Ascorbic dehydro

Micromodifications. Click et al. (G4) described a method for the determination of L-ascorbic, dehydro-L-ascorbic, and diketo-L-gulonic acids in microgram quantities of tissue, such as adrenal tissue, with the phenylhydrazine method. They also found slightly lower values by incubation during coupling at 100°C for 5 minutes but false, high values at 100° C for ten minutes, as compared to the values obtained at 37°G for 6 hours. 

Roe, Joseph H., Chemical Determinations of Ascorbic, Dehydro-ascurbic, and Diketogulonic Acids. 

This is probably an ascorbate-dehydro ascorbate redox buffer poised at -0.200 V vs SCE . 

The most significant chemical characteristic of L-ascorbic acid (1) is its oxidation to dehydro-L-ascorbic acid (L-// fi (9-2,3-hexodiulosonic acid y-lactone) (3) (Fig. 1). Vitamin C is a redox system containing at least three substances L-ascorbic acid, monodehydro-L-ascorbic acid, and dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid and the intermediate product of the oxidation, the monodehydro-L-ascorbic acid free radical (2), have antiscorbutic activity equal to L-ascorbic acid. 

The reversible oxidation of L-ascorbic acid to dehydro-L-ascorbic acid is the basis for its known physiological activities, stabiUties, and technical apphcations (2). The importance of vitamin C in nutrition and the maintenance of good health is well documented. Over 22,000 references relating only to L-ascorbic acid have appeared since 1966. 

This synthesis was the first step toward industrial vitamin production, which began in 1936. The synthetic product was shown to have the same biological activity as the natural substance. It is reversibly oxidized in the body to dehydro-L-ascorbic acid (3) (L-// fi (9-2,3-hexodiulosonic acid y-lactone), a potent antiscorbutic agent with hiU vitamin activity. In 1937, Haworth and Szent-Gyn rgyi received the Nobel Prize for their work on vitamin C. 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

The condensation reactions of aromatic o-diamines and sugars and sugar derivatives have been studied in detail and quinoxaline derivatives have been prepared recently from osones, osonehydrazones, and dehydro-L-ascorbic acidd ... 

It has long been recognized that ascorbate levels are low in patients with RA (Lunec and Blake, 1985) and ascorbate is predominantly found in the dehydro form. The presence of increased dehydroascorbate has been suggested to indicate its rapid oxidation by stimulated PMNs (Halliwell and Gutteridge, 1990). When ascorbate concentrations are lower than about 20 /tmol/1, as can occur in rheumatoid synovial fluid, the Fe(III) reducing effects of ascorbate outweigh its radical-scavenging effects. Ascorbate then causes increased OH formation and promotes lipid peroxidation (Blake et al., 1981). 

L-fhreo-2,3-Hexodiulosono-1,4-lactone (dehydro-L-ascorbic acid) 2-(p-bromophenyl)hydrazone DASCBH 37 386... 

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 scavenging of the radical HA (in the case of ascorbate). HA reduces relatively fast (in a non-rate determining step) another Fe(III) the product is dehydro-ascorbate. The rate expression for the production of dissolved iron(II) at a given pH is written as follows ... 

Dehydro- S-Adenosyl- S-Adenosyl-ascorbate methionine homocysteine... 

Reaction of dehydro-L-ascorbic acid and its analogs with two molecules of o-phenylenediamine or its substituted derivative gave the quinoxaline... 

Reaction of dehydro-L-ascorbic acid with guanidine derivatives gave the 2-aminoimidazole acyclo-C-nucleoside analog 1213, as shown in Scheme 241 (92T6385). 

Reduction of dehydro-c-ascorbic acid phenylhydrazone (40) with LiAlH4 resulted in hydrogenation of the hydrazone residue and cyclization to bicyclic compound 41, which was dehydrogenated with boiling acetic anhydride during acetylation to give diacetate 43, then partly hydrolyzed to monoacetate 42 (Scheme 7) (72JOC3523). 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

Asarone Sd EO 23 pj. pQ Qg%DC090 Ascorbic acid, dehydro Rt 2 C244 Ascorbic acid Rt 2093 Aspartic acid pH iei Astragalin Sd ... 

The trihydroxypropyl derivative 63 was prepared from dehydro-L-ascorbic acid phenylosazone (62) by opening the lactone ring with warm alkali and acidifying the mixture after a few minutes, whereupon the pyrazolinone 63 separated immediately.63 The structure of this compound was established by degradation,63 and confirmed by a study of its n.m.r. spectrum.64 The p-tolyl-, p-(bromophenyl)-, and p-(iodophenyl)-osazones of dehydro-L-ascorbic acid were also converted into the corresponding l-aryl-4-phenylazo-3-(trihydroxy-propyl)-5-pyrazolinone, and their acetylation and benzoylation products were prepared.64... 

The reduction of adrenochrome (1) with ascorbic acid (59) was first reported in 1948,158 although the nature of the reaction products (which may be of physiological importance, cf. ref. 159) was not determined until several years later. It was shown by Heacock and Laidlaw in 1958 that reduction mixtures of this type contained at least three indolic products,147 one of which was isolated and shown to be 5,6-dihydroxy- -methylindole (28).147 The major component of aqueous adrenochrome-ascorbic acid reaction mixtures has recently been shown to be a secondary product (60) (which was isolated as its di- and tetra-acetyl derivatives) produced by the interaction of the o-dihydroxy group of 28 with the a-dicarbonyl function of dehydro-... 

Ascorbic acid is readily oxidized to dehydro-ascorbic acid (Box 18-D Fig. 20-2, step e), which may be hydrolyzed to L-bisoxogulonic acid (step/). The latter, after decarboxylation and reduction, is converted to L-xylulose (steps g and h), a compound that can also be formed by a standard oxidation and decarboxylation sequence on L-gulonic acid (step z). Reduction of xylulose to xylitol and oxidation of the latter with NAD+ (steps j and k) produces D-xylulose, which can... 

Fruits and vegetables are the most common food sources for vitamin C (15-18). In food, vitamin C exists as two vitamers L-ascorbic acid (AA) and its oxidation product, dehydro-L-ascor-bic acid (DHAA) (Fig. 1) (15,17-19). Total Vitamin C is the sum of the AA and the DHAA contents. A stereoisomer, isoascorbic acid (IAA) is often added to food as an antioxidant. Both LAA and its oxidation product, dehydroisoascorbic acid (DHIAA), can interfere in Total Vitamin C determinations. 

L-Ascorbic Acid (AA) Dehydro-L Ascorbic Acid (DHAA)... 

AA = L-ascorbic Acid DHAA = dehydro-L-ascorbic Acid. 

---