Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)

The pair [IrCl6]2–/[IrCl6]3– has been demonstrated to be a good redox probe in biological systems while L-ascorbic acid (AA) is one of the most important antioxidants. D-isoascorbic acid (IAA) is an epimer of AA and is widely used as an antioxidant in various foods, beverages, meat, and fisher produ...

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Autores principales:	Han Zhang, Yanqing Xia, Peng Zhang, Liqian Hou, Ying Sun, Zhaozhan Lu, Yunfeng Tang, Hongwu Tian, Tiesheng Shi
Formato:	article
Lenguaje:	EN
Publicado:	Hindawi Limited 2021
Materias:	Chemistry QD1-999
Acceso en línea:	https://doaj.org/article/fbd215bbe26d4d1f82406534b305acb9
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spelling	oai:doaj.org-article:fbd215bbe26d4d1f82406534b305acb92021-11-29T00:56:24ZAntioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)2090-907110.1155/2021/5505741https://doaj.org/article/fbd215bbe26d4d1f82406534b305acb92021-01-01T00:00:00Zhttp://dx.doi.org/10.1155/2021/5505741https://doaj.org/toc/2090-9071The pair [IrCl6]2–/[IrCl6]3– has been demonstrated to be a good redox probe in biological systems while L-ascorbic acid (AA) is one of the most important antioxidants. D-isoascorbic acid (IAA) is an epimer of AA and is widely used as an antioxidant in various foods, beverages, meat, and fisher products. Reductions of [IrCl6]2– by AA and IAA have been analyzed kinetically and mechanistically in this work. The reductions strictly follow overall second-order kinetics and the observed second-order rate constants were collected in the pH region of 0 ≤ pH ≤ 2.33 at 25.0°C. Spectrophotometric titration experiments revealed a well-defined 1 : 2 stoichiometry, namely Δ[AA] : Δ[Ir(IV)] or Δ[IAA] : Δ[Ir(IV)] = 1 : 2, indicating that L-dehydroascorbic acid (DHA) and D-dehydroisoascorbic acid (DHIA) were the oxidation products of AA and IAA, respectively. A reaction mechanism is suggested involving parallel reactions of [IrCl6]2– with three protolysis species of AA/IAA (fully protonated, monoanionic, and dianionic forms) as the rate-determining steps and formation of ascorbic/isoascorbic and ascorbate/isoascorbate radicals; in each of the steps, [IrCl6]2– acquires an electron via an outer-sphere electron transfer mode. Rate constants of the rate-determining steps have been derived or estimated. The fully protonated forms of AA and IAA display virtually identical reactivity whereas ascorbate and isoascorbate monoanions have a significant reactivity difference. The ascorbate and isoascorbate dianions are extremely reactive and their reactions with [IrCl6]2– proceed with the diffusion-controlled rate. The species versus pH and the species reactivity versus pH distribution diagrams were constructed endowing that the ascorbate/isoascorbate monoanionic form dominated the total reactivity at physiological pH. In addition, the value of pKa1 = 3.74 ± 0.05 for IAA at 25.0°C and 1.0 M ionic strength was determined in this work.Han ZhangYanqing XiaPeng ZhangLiqian HouYing SunZhaozhan LuYunfeng TangHongwu TianTiesheng ShiHindawi LimitedarticleChemistryQD1-999ENJournal of Chemistry, Vol 2021 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Chemistry QD1-999
spellingShingle	Chemistry QD1-999 Han Zhang Yanqing Xia Peng Zhang Liqian Hou Ying Sun Zhaozhan Lu Yunfeng Tang Hongwu Tian Tiesheng Shi Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
description	The pair [IrCl6]2–/[IrCl6]3– has been demonstrated to be a good redox probe in biological systems while L-ascorbic acid (AA) is one of the most important antioxidants. D-isoascorbic acid (IAA) is an epimer of AA and is widely used as an antioxidant in various foods, beverages, meat, and fisher products. Reductions of [IrCl6]2– by AA and IAA have been analyzed kinetically and mechanistically in this work. The reductions strictly follow overall second-order kinetics and the observed second-order rate constants were collected in the pH region of 0 ≤ pH ≤ 2.33 at 25.0°C. Spectrophotometric titration experiments revealed a well-defined 1 : 2 stoichiometry, namely Δ[AA] : Δ[Ir(IV)] or Δ[IAA] : Δ[Ir(IV)] = 1 : 2, indicating that L-dehydroascorbic acid (DHA) and D-dehydroisoascorbic acid (DHIA) were the oxidation products of AA and IAA, respectively. A reaction mechanism is suggested involving parallel reactions of [IrCl6]2– with three protolysis species of AA/IAA (fully protonated, monoanionic, and dianionic forms) as the rate-determining steps and formation of ascorbic/isoascorbic and ascorbate/isoascorbate radicals; in each of the steps, [IrCl6]2– acquires an electron via an outer-sphere electron transfer mode. Rate constants of the rate-determining steps have been derived or estimated. The fully protonated forms of AA and IAA display virtually identical reactivity whereas ascorbate and isoascorbate monoanions have a significant reactivity difference. The ascorbate and isoascorbate dianions are extremely reactive and their reactions with [IrCl6]2– proceed with the diffusion-controlled rate. The species versus pH and the species reactivity versus pH distribution diagrams were constructed endowing that the ascorbate/isoascorbate monoanionic form dominated the total reactivity at physiological pH. In addition, the value of pKa1 = 3.74 ± 0.05 for IAA at 25.0°C and 1.0 M ionic strength was determined in this work.
format	article
author	Han Zhang Yanqing Xia Peng Zhang Liqian Hou Ying Sun Zhaozhan Lu Yunfeng Tang Hongwu Tian Tiesheng Shi
author_facet	Han Zhang Yanqing Xia Peng Zhang Liqian Hou Ying Sun Zhaozhan Lu Yunfeng Tang Hongwu Tian Tiesheng Shi
author_sort	Han Zhang
title	Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
title_short	Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
title_full	Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
title_fullStr	Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
title_full_unstemmed	Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)
title_sort	antioxidative reactivity of l-ascorbic acid and d-isoascorbic acid species towards reduction of hexachloroiridate (iv)
publisher	Hindawi Limited
publishDate	2021
url	https://doaj.org/article/fbd215bbe26d4d1f82406534b305acb9
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Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)

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