Ascorbic acid vitamin oxidation

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). 

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

Ascorbic acid (Vitamin C H A) has been widely used in pharmaceutical, chemical, cosmetic and food industry because of its bioactive and antioxidant properties [1]. Cu (II) ion is important nutrients found in many food and pharmaceutical products. Traces of transition metal ions like as Cu(II) ion also acts as catalyst in the oxidation... 

The SP procedure of water-soluble vitamins from multivitamin tablets is particularly challenging due to the diverse analytes of varied hydrophobicities and pfC. Water-soluble vitamins (WSVs) include ascorbic acid (vitamin C), niacin, niacinamide, pyridoxine (vitamin B ), thiamine (vitamin Bj), folic acid, riboflavin (vitamin B2) and others. While most WSVs are highly water soluble, riboflavin is quite hydrophobic and insoluble in water. Folic acid is acidic while pyridoxine and thiamine are basic. In addition, ascorbic acid is light sensitive and easily oxidized. The extraction strategy employed was a two-step approach using mixed solvents of different polarity and acidity as follows ... 

Interactions. Antacids inhibit iron absorption. Combination with ascorbic acid (Vitamin C), for protecting Fe + from oxidation to Fe +, is theoretically sound, but practically is not needed. 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

One problem with glucose monitors is that species such as ascorbic acid (vitamin C), uric acid, and acetaminophen (Tylenol) found in blood can be oxidized at the same potential required to oxidize the mediator in Reaction 17-13. To correct for this interference, the test strip in Figure 17-10 has a second indicator electrode coated with mediator, but not with glucose oxidase. Interfering species that are reduced at electrode 1 are also reduced at electrode 2. The current due to glucose is the current at electrode 1 minus the current at electrode 2 (both measured with respect to the reference electrode). Now you see why the test strip has two working electrodes. 

Ascorbic acid (vitamin C), the antiscurvy vitamin, is used by the body to form and maintain intercellular and skeletal material such as the collagen of fibrous tissue and the matrix of bone, dentin, and cartilage. Milk and milk products are not considered a significant source of this vitamin and should not be relied upon as such. Freshly drawn cow s milk contains about 2 mg ascorbic acid per 100 g milk, but as vitamin C is heat labile and easily destroyed by oxidation, the vitamin C content of pasteurized milk is reduced to about 0.94 mg/100 g. 

One of the best examples of this type of process is seen in the copper(u)-catalysed oxidation of ascorbic acid (vitamin C). Co-ordination of ascorbic acid (in the enediolate form) to copper(n) is a very favourable process and results in the formation of complex 9.2 containing a five-membered chelate ring (Fig. 9-2). 

The 5,6-O-isopropylidene acetal (152) of L-ascorbic acid has been prepared,340 and von Schuching and Frye341 prepared the corresponding cyclohexylidene acetal. These compounds were found to be more resistant than L-ascorbic acid toward oxidation, and the parent acid can be readily regenerated by acid hydrolysis. The derivative was used in the synthesis of 14C-labeled vitamin C. The C-2 and C-3 enols of L-ascorbic acid or its acetone derivative (152) can be readily methylated with diazomethane, yielding the corresponding dimethoxy analogues. 

P. A. Garcia, R. Velasco, and F. Barba, Role of trace metal ions. Kinetic and mechanism of the copper(II)-catalyzed oxidation of ascorbic acid (vitamin C) from protected derivatives of D-glucitol, Synth. Commun., 21 (1991) 1153-1161. 

A second solution is to use Harald Laban s AB C+ Pyro (available as Photographers Formulary Rollo Pyro). Adherents to Rollo Pyro claim that because of the use of ascorbic acid (vitamin C) as a preservative and a large quantity of pyro, it does not oxidize like traditional pyro formulas and can safely be used with JOBO rotary processing. Compared to other pyro/ cat developers, Rollo Pyro creates a light image stain. 

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-... 

L-Sorbose plays an important part as an intermediate in the synthesis of ascorbic acid (vitamin C). Reichstein and Griissner100 achieved this synthesis by the oxidation of 2,3 4,6-diisopropylidene-L-sorbose (XLVI) to the corresponding keto-L-gulonic acid derivative (XLVII), which gave 2-keto-L-gulonic acid (XLVIII) on hydrolysis. Isomerization produced ascorbic acid identical with the natural vitamin. 

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... 

Ascorbic acid is a vitamin in primates. In most other animals, it can be synthesized by a branch of the glucoronic acid pathway (Chapter 18). It is apparently not changed into any coenzyme in the human being and participates as a vitamin in a reducing capacity in several biochemical reactions. These include the post-translational hydroxylation of proline in collagen biosynthesis (Chapter 8) and in tyrosine metabolism (Chapter 20). Ascorbic acid is oxidized to dehydroascorbic acid, a diketo derivative of ascorbate. Scurvy is a deficiency disease caused by a shortage of dietary ascorbic acid. In children, this results in defective bone formation in adults, extensive bleeding occurs in a number of locations. Scurvy is to be suspected if serum ascorbic acid levels fall below 1 jug/mL. 

Other redox reagents include dinucleotides such as FAD (flavine adenine dinucleotide), lipoic acid, which we will meet when we discuss the chemistry of thiamine, and ascorbic acid (vitamin C)> which you met in Chapter 49. Ascorbic acid can form a stable enolate anion that can transfer a hydride ion to a suitable oxidant. 

Ascorbic acid is oxidized in the presence of air under neutral and alkaline conditions. At acid pH (for example, in citms juice), the vitamin is more stable. Because oxygen is required for the breakdown, removal of oxygen should have a stabilizing effect. For the production of fmit drinks, the water should be deaerated to minimize vitamin C loss. The type of container may also affect the extent... 

Studies on the antioxidant properties of anthocyanins on human low-density lipoprotein (LDL) and lecithin liposome systems in vitro showed that the inhibition of oxidation increased dose-dependently with antioxidant concentration. The oxidation was catalyzed by copper in the LDL system and the effects of the anthocyanins were explained by several antioxidant mechanisms including hydrogen donation, metal chelation and protein binding [33]. Anthocyanins also prevented the oxidation of ascorbic acid (vitamin C), through chelate formation with the metal ions, and finally by the formation of an ascorbic (copigment)-metal-anthocyanin complex [49]. 

---