Ascorbic acid reactions

After terminating the coupling reaction with ascorbic acid, reaction mixture clarified by precipitating protein with ethanol Turbidity decreased by final addition of caffein, sodium benzoate, and Teepol micromethod After the coupling reaction with ascorbic acid is terminated, azo color is extracted and read low blank the extract can be applied directly to a thin-layer plate and be separated rapidly to determine the ratio BMC BDC Micro versions assay artificially activated enzyme... 

The reduction of adrenochrome (1) with ascorbic acid (59) was first reported in 1948,158 although the nature of the reaction products (which may be of physiological importance, cf. ref. 159) was not determined until several years later. It was shown by Heacock and Laidlaw in 1958 that reduction mixtures of this type contained at least three indolic products,147 one of which was isolated and shown to be 5,6-dihydroxy- -methylindole (28).147 The major component of aqueous adrenochrome-ascorbic acid reaction mixtures has recently been shown to be a secondary product (60) (which was isolated as its di- and tetra-acetyl derivatives) produced by the interaction of the o-dihydroxy group of 28 with the a-dicarbonyl function of dehydro-... 

The figure shows the results of a bipotentiometric titration of ascorbic acid with If. Ascorbic acid (146 mg) was dissolved in 200 mL of water in a 400-mL beaker. Two Pt electrodes were attached to the K-F outlets of the pH meter and spaced about 4 cm apart in the magnetically stirred solution. The solution was titrated with 0.04 M If (prepared by dissolving 2.4 g of K1 plus 1.2 g of I2 in 100 mL of water), and the voltage was recorded after each addition. Prior to the equivalence point, all the If is reduced to I- by the excess ascorbic acid. Reaction B can occur, but Reaction A cannot. A voltage of about 300 mV is required to support a constant current of 10 pA. (The ascorbate dehydroascorbate couple does not react at a Pt electrode and cannot carry current.) After the equivalence point, excess If is present, so Reactions A and B both occur, and the voltage drops precipitously. 

In all of the ascorbic acid reactions substitution in the 5- and 5,6-positions does not interfere with its activity. Substitution in either the 2-or 3-position or both makes ascorbic acid no longer active as an oxygen scavenger or as an antioxidant. 

In 1963, Mali et al. ° showed that lactic acid reduced chromate at pH 4.3, and Samitz and Katz showed lactic acid reduction of chromate at pH 7 (in phosphate buffer) after incubation at 37 °C for 48 h. However, in our preliminary studies we found that in 1 M Tris HCl at pH 7.4 and 25 °C, the reaction occurs very, very slowly and is certainly negligible compared to cysteine and ascorbic acid reactions carried out under identical conditions. 

Ascorbic acid polypeptide Synonyms L-Ascorbic acid, reaction prods, with protein hydrolysates... 

Halliwell (22) suggests that the possible in vivo pro-oxidant effects of ascorbate are related to the availability of catalytic transition metal ions. The content of vitamin C in meals increases nonheme iron absorption (24). In patients with iron accumulation diseases such as hemochromatosis or thalassemia, this might lead to increased iron overload and deleterious clinical effects (22). Non-protein-bound iron, as far as it exists in the human body, can induce lipid peroxidation especially if it is present together with the pro-oxidative ascorbic acid (reactions 3 and 5). According to previous reviews (6,24), vitamin C ingestion enhances the iron absorption also in individuals with iron deficiency, but may have a rather small effect in individuals with normal iron status. 

The first term is the law observed in the absence of catalyst. The effect is dramatic, however, with associated rate constants ko and ki being 50.3 1 mol s and 3.43 x 10 1 mol s respectively, and may be ascribed to the formation of a Mo -BrOa" complex with no evidence for any change in the oxidation state of the catalyst during the course of the reaction. The kinetics and mechanism of the bromate-ascorbic acid reaction have also been reported, and the effects of phosphate on the oxidation of iodide by chlorate in the presence of catalytic concentrations of vanadium(iv) have been described, the latter systems being considered to involve vanadium(iv)-phosphate complexes. 

Brandt et al. [21] polymerized a solution of 20% acrylic acid in water with a crosslinker such as MBA or TMPTA and an initiator such as hydrogen peroxide-ascorbic acid. Reaction temperature was maintained within the range of 10 - 65°C during an exothermic polymerization and thereafter at 40°C for 3 hours. The resulting gel was chopped in the presence of dilute sodium hydroxide solution (70 mole%), allowed to stand at 40 C for 16 hours and then dried at 80 C under reduced pressure. The resulting dry absorbent polymer exhibited an absorbency for synthetic urine of 38 g/g and water soluble content of 9%. 

The kinetics and mechanism of the permanganate/ascorbic acid reaction are discussed in Z Phys. Chem. Leipzig.ISM1982)622. 

Since allylation with allylic carbonates proceeds under mild neutral conditions, neutral allylation has a wide application to alkylation of labile compounds which are sensitive to acids or bases. As a typical example, successful C-allylation of the rather sensitive molecule of ascorbic acid (225) to give 226 is possible only with allyl carbonate[l 37]. Similarly, Meldrum s acid is allylated smoothly[138]. Pd-catalyzed reaction of carbon nucleophiles with isopropyl 2-methylene-3,5-dioxahexylcarbomite (227)[I39] followed by hydrolysis is a good method for acetonylation of carbon nucleophiles. 

This experiment describes the use of FIA for determining the stoichiometry of the Fe +-o-phenanthroline complex using the method of continuous variations and the mole-ratio method. Directions are also provided for determining the stoichiometry of the oxidation of ascorbic acid by dichromate and for determining the rate constant for the reaction at different pH levels and different concentration ratios of the reactants. 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

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). 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Iron Absorption. A very important effect of ascorbic acid is the enhancement of absorption of nonheme iron from foods. Ascorbic acid also enhances the reduction of ferric iron to ferrous iron. This is important both in increasing iron absorption and in its function in many hydroxylation reactions (140,141). In addition, ascorbic acid is involved in iron metaboHsm. It serves to transfer iron to the Hver and to incorporate it into ferritin. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

It is shown that both Sb(III) and Bi(III) can speed reduction of 12-molybdophosphate (12-MPC) to the corresponding heteropoly blue (12-MPB) by ascorbic acid (AA). It is found that mixed polyoxometalates can be formed in solution which reduce considerably more rapidly than 12-MPC. Complete formation of mixed POM is observed only if significant excess of Me(III) ions is used in the reaction. POM responsible for blue color was synthesized by selective extraction. Chemical analysis of tetrabutyl-ammonium salt is in accordance with formula of (TBAl PMeflllfMo O j (Me = Sb, Bi). IR spectmm of mixed POM is identical to 12-MPC. 

The following reactions are to be expected with ascorbic acid and dehydroascorbic... 

FIGURE 6.17 Hydroxylation of proUne residnes is catalyzed by prolyl hydroxylase. The reaction requires -ketoglntarate and ascorbic acid (vitamin C). 

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