Autoxidation of L-Ascorbic Acid

Autoxidation reactions of L-ascorbic acid (H2A) have been the subject of intensive studies for decades. It was shown that some of the most 

The oxidation of dehydroascorbic acid to a higher oxidation product is relatively slow and could be neglected for all practical purposes in kinetic and stoichiometric studies. 

Earlier results regarding the kinetics and mechanism of the copper(II) catalysis are controversial. Reaction orders for [02], [Cu11], [H2A] and [H+] were reported in the following respective ranges 0.5 to 1, 0.5 to 1,0 to 1, and —2 to +1 (8). It is also disputed whether the redox cycling of the catalyst includes oxidation states +1 and +2 or +2 and +3. The discrepancies are too marked to be explained only by the differences in the experimental conditions applied. 

The first thorough study on the Cu(II) and Fe(III) catalyzed autoxida-tion of ascorbic acid was reported by Taqui Khan and Martell (6). These authors found evidence for a slow, overall second-order reaction between the HA- form of ascorbic acid and dioxygen in the absence of added catalyst. The corresponding rate constant was reported to be 0.57 M-1s-1 (aqueous solution, 25 °C, /x = 0.1 M KNO3). Later studies also supported the existence of such a reaction path (9), which was negligible under catalytic conditions. 

In the presence of Cu(II) or Fe(III), a two-term rate law was confirmed in which the two terms correspond to the reaction paths via the mono-and diprotonated forms of ascorbic acid. The reaction was found to be first-order with respect to [H2A]tot, [02] and [Mn+] with both catalysts  

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Kurata, T. Miyake N. Suzuki, E. Otsuka, Y. Autoxidation of L-ascorbic acid, and its significance in food processing. In Chemical Markers for the Quality of Processed and Stored Foods, Lee, T.C. Kim, H.J. (Eds.) American Chemical Society Washington, DC, 1996a pp. 137—145. 

MyakeN., Kim M., KurataT. Formation mechanism ofmonodehydro-L-ascorbic acid and superoxide anion in the autoxidation of L-ascorbic acid. Biosdence Biotechnology, and Agrochemistry, 61 1693-1695 (1997). 

The kinetics of autoxidation of L-ascorbic acid catalysed by Cu+ ions have been determined. Important features of the autoxidation, including the formation of hydrogen peroxide as a stable intermediate and of a ternary complex containing oxygen, copper(i), and ascorbate, were discussed. Reagents that complexed Cu+ ions were shown to inhibit the autoxidation. Cu+ apparently remains formally univalent throughout the entire reaction cycle and acts as an electron carrier between two substrate molecules. Other workers have shown that the metal-ion-catalysed oxidation of L-ascorbic acid at alkaline pH values is inhibited by superoxide dismutase. The kinetics and mechanism of the oxida-... 

Autoxidation of L-Ascorbic Acid and Its Significance in Food Processing... 

These amino reductones described above are usually very unstable reaction intermediate compounds and, therefore, isolation and elucidation of their precise chemical structures by ordinary experimental techniques are rather difficult. However, owing to the recent remarkable progress in computational chemistry, various types of molecular orbital methods are now applicable to obtain needed information about their precise structures and chemical reactivities. For instance, the optimized structure of L-ascqrbic acid, an important acid-reductone in food and biological systems, was obtained by both semi-empirical and ab initio molecular orbital methods (Abe et aL, 1987, 1992). Semi-empirical molecular orbital calculations were also used to elucidate the autoxidation mechanism of L-ascorbic acid (Kurata et aL, 1996a,b). 

Kurata, T. Miyake, N. Otsuka, Y. Formation of L-threonolactone and oxalic acid in the autoxidation reaction of L-ascorbic acid - Possible involvement of singlet oxygen. Biosci. Biotech. Biochem. 1996b, 60, 1212-1214. 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Figure 5. Variation of rate constants for the autoxidation of ascorbic acid as a function of concentration of Fe(III) chelates at 25°C and —log [H ] of 2.45 DTPA = diethylenetriaminepentaacetic acid CDTA = trsins l,2-diaminocyclohexanetetraacetic acid other terms as in caption of...

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