Kinetics ascorbic acid free radical

The chemistry of ascorbic acid free radicals is reviewed. Particular emphasis is placed on identification and charac-terization of ascorbate radicals by spectrophotometric and electron paramagnetic resonance techniques, the kinetics of formation and disappearance of ascorbate free radicals in enzymatic and nonenzymatic reactions, the effect of pH upon the spectral and kinetic properties of ascorbate anion radical, and chemical reactivity of ascorbate free radicals. 

During the past two decades ascorbic acid free radicals have become recognized and their kinetics studied (281-287) in the oxidation of ascorbic acid. Interactions between certain of the vitamins or ingredients... 

Other reactions of hemoglobin also permit a free radical interpretation, notably the coupled oxidation with ascorbic acid by molecular 02 which yields choleglobin, but further discussion requires a full kinetic analysis. Even though the denaturation reactions described above have not been examined kinetically it is worth emphasizing that their chief features can be explained by the formation of 02- as in the mechanism advanced for the autoxidation. The liberation of an activated O2 molecule is no longer required—02 is the active oxygen. 

With the exception of a study carried out with a partially characterized multicopper oxidase isolated from tea leaves (85), there has been very little detailed work concerned with the steady state kinetic behavior of laccases. Early work on the transient kinetics indicated, however, that (1) enzyme bound Cu + was reduced by substrate and reoxidized by O2, and (2) substrate was oxidized in one-electron steps to give an intermediate free radical in the case of the two electron donating substrates such as quinol and ascorbic acid. The evidence obtained suggested that free radicals decayed via a non-enzymatic disproportionation reaction rather than by a further reduction of the enzyme (86—88). In the case of substrates such as ferrocyanide only one electron can be donated to the enzyme from each substrate molecule. It was clear then that the enzjmie was acting to couple the one-electron oxidation of substrate to the four-electron reduction of oxygen via redox cycles involving Cu. 

Glickman, R.D., Sowell, R., Lam, K.-W., 1993. Kinetic properties of hght-dependent ascorbic acid oxidation by melanin. Free Radic. Biol. Med. 15, 453—457. 

The mechanism of the reduction of adrenochrome by ascorbic acid (80) in aqueous or methanolic solution has been investigated by spectroscopic [164] and polarographic [165] methods. E.P.R. studies indicated that free radicals were formed as intermediates. The reaction followed second-order kinetics, being first-order in adrenochrome concentration and first-order with respect to the concentration of undissociated ascorbic acid. The mechanism of the reaction was postulated by Mattok to be as shown below [164]. In methanol the ascorbic acid (80) was almost completely undissociated at the concentrations used, but in water a large amount of the ascorbate anion (81) was present. The addition of dehydroascorbic acid (82) increased the reaction rate in water, apparently by increasing the concentration of the undissociated ascorbic acid by an unknown mechanism. In methanol, on the other hand, the rate was reduced by dehydroascorbic acid probably due to a reduction in the concentration of ascorbic acid by a shift to the left in the equilibrium involving these two acids (i.e. (83) (82)- -(80). This factor is not important in the aqueous reaction as the concentration of the free radical (83) is very low in this case [164]. 

Kinetics of the oxidation of ascorbic acid (AA) by di- /-hydroxobis[(dioxalato-cobaltate(III)] ion has been obtained. Lack of acrylamide polymerization indicated that free radicals are not important in this reaction. The data have been interpreted in terms of an outer-sphere mechanism via an ion-pair intermediate. ... 

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