Dehydroascorbic acid, reaction with

Vitamin C can also be determined colorimetrically, after oxidation to dehydroascorbate, by reaction with dinitrophenylhydrazine. Under appropriate conditions, neither ascorbic acid itself nor potentially interfering sugars react with dinitrophenylhydrazine. However, diketogulonate, which has no vitamin activity, also reacts with dinitrophenylhydrazine under the same conditions. Unless diketogulonate is determined separately after reduction of dehydroascorbate to ascorbate, this method overestimates the vitamin. 

In recent years more utilities have begun to use ascorbic acid (vitamin C) for dechlorination (23). Vitamin C has long been used in the medical field for dechlorination of tap water prior to use for kidney dialysis treatment. Vitamin C reacts with chlorine to produce chloride and dehydroascorbate. The reactions with chlorine and chloramine are shown below ... 

It is clear from the foregoing that the oxidation of ascorbic acid may be catalyzed by all the well-known oxidase systems occurring in plants, but such biochemical studies furnish little evidence as to what extent these reactions proceed in vivo. All these oxidases carry oxidation only as far as the dehydroascorbic acid stage. The fact that ascorbic acid is always associated in fresh tissues with small but definite amounts of dehydroascorbic acid, combined with the probability that dehydroascorbic acid is continually being lost by irreversible conversion to 2,3-diketogulonic acid, makes it appear probable that there is a continuous oxidation of ascorbic acid in vivo. The low content of dehydroascorbic acid in most tissues is not inconsistent with this view, for the continuous oxidation may also be accompanied by an equally continuous reduction. 

The physicochemical properties of the water-soluble vitamins are extensively utilized in chemical methods. A method for quantitative vitamin C (ascorbic acid, AA) measurement in food and physiological samples is based on a reaction of the keto groups in dehydroascorbic acid (DHA) with o-phenylenediamine (OPD) to give a fluorescent quinoxaline. This method involves the oxidation of AA to DHA, followed by the measurement of total AA in the sample. The reductive capabilities of AA can especially be utilized for direct electrochemical (amperometric or coulometric) measurement when coupled with HPLC separation. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

SAKURAI H, POKORNY J, NGUYEN H T T, REBLOVA Z, VALENTOVA H and ISHII K (1997) Reaction of dehydroascorbic acid with aspartame , Potrav Vedy, 15 (1) 13-28. 

A chemical reaction subsequent to a fast (reversible) electrode reaction (Eq. 5.6.1, case b) can consume the product of the electrode reaction, whose concentration in solution thus decreases. This decreases the overpotential of the overall electrode process. This mechanism was proposed by R. Brdicka and D. H. M. Kern for the oxidation of ascorbic acid, converted by a fast electrode reaction at the mercury electrode to form dehydro-ascorbic acid. An equilibrium described by the Nernst equation is established at the electrode between the initial substance and this intermediate product. Dehydroascorbic acid is then deactivated by a fast chemical reaction with water to form diketogulonic acid, which is electroinactive. 

The intramolecular electron transfer leads to fast formation of semi-quinone and the lower oxidation state metal ion. The catalytic cycle is completed by fast reoxidation of the metal ion. Significant deviations from this model were observed at low dioxygen concentrations and it was suggested that another oxidation path becomes operative under such conditions. Although earlier they had been proposed to participate (10), side reactions with dehydroascorbic acid could be excluded. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

A number of Maillard reaction products have been found to be mutagenic or carcinogenic (Lee and Shibamoto, 2002). Reactions of the lysine residue with other food components, including dehydroascorbic acid, result in crosslinking of the heated protein (Fayle et al., 2000). 

The figure shows the results of a bipotentiometric titration of ascorbic acid with If. Ascorbic acid (146 mg) was dissolved in 200 mL of water in a 400-mL beaker. Two Pt electrodes were attached to the K-F outlets of the pH meter and spaced about 4 cm apart in the magnetically stirred solution. The solution was titrated with 0.04 M If (prepared by dissolving 2.4 g of K1 plus 1.2 g of I2 in 100 mL of water), and the voltage was recorded after each addition. Prior to the equivalence point, all the If is reduced to I- by the excess ascorbic acid. Reaction B can occur, but Reaction A cannot. A voltage of about 300 mV is required to support a constant current of 10 pA. (The ascorbate dehydroascorbate couple does not react at a Pt electrode and cannot carry current.) After the equivalence point, excess If is present, so Reactions A and B both occur, and the voltage drops precipitously. 

