Dehydroascorbic acid and

Lunec, J. and Blake, D.R, (1985). The determination of dehydroascorbic acid and ascorbic acid in the serum and synovial fluid of patients with rheumatoid arthritis. Free Rad. Res. Commun. 1, 31-39. 

Most of the work reported with these complexes has been concerned with kinetic measurements and suggestions of possible mechanisms. The [Ru(HjO)(EDTA)] / aq. HjOj/ascorbate/dioxane system was used for the oxidation of cyclohexanol to cw-l,3-cyclohexanediol and regarded as a model for peroxidase systems kinetic data and rate laws were derived [773], Kinetic data were recorded for the following systems [Ru(Hj0)(EDTA)]702/aq. ascorbate/dioxane/30°C (an analogue of the Udenfriend system cyclohexanol oxidation) [731] [Ru(H20)(EDTA)]70j/water (alkanes and epoxidation of cyclic alkenes - [Ru (0)(EDTA)] may be involved) [774] [Ru(HjO)(EDTA)]702/water-dioxane (epoxidation of styrenes - a metallo-oxetane intermediate was postulated) [775] [Ru(HjO)(EDTA)]7aq. H O /dioxane (ascorbic acid to dehydroascorbic acid and of cyclohexanol to cyclohexanone)... 

Various workers (Parks 1974) have observed a correlation between the oxidation of ascorbic acid to dehydroascorbic acid and the development of an oxidized flavor. Smith and Dunkley (1962A) concluded, however, that ascorbic acid oxidation cannot be used as a criterion for lipid oxidation. Their studies showed that although ascorbic acid oxidation curves for homogenized and pasteurized milk were similar, the homogenized samples were significantly more resistant to the development of an oxidized flavor. Furthermore, whereas pasteurization caused an appreciable decrease in the rate of ascorbic acid oxidation compared to raw milk, the pasteurized samples were more susceptible to oxidation. 

WA Behrens, R Madere. Ascorbic acid, isoascorbic acid, dehydroascorbic acid, and dehydroisoascor-bic acid in selected food products. J Food Comp Anal 7 158-170, 1994. 

Dehydroascorbic Acid and Ascorbic Acid. The oxidized and reduced forms of vitamin C (dehydroascorbic acid and ascorbic acid, respectively) have redox characteristics that are similar to those for o-quinone/catechol systems.13 Al-... 

The first step in the mechanism involves the reduction of Cu(II) to Cu(I) by ascorbyl-6-hexadeeanoate giving dehydroascorbic acid and a weak acid HY benzoic acid). In fact this stage of the process has no importance since Cu(I) benzoate may directly be used to initiate the polymerization by reducing the pyridinium salt. The strong Bronsted acid formed attacks the monomer and initiates the polymerization. Notably, lower polymer yields were obtained by using pyridium salt rather than iodonium salt. 

Gioia, M.G. et al. Development and validation of a liquid chromatographic method for the determination of ascorbic acid, dehydroascorbic acid and acetaminophen in pharmaceuticals. J. Pharm. Biomed. 2008, 48, 331-339. 

Park JB and Levine M (2000) Intracellular accumulation of ascorbic acid is inhibited by flavonoids via blocking of dehydroascorbic acid and ascorbic acid uptakes in HL-60, U937 and lurkat cells. Journal of Nutrition 130,1297-1302. 

Disposition in the Body. Readily absorbed after oral administration the proportion of a dose absorbed tends to decrease with increasing dose it is widely distributed in the body tissues. The concentration of ascorbic acid is higher in leucocytes and platelets than in erythrocytes and plasma. Ascorbic acid is metabolised to dehydroascorbic acid, 2,3-diketogulonic acid, oxalate, and carbon dioxide some conjugation with sulphate occurs to form ascorbate-3-sulphate. Ascorbic acid in excess of the body s requirements is rapidly eliminated in the urine. About 85% of an intravenous dose, given to subjects previously saturated with the vitamin, is excreted in the urine in 24 hours, with about 70% of the dose excreted unchanged and 15% as dehydroascorbic acid and diketogulonic acid. The amount normally present in the body is in excess of 1.5 g. 

