Abstract
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.
Highlights
Vitamin C is a general name for all the compounds which possess the fully or partially biological activity of L-ascorbic acid (AA) [1,2,3,4,5,6]
Vitamin C is widely found in plants and in animals, and nowadays it is available as caplets, tablets, capsules, and drink mixes and in multivitamin and antioxidant formulations [1,2,3]
An outer-sphere electron transfer was suggested for the oxidation of AA by [IrCl6]2, the reaction course was not exploited previously [25, 26]. e rapid scan spectra were recorded for the oxidations of AA/isoascorbic acid (IAA) by [IrCl6]2– in this work; Figure 1 displays such spectra in the case of AA
Summary
Vitamin C is a general name for all the compounds which possess the fully or partially biological activity of L-ascorbic acid (AA) [1,2,3,4,5,6]. E purposes of the present work were to study the redox reactions in a pH range as wide as possible, to derive the rate constants of the rate-determining steps in a good accuracy, to propose a convincing reaction mechanism, and to find out the antioxidative difference between AA and IAA.
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