Redox systems ascorbate

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. 

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). 

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. 

Grafting of polyacrylamide onto guar gum [431] and Ipomoea gum [178] in aqueous medium initiated by the potassium persulphate/ascorbic acid redox system was performed in the presence of atmospheric oxygen and Ag" " ions. After grafting, a tremendous increase of the viscosity of both gum solutions was achieved, and the grafted gums were found to be thermally more stable. 

Carbon electrodes exhibit a wide range of electron transfer rates for benchmark redox systems, depending on carbon material and surface history. Two examples are shown in Figure 10.2, which compares two carbon surfaces with very different k° for Fe(CN) /4. In some cases, the variations in electrode kinetics have been particularly important to analytical applications. For example, carbon paste and carbon fiber electrodes have been used to monitor neurotransmitters in living animal brains [5,6]. The determination of catechol transmitters in the presence of relatively large amounts of interferents (e.g., ascorbate) de-... 

It must be emphasized that redox systems do not necessarily consist of ions. Although most inorganic systems are made up wholly or partly of ions, a number of organic redox systems are in fact equilibria between molecules. The dehydroascorbic acid (C6H606) and ascorbic acid (C6H806) redox system for example... 

Crivello and Lam [69] have reported that the diaryliodonium salt-ascorbate redox system readily initiates the cationic polymerization of appropriate monomers. N-Alkoxy pyridinium salts were also shown [70] to participate in this redox process. The polymerization mechanism depicted below is quite similar to that described for the iodonium salts (Scheme 17). 

The ECSOW system has also been applied to a biomimetic redox system, i.e., the oxidation of L-ascorbic acid in W by chloranil added to NB [38]. A comparison of the cyclic voltammograms obtained with the ECSOW system and the O/W interface has provided important suggestions on the possible reaction mechanism at the OAV interface. Thus, the ECSOW system would offer important clues to clarify ET processes at O/W interfaces. 

Redox systems which have been the subject of recent examlnC atlon include potassiian permanganate - tartaric acid ( ), and potassium persulfate — ascorbic acid.( ) Whilst experiments were with the water soluble acrylamid they should be adaptable to emulsion conditions. The ascorbic acid reductant is of inters est as it is not interfered with by air or monomer stabilisers. 

A comprehensive review of spectrophotometric methods for the determination of ascorbic acid (1) was presented. Most of the methods are based on the reducing action of ascorbic acid, making use of an Fe(III)-Fe(II) redox system, and to a lesser extent Cu(II)-Cu(I), V(V)-V(IV) and phosphomolybdate/phosphotungstate-molybdenum/tungsten blue redox systems. A kinetic spectrophotometric method for the determination of L-ascorbic acid and thiols (RSH) was developed, whereby the absorbance of the Fe(II)-phen complex formed during the reaction of 1 or RSH with Fe(III)-phen was continuously measured at 510 nm by a double beam spectrophotometer equipped with a flow cell. The linearity range for 1 was 4-40 p,M and for RSH 8-80 xM. The method was validated for pharmaceutical dosage forms . 

Ascorbic acid (1) is most commonly used for testing the performance of electrodes in redox systems. Thus, a Ag-Ag ascorbate selective electrode was constructed with view to use it for vitamin C determination. Its reproducibility and stability was satisfactory and ascorbate ion concentration could be determined in neutral, alkaline and alcoholic media" . A voltametric study was carried out for the evaluation of graphite-epoxy composite (GEC) electrodes for use in the determination of ascorbic acid and hydroquinone. They were compared with mercury and CPE in similar operating conditions of pH and supporting electrolytes. Like all redox electrodes, also GEC electrodes deteriorate on exposure to air or after repeated usage, and the surface had to be renewed for activation. GEC electrodes were found to be adequate for redox system analyses"". The electrocatalytic oxidation of 1 is an amplification method for determination of specific miRNA strands using the An biosensor described in Table 1 . 

This kind of redox system leads also to other free-radical species and the obtained polymers are not purely hydroxytelechelic. Hydroxylamine/mineral acid (HC1, H2S04)/H202 55 59,60), NaHS03/H202 55), ascorbic acid/H202 61), thiourea (or N-substituted thiourea)/H202 62 -64) systems have been suggested. The last one yields mostly hydroxyl-terminated polymers. 

Another redox system, ethyl eosin/ascorbic acid in aqueous methanol solution, has been proposed 74,75). In fact, hydrogen peroxide is generated and its association with ascorbic acid initiates the polymerization. 

Hydroxytelechelic poly(vinyl acetate)s have been synthesized with redox system such as ethyl eosine-ascorbic acid-visible light in aqueous methanol74). The irradiation of the dye-acid system leads to hydrogen peroxide formation and then to the generation of hydroxyl radicals which initiate polymerization. The following initiation mechanism has been suggested 74,75)... 

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].

A third type of polymers is polymerized methyl methacrylate or methacrylamide (768-773), Strubell (768) has carried out polymerization of methyl methacrylate with an ascorbic acid-benzoyl peroxide system. In an aqueous polymerization of methyl methacrylate, Misra and Gupta (770) used the redox system of potassium peroxydisulfate and ascorbic acid. A similar system was reported by Pattnaik et al. (773). 

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

In addition to the ketoses, Dillon (9) showed that the terminal galactose units and the nonreducing sugar fraction could be oxidized, oxidation of the latter resulting in depolymerization. Although carrageenin has no marked antioxidant value in itself, a redox system combining it with ascorbic acid is very effective (38). 

Many different redox systems have been used in the emulsion polymerization of vinyl acetate. Further investigations on the use of persulfate-bisulfite, hydrogen peroxide-ascorbic acid, tert-butyl hydroperoxide with various water-soluble as well as monomer-soluble reducing agents, etc., should be carried out. 

General metabolic significance. Ascorbic acid participates in numerous biological events concerning electron transport reactions, hydroxylations, oxidative catabolism of aromatic amino acids one of the most important biologic redox systems (no coenzyme function). 

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