Ascorbate, reactions with

Sodium ascorbate is the sodium salt of ascorbic acid. Most of sodium ascorbate reactions with chlorinated waters are similar to those of ascorbic acid. However, a key difference in dechlorination using sodium ascorbate is that it does not lower the water pH. Several utilities in the Pacific Northwest have evaluated the use of sodium ascorbate for neuhalizing chlorine from potable water releases. The pH of sodium ascorbate is approximately neuhal. The expected reaction of sodium ascorbate with chlorine is shown below ... 

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). 

Deoxy-6-fluoro-L-ascorbic acid ° (314) was prepared from methyl 2,3-<9-isopropylidene-6-0-tosyl-a-L-gulosonate (313) by reaction with KP followed by isomerization of the product (with H" " cation-exchange resin). 

Mirvish (53,54) discovered that vitamin C could inhibit ni-trosation reactions. The purely chemical interaction of ascorbic acid with nitrite has been studied for theoretical reasons and because of its importance in the preservation of foods. This interaction has received increased attention for minimizing the presence of nitrosamines and nitrosamides in the environment, and especially in foods. We have studied the relationship in gastric carcinogenesis between high levels of nitrite, including pickling, and of vitamin C as a protective and inhibiting element. 

Nelan, D. R. Robeson, C. D. The oxidation product from a-tocopherol and potassium ferricyanide and its reaction with ascorbic and hydrochloric acids. J. Am. Chem. Soc. 1962, 84, 2963-2965. 

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 IrIV anion, [Ir(H20)Br5], oxidises ascorbic acid at 20.0 °C.51 This reaction is first order with respect to ascorbic acid concentration and first order with respect to the Irlv anion. Comparison of hexabromo-, hexachloro, aquopentachloro-, and di-aquotetracholoiridium(IV) reactions with ascorbic acid shows that replacing a halide ion with a water molecule increases the standard reduction potential of the IrIV complex and increases the rate of reaction. 

Bors et al. [175] determined the rate constants and equilibrium constants for the reactions of flavonoids with ascorbate (Reaction (18)) by a pulse-radiolysis method and on their basis calculated the one-electron oxidation potentials of flavonoids (Table 29.9). 

The absence of substituents with free radical scavenging properties in most of the (3-blockers makes doubtful their efficacy as powerful antioxidants. Arouma et al. [293] tested the antioxidative properties of several 3-blockers in reactions with superoxide, hydroxyl radicals, hydrogen peroxide, and hypochlorous acid. It was demonstrated that most of the compounds tested were inactive in these experiments. Nonetheless, propranolol, verapamil, and flunarizine effectively inhibited iron ascorbate-stimulated microsomal lipid peroxidation and all drugs (excluding flunarizine) were effective scavengers of hydroxyl radicals. Contrary to Janero et al. [292], these authors did not find the inhibition of xanthine oxidase by propranolol. It was concluded that 3-blockers are not the effective in vivo antioxidants. 

The disturbance of balance between superoxide and nitric oxide occurs in a variety of common disease states. For example, altered endothelium-dependent vascular relaxation due to a decrease in NO formation has been shown in animal models of hypertension, diabetes, cigarette smoking, and heart failure [21]. Miller et al. [22] suggested that a chronic animal model atherosclerosis closely resembles the severity of atherosclerosis in patients. On the whole, the results obtained in humans, for example, in hypertensive patients [23] correspond well to animal experiments. It is important that endothelium-dependent vascular relaxation in patients may be improved by ascorbic acid probably through the reaction with superoxide. 

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). 

Ruhho, H., Radi, R., Ansehni, D., Kirk, M., Bames, S., Butler, J., Fiserich, J. P., and Freeman, B. A., 2000, Nitric oxide reaction with hpid peroxyl radicals spares alpha-tocopherol during hpid peroxidation. Greater oxidant protection from the pair nitric oxide/alpha-tocopherol than alpha-tocopherol/ascorbate, J. Biol. Chem. 275 10812-10818. 

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