Oxidation ascorbic acid-ascorbate

Oxidizing ascorbic acid requires two electrons, and reducing 13 to R also requires two electrons. Thus... 

Fewer examples are reported for organic electrode reactions some alkyl halides were catalytically reduced at electrodes coated with tetrakis-p-aminophenylporphy-rin carboxylate ions are oxidized at a triarylamine polymer and Os(bipy)3 in a Nafion film catalytically oxidizes ascorbic acid Frequently, modified electrodes fail to give catalytic currents for catalyst substrate combinations that do work in the homogeneous case even when good permeability of the film is proven... 

Molecular Characterization It has been repotted that o-qulnones oxidize ascorbic acid In homogeneous solutions (25). Surface qulnones have also been reported to exist on activated carbon surfaces (16). However, cyclic voltarammetry Is not sufficiently sensitive to allow an unambiguous Identification of the reversible wave ascribed to surface qulnones (16). Therefore, differential pulse voltammetry (DPV) and square wave voltammetry were employed. 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Hydroxypyridine and pyridin-2-one are sufficiently reactive to undergo Elbs oxidation, and in both cases the substituent directs hydroxylation mainly para (Scheme 40) (58JA3717). Quinoline may be converted into 3-hydroxyquinoline (6% yield) by Udenfriend oxidation (ascorbic acid and oxygen in the presence of iron(II)) which is believed to involve attack by OH+ rather than radicals (54JBC(208)74i). 

Riboflavin absorbs light maximally at about 450nm and in doing so can be excited to a triplet state. This excited form of riboflavin can interact with triplet 02 to form a superoxide anion OJ (or H202 at low pH). Excited riboflavin can also oxidize ascorbate, a number of amino acids and proteins and orotic acid. Riboflavin-catalysed photo-oxidation results in the production of a number of compounds, most notably methional (11.1) which is the principal compound responsible for the off-flavour in milk exposed to light. 

There is a third enzyme expressed in norepinephrine neurons that converts dopamine into more norepinephrine therefore, this enzyme is not expressed by dopamine neurons. The enzyme is stored within the synaptic vesicles and lies in wait for the entry of dopamine molecules once they have been synthesized in the cytoplasm of the neuron. In addition to this third enzyme, the vesicles contain copper and the anti-oxidant ascorbic acid, also known as Vitamin C. Copper is required for the enzyme to function appropriately. The ascorbic acid maintains the integrity of norepinephrine within the vesicle in the same way that ascorbic acid added to processed meats such as hotdogs... 

The ease of oxidation of reduced ascorbic acid is the basis for a simple method of analysis by dye titration (58j. Ascorbic acid as it occurs in citrus juice is in the reduced form. When subjected to oxidation, ascorbic acid changes to the dehydro form. Dehydroascorbic acid has nearly the same physiological activity as the reduced form and is easily converted to the latter. Further oxidation of the dehydroascorbic acid converts it to 2,3-diketo-gulonic acid. This reaction is irreversible, and the oxidized product is devoid of biological activity. These reactions are shown in Figure 2. Nearly 90 percent or more of the vitamin C found in citrus juice and citrus products is in the reduced form (Table X) (59). 

Ascorbic acid as a water-soluble vitamin (vitamin C) is an essential component in the human diet. As one of many anti-oxidants (vitamin E and 3-carotene are examples of fat-soluble anti-oxidants), ascorbic acid is required for the growth and repair of tissues in all parts of the body. It is necessary to form collagen, an important protein used to make skin, scar tissue, tendons, ligaments, and blood vessels. 

Ascorbic acid inhibits light-induced yellowing for a finite time, 1 to 2 hours when irradiated with near-uv light with an intensity of 9.2 mW/cm2 (6). This limitation has been attributed in part to photooxidation of ascorbic acid. In addition to air oxidation, ascorbic acid is oxidized by photochemically produced peroxyl radicals, superoxide radical anion and singlet oxygen (27,28). If ascorbic acid is to be an effective inhibitor of light-induced yellowing it s oxidation must be slowed. 

Disaccharides and polysaccharides are also oxidized by the same degradative oxidation at first they react rapidly then, when the point of branching is reached, they react much more slowly. When subjected to similar oxidations, ascorbic acid affords threonic acid and oxalic acid, and 2-deoxy sugars yield 2-deoxyaldonic acids and lower alditols. 

