Redox systems ascorbic acid

Simms et a/. [76] recently used ATRP of nBMA in the miniemulsion systems, using a redox initiation system (ascorbic acid/ hydrogen peroxide) with a CuBr /EHAb-TREN catalyst. A high MW was obtained with Mn=10 g/mol (number average molecular weight) and polydispersity index (PDI)=1.25. In addition, the conversions above 80% were achieved in 8 h, as well as the particle size of 100 nm. 

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. 

The formation of the [M(HA)](" 1>+ complex was confirmed in independent pH-metric experiments in the case of copper(II). These studies also provided evidence that ascorbic acid is coordinated to the metal center in its monoprotonated form. Because of relatively fast redox reactions between iron(III) and ascorbic acid, similar studies to confirm the formation of [Fe(HA)]2+ were not feasible. However, indirect kinetic evidence also supported the formation of the [M(HA)](" 1>+ complex in both systems (6). 

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). 

An example is adrenodoxin reductase (see chapter banner, p. 764), which passes electrons from NADPH to cytochrome P450 via the small redox protein adrenodoxin. This system functions in steroid biosynthesis as is indicated in Fig. 22-7.209a b Other flavin-dependent reductases have protective functions catalyzing the reduction of ascorbic acid radicals,210 211 toxic quinones,212-214 and peroxides.215-218... 

The emulsion copolymerization of BA with PEO-MA (Mw=2000) macromonomer was reported to be faster than the copolymerization of BA and MMA, proceeding under the same reaction conditions at 40 °C [100]. Polymerizations were initiated by a redox pair consisting of 1-ascorbic acid and hydrogen peroxide in the presence of a nonionic surfactant (p-nonyl phenol ethoxylate with 20 moles ethylene oxide). In the macromonomer system, the constant-rate interval 2 [9,10] was long (20-70% conversion). On the other hand, the interval 2 did not appear in the BA/MMA copolymerization and the maximum rate was lower by ca. 8% conversion min 1 and it was located at low conversions. 

Spectrophotometric techniques combined with flow injection analysis (FIA) and on-line preconcentration can meet the required detection limits for natural Fe concentrations in aquatic systems (Table 7.2) by also using very specific and sensitive ligands, such as ferrozine [3-(2-bipyridyl)-5,6-bis(4-phenylsulfonic acid)-l,2,4-triazine], that selectively bind Fe(II). Determining Fe(II) as well as the total Fe after on-line reduction of Fe(III) to Fe(II) with ascorbic acid allows a kind of speciation.37 A drawback is that the selective complexing agents can shift the iron redox speciation in the sample. For example, several researchers have reported a tendency for ferrozine to reduce Fe(III) to Fe(II) under certain conditions.76 Most ferrozine methods involve sample acidification, which may also promote reduction of Fe(III) in the sample. Fe(II) is a transient species in most seawater environments and is rapidly oxidized to Fe(III) therefore, unacidified samples are required in order to maintain redox integrity.8 An alternative is to couple FIA with a chemiluminescence reaction.77-78... 

GSH has been proposed to be part of the thiol cycling in mammalian cells that may transduce oxidative stress redox signaling into the induction of many genes involved in proliferation, differentiation, and apoptosis [15], Studies with pure chemical systems have confirmed the reduction of V(V) maltol compounds by GSH or ascorbic acid [16], Putative glutathione transferase enzymes that bind vanadium have been isolated from an ascidian that accumulates vanadium in specialized cells to over 350 mM [17],... 

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-... 

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

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... 

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. 

It is evident from these considerations that the use of a less hydrophobic redox species in the O phase makes the homogeneous ET occur more favourably. Another example of the IT mechanism has been found in the ET between L-ascorbic acid in W and chloranil (with Ko = 900) in NB or DCE. This has been confirmed using potential-controlled polarography [47], potential modulated reflectance spectroscopy [46], microflow coulometry [39], ECSOW system [38] and digital simulation of cyclic voltammograms [48]. 

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)... 

Electrochemical detection involves the induction of a change in redox state (electrolysis) by application of an electrical potential to an electrode (71). Compounds that can be readily detected by this means are termed electroactive. Under physiological conditions, these compounds tend to be in their reduced state in the nervous system because of the rich level of antioxidants (e.g., ascorbic acid) and, thus, can be oxidized by application of a positive potential to the electrode. The evolved electrons are detected at the electrode in the form of electrical current. This current is proportional to the number of electroactive molecules at the surface of the electrode, and therefore it is proportional to their concentration in the bulk solution. By implanting an electrode in the extracellular space close to the release site and detecting changes in the local (extracellular) concentration of the neurotransmitter, neurotransmitter release can be monitored. The key advantage of this approach is the high temporal resolution that can be in the millisecond domain. Neurotransmitters that can be detected this way include dopamine, norepinephrine, epinephrine, serotonin, and melatonin. 

Many different redox reactions i acidic solutions are catalyzed by the same substances that catalyze hydrogen peroxide reactions. For example, Bognar and Jellinek determined traces of V(V), Fe(III), and osmium tetroxide using a chlorate-bromide-ascorbic acid-o-tolidine system and the Landolt effect. 

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