Dehydroascorbic Acid hydration

Detailed reaction mechanisms are in fact more complex. In both cases, the primary product of oxidation of ascorbic acid is an ascorbyl radical (HA ) and its anion (HA ) stabilised by resonance, respectively. It is relatively inert and does not react with oxygen, but comparatively quickly (the reaction half-life is about 0.2 s) provides an equimolar mixture of ascorbic acid and dehydroascorbic acid (or bicychc dehydroascorbic acid hydrate. Figure 5.27) by disproportionation ... 

Oxidation of ascorbic acid by singlet oxygen gives L-threonic acid 1,4-lactone and oxalic acid, analogously to the reaction catalysed by ascorbate 2,3-dioxygenase. Intermediates of this reaction are hydroperoxides of ascorbic acid and of dehydroascorbic acid hydrate (Figure 5.29). 

The biological functions of vitamin C appear to be related principally to its well-established reducing properties and easy one-electron oxidation to a free radical or two-electron reduction to dehydroascorbic acid. The latter is in equilibrium with the hydrated hemiacetal shown at the beginning of this box as well as with other chemical species.1 Vitamin C is a weak acid which also has metal complexing properties. 

Dehydroascorbic acid (DHA) is the first stable oxidation product of i.-ascorbic acid (AA), DHA can be easily and quantitatively prepared by air oxidation of AA over charcoal in ethanol DHA is stable for days in aqueous solution of pH 2-4. H NMR and NMR studies show that the principle species of DHA is the bicyclic hydrate, 3,6-an-hydro-L-xy o-hexalono-l,4-lactone hydrate. This finding is confirmed by synthesis and spectral studies of related compounds. DHA contains equilibrium concentrations of various dehydrated and open side-chain forms, but these species are too small to detect using NMR spectroscopy. 

Dehydroascorbic acid is now readily prepared in pure form by the Ohmori-Takagi method in which ascorbic acid is dissolved in ethanol and oxygenated in the presence of charcoal. The structure of dehydro-L-ascorbic acid is solvent dependent. In water, dehydro-L-ascorbic acid exists almost exclusively in the bicyclic hydrated monomer in which 6-OH... 

Ascorbic acid is quoted as an example of a first-order consecutive reaction. This substance is oxidized electrochemically to an unstable electro-active intermediate, transformed by a fast chemical reaction with rate constant k into the electro-inactive dehydroascorbic acid. It is assumed that the electro-inactive form is hydrated. The halfwave potential of the anodic wave is independent of ascorbic acid... 

Dehydroascorbic Acid. L-rftreo-2,3-Hexodiulo-sonic acid -/-lactone. CtH Of mol wt 174.11. C 41.39%, H 3.47%, O 55.14%. The reversibly oxidized form of ascorbic acid. Prepd by the action of benzoquinone on ascorbic acid Ohle, Erl bach, Ber. 67, 555 (1934) Moll, Wieters, E. Merck s Jahresber, 50, 65 (1936) by the action of iodine Herbert et al, J. Chem. Soc. 1933, 1270 Kenyon, Munro, ibid. 1948, 158 by oxidn with perr-naphthindan -2,3,4-trione hydrate Moubasher, J. Biot. Chem. 176, 533 (1948) see also Muller-Mulot, Z. Physiol Chem. 351, 52 (1970). Isomerization and formn of derivs Egge, Tetrahedron Letters 1969, SOI. Structure studies Teichmann, Ziebarth, Z. Prakt. 

Fine needles, dee 225". Sol in water at tdf. In soln the two carbonyl groups (in position 2 and 3) assume the hydrated form —C(OH),—C(OH)j—. Practically neutral reaction. pKa 3.90. lascorbic acid. Detailed stability data Bogdanski. Bogdanska. Bull. Acad. Polonaise Sci. (II classe) 3, 41 (1955). See also Velisek et al. Coll Czech. Chem. Commute 37, 1465 (1972). Undecomposed dehydroascorbic acid in soln is easily converted to ascorbic acid by reduction with sulfurous acid. Has same antiscorbutic activity in humans as ascorbic acid (upon oral ingestion). 

Ascorbic acid is oxidized by plants with ascorbate oxidase to yield dehydroascorbic acid (3,6-anhydro-L-Ay/o-hex-ulono-1,4-lactone hydrate) (76) (Loewus, 1980, 1988). In plants, ascorbic acid is rarely accumulated, but is usually converted into tartaric (77) and oxalic (78) acids (Fig. 15.14). 

