Oxidation ascorbic acid

On the other hand, some PCSs lave demonstrated an effect which could be named photoinhibition of the catalytic oxidation process . Thus, it can be seen from Fig. 22 that poly(propionitrile) catalyzing the ascorbic acid oxidation in darkness manifests suppessed catalytic activity on exposure to light. 

SARMA A D, SREELAKSHMi Y and SHARMA R (1997) Antioxidant ability of anthocyanins against ascorbic acid oxidation, Phytochemistry, 45, (4), 671-4. 

The presence of o-qulnone surface waves seems, at the present time, to be coincidental to activation particularly In the case of ascorbic acid oxidation. On the other hand. Its presence may serve as a criterion of cleanliness and activation. Thus, the surface waves at 0.250 and 0.190 are Indicators or signatures for active GCE electrodes and should be used as diagnostic for a clean GCE surface as Is the hydrogen fine structure for platinum (31). It Is unfortunate that the o-qulnone peaks do not appear to be proportional to the surface area as Is the platinum fine structure. 

Another electro-oxidation example catalyzed by bimetallic nanoparticles was reported by D Souza and Sam-path [206]. They prepared Pd-core/Pt-shell bimetallic nanoparticles in a single step in the form of sols, gels, and monoliths, using organically modified silicates, and demonstrated electrocatalysis of ascorbic acid oxidation. Steady-state response of Pd/Pt bimetallic nanoparticles-modified glassy-carbon electrode for ascorbic acid oxidation was rather fast, of the order of a few tens of seconds, and the linearity was observed between the electric current and the concentration of ascorbic acid. 

The problem of selectivity is the most serious drawback to in vivo electrochemical analysis. Many compounds of neurochemical interest oxidize at very similar potentials. While this problem can be overcome somewhat by use of differential waveforms (see Sect. 3.2), many important compounds cannot be resolvai voltammetrically. It is generally not possible to distinguish between dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) or l tween 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA). Of even more serious concern, ascorbic acid oxidizes at the same potential as dopamine and uric acid oxidizes at the same potential as 5-HT, both of these interferences are present in millimolar concentrations... 

Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

Various workers (Parks 1974) have observed a correlation between the oxidation of ascorbic acid to dehydroascorbic acid and the development of an oxidized flavor. Smith and Dunkley (1962A) concluded, however, that ascorbic acid oxidation cannot be used as a criterion for lipid oxidation. Their studies showed that although ascorbic acid oxidation curves for homogenized and pasteurized milk were similar, the homogenized samples were significantly more resistant to the development of an oxidized flavor. Furthermore, whereas pasteurization caused an appreciable decrease in the rate of ascorbic acid oxidation compared to raw milk, the pasteurized samples were more susceptible to oxidation. 

Smith, G. J. and Dunkley, W. L. 1962C. Ascorbic acid oxidation and lipid peroxidation in milk. J. Food Sci. 27, 127-134. 

GC is arguably the most common carbon electrode material in current use, and its applications are too extensive to list here in any comprehensive fashion. Examples are listed here to illustrate GC electrode performance, particularly for voltammetry. Figure 10.12 shows the effect of vacuum heat treatment on ascorbic acid oxidation on initially polished GC [41]. Note that the less positive peak... 

Shtamm E.V., Catalysis of Ascorbic Acid Oxidation with Copper Ions , PhD Thesis, MGU, Moscow, 1975, 120 pp. (in Russian). 

The first, called the Maillard reaction,1 occurs between a carbonyl compound, which here is usually a reducing sugar, and an amine, which here is usually an amino acid, a peptide, or a protein. The second is caramelisation, a reaction where the sugars react on their own, but normally requires more drastic conditions. (Some discuss this under the heading of active aldehydes.) The third is ascorbic acid oxidation. The last, although it need not involve any enzyme at all, is nearest to enzymic browning, since it often does involve ascorbic acid oxidase, which, however, does not affect the phenols, which are the normal substrate in enzymic browning, but may involve other enzymes, e.g., laccase or peroxidase. 

Here, much attention will be given to the Maillard reaction, since one can consider caramelisation and ascorbic acid oxidation as special cases of it. Also, the Maillard reaction is the one of physiological significance. 

Interaction of ascorbic acid (vitamin C) with the silica surface was complicated by the fast oxidation of ascorbic acid in aqueous and ethanol solutions because of dissolved oxygen.12,13 However, both unmodified and modified silica increase the oxidation resistance of vitamin C. In particular, the rate of ascorbic acid oxidation to dehydroascorbic acid was found to be much less in the presence of unmodified or modified silica (Figure 4). Vitamin C is stabilized in the presence of silica, apparently due to interaction of the vitamin with the surface of highly-disperse silica particles, as confirmed by the results... 

Krukovsky, V.N., Guthrie, E.S. 1945. Ascorbic acid oxidation, a key factor in the inhibition or promotion of the tallowy flavor in milk. J. Dairy Sci. 28, 565-579. 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

Lyne and O Neil [117] reported the in vivo detection of dopamine using stearate-modified carbon-Nujol paste electrodes. Prior to their work, the detection of dopamine by voltammetric techniques was hindered primarily due to the coexisting ascorbic acid in the extracellular fluid of the mammalian brain. Ascorbic acid oxidizes at electric potentials similar to that of dopamine on many electrode materials. These authors found that the use of stearate-modified carbon-Nujol paste electrodes retards the electro-oxidation of anionic species (such as ascorbate) to such an extent that the cationic dopamine species could be detected in their presence. 

Ascorbate oxidase Ascorbic acid oxidation and reduction... 

The use of metaphosphoric acid solutions for the extraction of ascorbic acid from plant and animal tissues was first proposed in 1935 (29). Metaphosphoric acid, along with trichloroacetic acid, remain as the reagents of choice. Besides the decreased tendency for hydrolysis of the lactone ring, metaphosphoric acid inhibits the catalytic oxidation of ascorbic acid by metal catalysts, such as copper and iron ions, and it inactivates the enzymes that oxidize ascorbic acid. Oxidation of ascorbic acid, which apparently is the result of the action of oxyhemoglobin, may occur when animal tissues are ground with metaphosphoric acid. This... 

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