A cofartor can be any chemical required by an enzyme, that is, a metal ion, coenzyme, lipid, or accessory pmtein. A coenzyme is a small organic molecule required by an enzyme that participates in the chemistry of catalysis. Most of the coenzymes work according to the following principle. They shuttle back and forth between two or more different forms. I lere, one of the forms may be considered to be the coenzymatically active form, and the other may be seen as requiring regeneration to the active form. This is the case for folate, vitamin B, riboflavin-and niacin-based cofactors, ascorbic acid, and vitamin K. The coenzymatically active (inactive) forms of three of these coenzymes are tetrahydrofolate (dihydro-folate), ascorbic acid (dehydroascorbic acid), and vitamin KH2 (vitamin K). 

On the other hand, a monocyclic derivative was obtained when a solution of dehydroascorbic acid and p-bromophenylhydrazine in di-methylacetamide was allowed to stand overnight at room temperature (25). The compound was precipitated with water and recrystallized from absolute alcohol its molecular structure is shown in Figure 11. The lactone ring is of the envelope type with C4 deviating by 0.126 A from the plane defined by Cl, Ol, C2, and 04. The side-chain is very nearly staggered and with 05 and C6 anti,... 

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 ... 

We thank Paul Seib for the samples of D-ert/thro-dehydroascorbic acid and 6-bromo-6-deoxy-L-dehydroascorbic acid. 

Fayle, S.E. et al., Crosslinkage of proteins by dehydroascorbic acid and its degradation products, Food Chem., 70, 193, 2000. 

On the basis of their redox thermodynamics and reaction chemistry with 02"- in aprotic media, ascorbic acid S and some catechols may be subject to an (O2 -)-catalyzed auto-oxidation to dehydroascorbic acid and o-quinones, respectively. 

In order to better understand the effect of ascorbic acid on non-enzymatic browning of citrus juices, we oxidized endogenous ascorbic acid in grapefruit with ascorbic acid oxidase. The enzyme oxidized ascorbic acid to dehydroascorbic acid and H2O without forming H2O2 (19). 

Very little information is available about the species which react in the further decomposition of ascorbic add. Apparently, both dehydroascorbic acid and 2,3-diketogulonic acid can be oxidized directly, since both oxalylthreonic acid and the same two free acids have been identified. The former would result from oxidative fission of the chain while the lactone ring was intact. Even less is known of the oxidation mechanism. The oxidation occurs with iodine and with acid permanganate (H12) and also with oxygen or peroxide (R23). Recent studies on the similar decomposition of the enols of aryl pyruvates to aryl aldehydes plus oxalic acid identified these reactions as examples of the direct attack... 

Figure 3.3 Cleavage of dehydroascorbic acid and establishment of ascorbic acid configuration...

In many redox reactions of L-ascorbic acid which have been studied it is assumed that dehydroascorbic acid is the only product. This is often not the case and the oxidation proceeds further. A technique for unambiguously determining the concentration of L-ascorbic acid, dehydroascorbic acid, and some or all of the further oxidation products with precision, accuracy, and sensitivity in the same solution still eludes us. 

It is frequently desirable to analyse for dehydroascorbic acid and l-ascorbic acid in the same solution. The former has an u.v. absorbance at 220 nm, but with a much smaller molar absorbtivity of 720. Thus any attempt at a direct spectrophotometric determination will be factor of about 20 times less sensitive than for L-ascorbic acid. We will see later that these facts have far reaching consequences in the use of chromatographic techniques for the separation and subsequent detection of L-ascorbic acid and dehydroascorbic acid. 

The complexity of the redox processes involving vitamin C have not prevented numerous studies of the oxidation of L-ascorbic acid and some on the oxidation and reduction of dehydroascorbic acid and other oxidation products. One of the reasons for the great interest in this subject is that the role of vitamin C in living systems is certainly connected to its oxidation-reduction behaviour and this may ultimately be the key to understanding the biological mechanisms of the actions of vitamin C. In this. section we will examine the oxidation and reduction of vitamin C by a wide variety of reagents, with emphasis on the behaviour of L-ascorbic acid since this is the most studied compound in this respect. 

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. 

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