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Spectrophotometric studies of the reaction of quercetin with peroxynitrite at different pH

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Gebicka L., Stawowska K.
Open Chemistry
|
2012
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vol. 10
|
issue 1
187-193
EN
Peroxynitrite (ONOOH/ONOO-) which is formed in vivo under oxidative stress is a strong oxidizing and nitrating agent. It has been reported that several flavonoids, including quercetin, inhibit the peroxynitrite-induced oxidation and/or nitration of several molecules tested; however, the mechanism of their protective action against peroxynitrite is not univocally resolved. The kinetics of the reaction of quercetin with peroxynitrite was studied by stopped-flow as well as by conventional spectrophotometry under acidic, neutral and alkaline pH. The obtained results show that the protective mechanism of quercetin against peroxynitrite toxicity cannot be explained by direct scavenging of peroxynitrite. We propose that quercetin acts via scavenging intermediate radical products of peroxynitrite decomposition (it is an excellent scavenger of ·NO2) and/or via reduction of target radicals formed in the reaction with peroxynitrite.
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A comparative study on the antioxidant efficiency of nine compounds commonly used as standards in antioxidant assays of extracts from medicinal plants and functional foods

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Quessaugy K., Cumbane P.
World News of Natural Sciences
|
2023
|
vol. 46
30-45
EN
The detection of antioxidant activity in plant extracts or in pure compounds can be performed by a large number of methods with different reaction mechanisms, however, the criteria for choosing comparative standards are still not consensual. Thus, the present work intends to compare the antioxidant efficiency of nine substances, namely, gallic acid (GA), pyrogallol (PyG), propyl gallate (nPG), tannic acid (TA), quercetin (Qtn), rutin (Rut), ascorbic acid (Asc), Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and butyl hydroxytoluene (BHT) using methods, (1) Ferric Reducing Antioxidant Power (FRAP) Assay, (2) Ferric Reducing Power (FRP), (3) Ferric-Ferrozine Antioxidant Capacity (FFAC), (4) Total Phosphomolybdenum Antioxidant Capacity (TAC) and (5) Radical Cation Elimination Assay 2,2' Azinobis(3 Ethylbenzothiazoline 6 Sulfonic Acid) (ABTS). Antioxidant efficacy by the 1,1-diphenyl-2-picrylhydrazine free radical scavenging method was previously described in a preliminary study. The results show that the maximum effectiveness was exhibited by PyG in the ABTS (0.425 ± 0.005 µM) and TAC (0.872 ± 0.075 µM) methods, Qtn in the FRP (5.776 ± 0.020 µM) and FFAC (20.390 ± 0.291 µM) methods and GA in the FRAP (6.765 ± 0.086 µM) and DPPH (1.105 ± 0.003 µM) methods. The results found in this study reveal that the effectiveness of a standard depends on the method applied, and the antioxidant activity of the same standard may present differences between the methods, which suggests that the selection of a comparative standard for the antioxidant activity tests of the extracts of plants or functional foods must be made according to the method to be applied.
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