In aqueous solutions, the EC mechanism proposed by Ruiz [13] for the oxidation of AA at low pH is widely accepted. It involves two consecutive one-electron transfer processes to form dehydroascorbic acid immediately followed by irreversible hydration to give the final product 2-3 diketogluco-nic acid. Although the electrochemical reaction is reversible at Hg electrodes [13], the large overpotential needed at carbon electrodes renders the oxidation of AA to be irreversible and the anodic potential (--300 mV at pH 3.9) is considerably higher than its standard value [14, 27], Figure 1. 

If the shape of the wave shows a reversible process, when the limiting current is diffusion controlled, but the half-wave potentials differ from those measured at the equilibrium conditions and are dependent on the drop-time ( x/2 = const. 2-303RTInFlogiff, then the product of the reversible electrode reaction is inactivated by a fast chemical reaction, e.g. in the case of the anodic waves for ascorbic acid.< > > It was supposed that the inactivation reaction is the hydration of dehydroascorbic acid, but the polarographic behaviour of dehydroascorbic acid does not agree with this simple explanation. 

The nature of the intermediates and products formed on oxidation of L-ascorbic acid and its 6-benzoate and 6-palmitate by iodine have been investigated by i C-nmr in both D2O and d6-DMSO. In D2O, there was evidence for the formation of the 3,6-hemiketal-hydrate 63 in equilibrium with the bis-hydrate 64 of dehydroascorbic acid. A similar bis-hydrate was formed from the 6-(9-benzoyl deriv.itive in D2O, and this formed a bis-anhydrodimer on standing. In d -DMSO, the 6-O-acyl derivatives formed the 2,3-dione (dehydroascorbate), which formed the bis-hydrate on adding water. 2 Thg hydrogen-ion dependence in the oxidation of L-ascorbic acid by hexacyanoferrate(III) in aqueous ethanol has been investigated the rate law was derived and the... 

Ascorbic acid (I) has an acidic hydroxyl group (pKi = 4.04, pK2 = 11.4 at 25 °C). Its UV absorption depends on the pH value (Table 6.9). Ascorbic acid is readily and reversibly oxidized to dehydroascorbic acid (II), which is present in aqueous media as a hydrated hemiketal (IV). The biological activity of II is possibly weaker than that of I because the plasma and tissue concentrations of II are considerably lower after the administration of equal amounts of I and II. The activity is completely lost when the dehydroascorbic acid lactone ring is irreversibly opened, converting II to 2,3-diketogulonic acid (III), cf. Formula 6.18 ... 

Ascorbyl radical is acid pK = —0.96), which exists in solution as a resonance stabilised anion (as a bicyclic compound, apparently with a double bond between carbons C-2 and C-3) and with the unpaired electron located in the C-4 region. Dehydroascorbic acid occurs as the bicycHc hydrated monomer (3,6-anhydro-L-xylo-hexulono-l,4-lactone hydrate, Figure 5.27) in aqueous solutions. 

Oxidation of ascorbic acid by hydrogen peroxide proceeds via ascorbyl radical and dehydroascorbic acid (apparently its hydrate), which yields 2,3-dioxo-L-gulonic acid. Hydrogen peroxide further oxidises 2,3-dioxo-L-gulonic acid to give unstable 2,3,5-trioxo-L-gulonic acid (Figure 5.29), which decomposes and produces other... 

Ascorbic acid is generally a more effective antioxidant, when used in combination with tocopherols. They then preferentially react with Hpid free radicals and the resulting tocopheryl radicals are, on the oil-water interface, reduced back to tocopherols by ascorbic acid. Ascorbyl pahnitate yields, via the corresponding radical, dehydroascorbyl pahnitate, which, unlike dehydroascorbic acid, cannot form a cycHc hydrate. Ascorbic acid also reacts similarly with toxic forms of oxygen, such as a hydroxyl radical (HO ), an anion of superoxide radical (Oj ) and singlet oxygen ( 02). Simultaneously, aU of these reactions slowdown the oxidation of Hpids ... 

Like other vicinal dicarbonyl derivatives of sugars, dehydroascorbic acid is involved in the MaiUard reaction. Dehydroascorbic acid (or its bicyclic hydrate. Figure 5.27) is a y-lactone that is readily hydrolysed under a base catalysis to its parent unstable compound that undergoes a series of irreversible reactions. These reactions result in loss of vitamin C and the formation of coloured products, and the discoloration of fruit and vegetable products. 

A reversible thickening method of UPRs was developed. The method consisted of the use of thermally breakable fimctional groups derived from dike-togulonic acid and a salt-forming diamine [238]. Diketogulonic acid itself was prepared by hydratation of dehydroascorbic acid, being the product of oxidation of L-ascorbic acid with H2O2 (Fig. 23). 

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

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