Ascorbate oxidase is a tetramer each subunit has 552 amino acids and contains 4 copper ions, the type-I blue copper center and the adjacent trinuclear center (arranged as a type-n center and a type-in dinuclear center) separated by /S-sheets (Figure 20) °. Ascorbate is oxidized to dehydroascorbate by dioxygen however, it is not bound directly to the metal center to be oxidized, but is proposed to bind near the type-I Cu site which may facihtate electron transfer to oxygen, presumably in the tri-Cu cluster site. Since humans cannot synthesize ascorbic acid, conservation of this important compound is highly regulated. For example, the oxidized ascorbate can be transported into red blood... 

Rose, R. C, Choi, J.-L., and Koch, M. J, (1988), Intestinal transport and metabolism of oxidized ascorbic acid (dehydroascorbic acid). Am. ]. Phifiloi, 254, OS24r-C828. 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

The case of the reduction of nitro compounds leads to their use as oxidizing agents. The use of nitrobenzene in the Skraup reaction is well known. o-Dinitro-benzene was reported to be a good agent for oxidizing ascorbic acid to diketone [1641. 

A survey of work since 1975 on the derivatization of ascorbic acid is reviewed from the perspective of the organic chemistry of ascorbic acid. Recent advances in the control of regioselectivity of alkylative derivatization of ascorbic acid have been made possible by the utilization of di- and trianions of ascorbic acid. Their use has allowed the facile synthesis of inorganic esters of ascorbic acid. New synthesis of acetal and ketal, side-chain oxidized, and deoxy derivatives are reviewed. The total synthesis of a new side-chain oxidized ascorbic acid derivative, 5-ketoascorbic acid, is reported. 

In basic solutions ascorbate is apparently oxidized preferentially by the electron transfer process, which goes to completion in less than 2 fts after termination of the electron pulse (see Structure I). In nitrous-oxide-saturated acid solutions (pH 3.0-4.5), A and two other species which were shown to be OH-radical adducts were observed (37), thus confirming earlier observations (18,19,23, 25). The ascorbate radical anion was identified by its doublet of triplets spectrum that maintains its line position from pH 13 to 1. One OH-radical adduct (IV) shows a doublet, the lines of which start to shift below pH 3.0 it has a pK near 2.0, a decay period of about 100 fxs, and probably does not lead to formation of A". The other OH-radical adduct (II) is formed by addition of the OH radical to the C2 position its ESR parameters are = 24.4 0.0002 G and g == 2.0031 0.0002. Time growth studies suggest that this radical adduct converts to the ascorbate anion radical (III) with r 15 fxs, and accounts for 50% of the A signal intensity 40 fxS after termination of the electron pulse. The formation of the three radicals can be summarized as shown in Scheme 1. 

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... 

In order to better understand the effect of ascorbic acid on non-enzymatic browning of citrus juices, we oxidized endogenous ascorbic acid in grapefruit with ascorbic acid oxidase. The enzyme oxidized ascorbic acid to dehydroascorbic acid and H2O without forming H2O2 (19). 

Ascorbic Acid-dependent DPNH-Oxidase Microsomes of adrenal medullas contain an enzyme which oxidizes DPNH but not TPNH, and like the similar enzyme from plant tissues (K2, N2) utilizes oxidized ascorbic acid but not dehydroascorbic acid as the acceptor. Complexed Fe + in ferricyanide or cytochrome c can also serve... 

This system cannot now be fitted into known pathways of electron transport, principally because a system that oxidizes ascorbic acid in animal tissues is unknown. The fact is clear, however, that ascorbic acid is oxidized to dehydroascorbic acid in animal tissues and that dehydro-ascorbic add can be reduced. At the very least, this system emphasizes the potential eflBciency of a form of ascorbic acid as an electron acceptor (K3). 

Further oxidation ofdehydroascorbic acid, using sodium hypoiodite in alkaline solution, yielded oxalic acid and L-threonic acid, the latter being identified by its conversion into the known substances l-dimethoxysuccinamide and tri-O-methyl-L-threonamide. This established the stereochemical relationship of natural ascorbic acid to the l-series of sugars and also confirmed that the lactone carbonyl was directly adjacent to the ene-diol grouping (Figure 3.3). 

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