Dehydroascorbic acid reduction

This reaction (Eq. 18) occurs spontaneously above pH 6. The similar reaction with H2S is used in the analysis of dehydroascorbic acid by dye reduction. Such spontaneous reactions are believed to account for the dehydroascorbic acid reduction observed in animal tissue extracts, and no certain evidence of an animal tissue enzyme for this reaction has been obtained (B26, S12). Many plants contain such an enzyme, however. 

Freshly prepared mitochondria contain ascorbate, as do mitoplasts, that lack the outer mitochondrial membrane (Li etal. 2001). Both mitochondria and mitoplasts rapidly take up oxidised ascorbate as dehydroascorbic acid and reduce it to ascorbate. Ascorbate concentrations in mitochondria and mitoplasts rise into the low micromolar range during dehydroascorbic acid uptake, although uptake and reduction are opposed by ascorbate efflux. Mitochondrial dehydroascorbic acid reduction depends mainly on GSH, but mitochondrial thioredoxin reductase may also contribute. Reactive oxygen species generated within mitochondria oxidise ascorbate more readily than they do GSH and a-tocopherol. 

Enzymatic dehydroascorbic acid reduction activity is also found in a number of tissues (Khatami et al, 1986 Rose and Bode, 1992). Stomach, brain, adrenals, small intestine, and lung have at least as much activity as the liver in guinea pigs. The kidney has long been known to serve in reabsorption of ascorbic acid renal... 

Ascorbic acid, for example, is oxidized to dehydroascorbic acid with reduction of the iron(III) ions. The Fe(II) ions so produced react with 2,2 -bipyridine with formation of a colored complex. 

Solution Abbreviating dehydroascorbic acid as D, and ascorbic acid as H2A, we rewrite the reduction as... 

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. 

Analysis of vitamin C has been the topic of numerous papers. Most of the methods analyze L-ascorbic acid by HPLC, before and after reduction of the dehydroascorbic acid present.338 The concentration of the dehydroascorbic acid is calculated by subtraction. A later work describes a method, which combines iodometry with a voltammetric technique to detect the endpoint of the titration.339 The results are comparable to those obtained by HPLC and can be applied to vegetable and fruit samples. 

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. 

Cu2(L )(NCS)2] (66), which has been shown (c.f. Fig. 8) [43] to contain two tetrahedrally coordinated Cu(I) ions held 2.796(8) A apart and linked inter-molecularly via the sulfur atoms of the thiocyanate ions. For the complexes 63 and 64, the reduction product is the diamagnetic complex [Cu2(L )(MeCN)2](Y)2 (67) (Y = CIO4 or BPh4) in which each three coordinate Cu(I) ion is bonded to two of the four macrocyclic nitrogen atoms and to the nitrogen of one of the two MeCN molecules (Fig, 9) [43]. In the presence of certain substrates the reduction of 63 or 64 is accompanied by substrate oxidation. For example, PhSH, PhC=CH, hydrazobenzene, catechols, hydroquinone, and ascorbic acid afford PhSSPh, PhC=CCsPh, azobenzene, o-quinones, p-quinone, and dehydroascorbic acid, respectively, together with the reduced species 67 and/or other copper complexes... 

Figure 12. Ascorbic acid-dehydroascorbic acid redox system (a) oxidation of ascorbate to semidehydroascorbic acid, (b) disproportionation of semidehydroascorbic acid, and (c) reduction of dehydroascorbic acid [From (100), with permission].

These differences are correlated with differing functions. The intracellular reduced thioredoxins are thermodynamically the best reductants of disulfide linkages in proteins and they help keep intracellular proteins reduced. Glutaredoxin can be reduced efficiently by reduced glutathione or by NADPH and glutathione reductase and can, in turn, reduce cysteine and the oxidized form of vitamin C, dehydroascorbic acid (Box The peri-... 

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). 

Ascorbate is a reactive reductant, but its free radical is relatively nonreactive (2) see Table I) and decays by disproportionation to ascorbic acid/ascorbate and dehydroascorbic acid ... 

The predicted shift was verified (Kem, 1954), but the height of the reduction waves of dehydroascorbic acid found (Ono et ai., 1953) and their half-wave potentials do not agree with this theory, indicating that the reaction scheme is more complex. 

Ascorbic acid, for example, is oxidized to dehydroascorbic acid with reduction of -... 

Polarography has had limited use in the estimation of ascorbic acid, since early investigations disclosed no advantage over the simpler methods of chemical titration, even as regards specificity. A recent application of this method to vegetables and fruits was reported by Krauze and Boxyk (K16). No dehydroascorbic acid was found in the samples after reduction by H2S. 

Certain microorganisms, such as Escherichia coli and Staphylococcus albus strains will reduce dehydroascorbic acid. This reduction has been used for the estimation of ascorbic acid after its oxidation in extracts by ascorbic acid oxidase (S27) and also in the chemical reduction methods as a way of reducing dehydroascorbic acid to ascorbic acid. The reduction is not specific for L-ascorbic acid, but interfering compounds are unlikely in natural products. The use of this reducing action of bacteria has been improved by Mapson and Ingram (M8). 

I. Dehydroascorbic Acid Determination by Reduction. A disadvantage of the dichlorophenolindophenol reduction method is that the dehydroascorbic acid can be determined only after its reduction to ascorbic acid. The reducing agent used may interfere in the subsequent reduction of the dye. Since ascorbic acid exists mainly in the reduced form in most tissues, this is not a serious disadvantage for the reduction method in the usual analysis. The reduction of dehydroascorbic acid to ascorbic acid is usually performed vi ith HaS by treatment for 15 min between pH 1.2 and 4.7 (R20). The completeness of the reduction was checked by the dinitrophenylhydrazine method. 

---