Catalysis ascorbic acid oxidation

Shtamm E.V., Catalysis of Ascorbic Acid Oxidation with Copper Ions , PhD Thesis, MGU, Moscow, 1975, 120 pp. (in Russian). 

The specific enzymatic catalysis of ascorbic acid oxidation is known in plants but not in animal tissues. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

Figure 6. Simulated cyclic voltammogram for the oxidation of ascorbic acid without Inclusion of ec catalysis by the surface qulnone functionalities. Filled circles represent the simulated data and an experimental curve Is shown with a line for comparison. A scan rate of 100 mV s was assumed for experimental and simulated data.

The most important point during sample preparation is to prevent oxidation of ascorbic acid. Indeed, it is easily oxidized by an alkaline pH, heavy metal ions (Cu and Fe ), the presence of halogens compounds, and hydrogen peroxide. The most suitable solvent for this purpose is metaphosphoric acid, which inhibits L-ascorbic oxidase and metal catalysis, and it causes the precipitation of proteins. However, it can cause serious analytical interactions with silica-based column, e.g., C18 or amino bonded-phases [542] and it could co-elute with AA. 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

The work described here on the Cu(II)- and Fe(III)-catalyzed autoxidation of ascorbic acid has been extended to catalytic systems involving vanadyl (12) and uranyl (13) ions. On the basis of the results described above it would seem that there are potentially many other metal ions that are capable of undergoing redox reactions with the ascorbate ion, and that may function as catalysts in the autoxidation of ascorbic acid. Analogous mechanisms may also apply to systems involving metal-ion catalysis of ascorbate oxidation in which the primary oxidant is a reagent other than molecular oxygen. 

Ingold (6) did not review ascorbic acid per se, but did present a chapter on metal catalysis including the metal content of vegetable oils and the eflFect of valence state of the metals on oxidation of fats and oils. He reports cobalt, manganese, copper, iron, and zinc at the higher valences acted catalytically to oxidize many substrates. The author discussed the antioxidant activity of ascorbic acid in radiation-induced free radicals, fats in emulsions, fluid milk, and frozen fish. He also discussed the quandary of metal reactions vs. valence state. 

Metal Effects and Prooxidant Action. Ascorbic acid is prooxidant in some situations. Kanner et al. (28) showed that Cu increased the oxidation of linoleate using loss of 8-carotene as an indicator. However, when sufficient ascorbic acid was added to his system, copper catalysis was reversed. Furthermore, when Fe was added, the addition of ascorbic acid increased the prooxidant effect. Previous publications (29) have discussed the deactivation of copper catalysis by ascorbic acid, but in iron-catalyzed oxidation, Fe " initiates oxidation of lipid (2). Fe is formed from Fe by ascorbic acid. Many foods contain metals, and the... 

Oxidation of L-ascorbic acid itself, dehydroascorbic acid, and the further oxidation products appears to be subject to metal ion catalysis of which copper(ii) is the most potent. There have been many investigations of the reactions of L-ascorbic acid under these conditions, but very few of the catalytic oxidation of the other compounds. Many of the studies have sought to establish the role of the metal ion, usually copper(ii) in the mechanism of the reaction. Although the picture is somewhat clearer today, there remains disagreement about the role of the metal ions in the process. 

Oxidation of methionine to methionine sulfoxide in small peptides was catalyzed by Fe3+ and promoted by ascorbic acid. Intramolecular catalysis by a histidine residue was involved in this oxidation, and its effect was maximal when the histidine and methionine residues were separated by one residue.822... 

The catalysis of P4VP-Cu complexes during the oxidation of such substrates as 3,4-dioxycinnamic, salicylic and ascorbic acids and substituted phenols has been discussed [93]. The rate of oxidation of the acids depends on the pH of the medium, its maximum being attained at pH 2.5, 3.0 and 4.2, respectively. A maximum is reached at these pH values because at these values, the protonation of uncomplexed nitrogen atoms occurs. Moreover, the electrostatic interaction between the polyelectrolyte and the anionic substrate raises the nitrogen concentration near the active center. In the case of a positively charged substrate such as paraphenylene diamine, however, the oxidation rate sharply drops upon an increase in the solution acidity due to the electrostatic repulsion of the like-charged polyelectrolyte and substrate. 

PANI/DBSA nanoparticles have also been studied for their oxidation of ascorbic acid [82]. The nanoparticles were drop-coated onto screen-printed carbon paste electrodes. The peak current response was achieved at approx. 250 mV vs. Ag/AgCl. However, the device also showed sufficient catalysis at 0.0 V vs. Ag/AgCl to allow measurements to be made at this potential where it was capable of detection of 8.3 pM ascorbic acid and was linear in the range of 0.5 to 8 mM. 

There are few references in the literature to the inhibition of oxidase enzymes by naturally occurring substances. The presence of a substance in many fruits and vegetables which effectively inhibits the oxidation of ascorbic acid, whether this is brought about by ascorbic oxidase, poly-phenolase, peroxidase, or by inorganic Cu, has been reported by Somogyi (1944), but the nature of this substance or substances was not identified. Damodaran and Nair (1936) isolated a tannin from the Indian gooseberry (Phyllanthus emblica) which inhibited the oxidation of ascorbic acid in the press juice. Since the protective effect of this substance could be overridden by the addition of Cu, they concluded that its action depended on the suppression of metal catalysis. 

In fact, the earliest application of kinetic methods was to determine trace levels of substances exerting catalytic activity in oxidation-reduction reactions involving multiple electron transfers (1885-trace level V on its catalysis of the oxidation of aniline). For example, the reduced form of many triphenylmethane dyes is colorless , and loses two electrons on oxidation to the dye. The rate of reaction with such oxidants as 104 is relatively slow, but can be catalyzed by trace levels of transition metal ions which involve single electron transfer in their own redox steps. Thus, trace levels of manganese can be determined by the proportionality of the rate of oxidation of leuco-malachite green by iodate at less than micromolar concentrations. Similarly, trace levels of Cu ", < 10 M, can be determined from the catalytic effect on the atmospheric oxidation of ascorbic acid. Such systems can be written as a generalized redox reaction... 

Based on the selective catalysis of Au-NPs, selective electrochemical analysis could also be achieved as, for example, in the dopamine electrochemical detection in presence of ascorbic acid. In this case, Au-NPs can be used as selective catalysts since their presence induces the decreasing of ascorbic acid overpotential and the effective separation of the oxidation potentials of ascorbic acid and dopamine [13]. 

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