However, its presence is not the only determinant of whether or not oxidative deterioration occurs. Olson and Brown (1942) showed that washed cream (free of ascorbic acid) from susceptible milk did not develop an oxidized flavor when contaminated with copper and stored for three days. Subsequently, the addition of ascorbic acid to washed cream, even in the absence of added copper, was observed to promote the development of an oxidized flavor (Pont 1952). Krukovsky and Guthrie (1945) and Krukovsky (1961) reported that 0.1 ppm added copper did not promote oxidative flavors in milk or butter depleted of their Vitamin C content by quick and complete oxidation of ascorbic acid to dehydroascorbic acid. Krukovsky (1955) and Krukovsky and Guthrie (1945) further showed that the oxidative reaction in ascorbic acid-free milk could be initiated by the addition of ascorbic acid to such milk. Accordingly, these workers and others have concluded that ascorbic acid is an essential link in a chain of reactions resulting in the development of an oxidized flavor in fluid milk. 

The radical form 9.4 has an unpaired electron and may undergo fast reactions with redox partners that also undergo one-electron processes. Such a redox partner is the triplet radical, dioxygen. The copper complex of ascorbic acid undergoes rapid aerial oxidation to give the dione, dehydroascorbic acid, which may be viewed as being derived by electron loss from the radical (Fig. 9-4). 

Figure 9-4. The reaction of dioxygen with the copper complex of ascorbic acid generates a copper complex of dehydroascorbic acid.

A. A. Frimer and P. Guilinsky-Sharon, Reaction of superoxide with ascorbic acid derivatives insight into the superoxide-mediated oxidation of dehydroascorbic acid, J. Org. Chem., 60 (1995) 2796-2801 and references therein. 

The reaction of nitrous acid with ascorbic acid (41) also involves 0-nitrosa-tion. The final product is dehydroascorbic acid which arises from the nitrite by a series of rapid reactions involving homolytic fission and forming nitric oxide (Dahn et al., 1960). 

The term vtiamin C refers to ascorbic acid (the fully reduced form of the vitamin) and to dchydroascorbic acid- Removal of one electron from ascorbic acid yields semidehydroascorbic acid (ascorbate radical). This form of the vitamin is a free radical it contains an unpaired electron- The structures of free radicals are written with large dots. The removal of a second electron yields dehydroascorbic acid. Conversion of ascorbate to dehydroascorbate, via the removal of two electrons, can occur under two conditions (1) with use of ascorbic add by ascorbate-dependent enzymes and (2) with the spontaneous reaction of ascorbate with oxygen. Semidehydroascorbate is an intermediate in this conversion palhway... 

An improved synthesis of dehydroascorbic acid has been reported (42). The oxidation of ascorbic acid in absolute methanol with oxygen over activated charcoal catalyst is reported to aflFord 28 in 95% yield. Dehydroascorbic acid has been characterized in solution as the monomer, 28 (43), and as the dimer (44,45) and its tetra acetyl derivative 29 (46). Several studies of mono- and di-hydrazone (48-53) and osazone (54) derivatives of dehydroascorbic acid have been reported. Hydrazone derivatives of dehydroascorbic acid have been used in the reductive synthesis of 2,3-diaza-2,3-dideoxy- and 2-aza-2-deoxyascorbic acid derivatives 30, 31, and 32 (55,56). Recently the reaction product of dehydro-L-ascorbic acid and L-phenylalanine in aqueous solution has been isolated and identified as tris(2-deoxy-2-L-ascorbyl)amine, 33, based on spectral and chemical data and its symmetry properties (57). 

On the other hand, reaction rates of ascorbate/ascorbic acid with molecular oxygen are low and pH dependent [10" -5 M h at pH 4-10 (43, 44)] detection of 02 may be diflBcult because of its low concentration. Ascorbate free radicals in such solutions could arise from a secondary reaction between dehydroascorbic acid and ascorbate ... 

Liquid Dosage Forms. In dry form and at very low moisture content, L-ascorbic acid is very stable, but in solution exposed to air or oxygen it is subject to oxidation accelerated by dissolved trace minerals (copper and iron) and light exposure, l-Ascorbic acid is a reducing agent and is subject to oxidative decomposition in solution. This proceeds first to dehydroascorbic acid, which has full vitamin C activity, but continues to diketogulonic acid and various other breakdown products. The degradation reactions are complex and vary with aerobic or anaerobic... 

DHA can be reduced to RAA by chemical agents, such as hydrogen sulfide or enzymatically, by dehydroascorbic acid reductase. The conversion of DHA to diketogulonic acid (DKG) is irreversible and occurs both aerobically and anaerobically, particularly during heating. This reaction results in loss of biological activity. The total oxidation of RAA may result in the formation of furfural by decarboxylation and dehydration. With subsequent polymerization, the formation of dark-colored pigments results. These compounds affect the color and flavor of certain foods, such as citrus juices, and decrease nutritive value. 

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