reaction of ?-tocopherol (?-TocH) by ubiquinol-10 has been performed in the presence of four kinds of alkali and alkaline earth metal salts (LiClO(4), NaClO(4), NaI, and Mg(ClO(4))(2)) in methanol and acetonitrile solutions, using double-mixing stopped-flow spectrophotometry. The second-order rate constants (k(r)s) for the reaction of ?-tocopheroxyl (?-Toc•) radical with ubiquinol-10 increased and decreased notably with increasing concentrations of metal salts in methanol and acetonitrile, respectively. The k(r) values increased in the order of no metal salt < NaClO(4) ~ NaI < LiClO(4) < Mg(ClO(4))(2) at the same concentration of metal salts in methanol. On the other hand, in acetonitrile, the k(r) values decreased in the order of no metal salt > NaClO(4) ~ NaI > LiClO(4) > Mg(ClO(4))(2) at the same concentration of metal salts. The metal salts having a smaller ionic radius of cation and a larger charge of cation gave a larger k(r) value in methanol, and a smaller k(r) value in acetonitrile. The effect of anion was almost negligible in both the solvents. Notable effects of metal cations on the UV-vis absorption spectrum of ?-Toc• radical were observed in aprotic acetonitrile solution, suggesting complex formation between ?-Toc• and metal cations. On the other hand, effects of metal cations were negligible in protic methanol, suggesting that the complex formation between ?-Toc• and metal cations is hindered by the hydrogen bond between ?-Toc• and methanol molecules. The difference between the reaction mechanisms in methanol and acetonitrile solutions was discussed on the basis of the results obtained. High concentrations of alkali and alkaline earth metal salts coexist with ?-TocH and ubiquinol-10 in plasma, blood, and many tissues, suggesting the contribution of the metal salts to the above regeneration reaction in biological systems.
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What is Visualize?

JoVE Visualize is a tool created to match the last 5 years of PubMed publications to methods in JoVE's video library.

How does it work?

We use abstracts found on PubMed and match them to JoVE videos to create a list of 10 to 30 related methods videos.

Video X seems to be unrelated to Abstract Y...

In developing our video relationships, we compare around 5 million PubMed articles to our library of over 4,500 methods videos. In some cases the language used in the PubMed abstracts makes matching that content to a JoVE video difficult. In other cases, there happens not to be any content in our video library that is relevant to the topic of a given abstract. In these cases, our algorithms are trying their best to display videos with relevant content, which can sometimes result in matched videos with only a slight relation.