Metal-chelate-catalyzed autoxidation

Scheme 3. Proposed mechanism for metal-chelate-catalyzed autoxidation of ascorbic acid.

Kate Laws for Metal-Ion- and Metal-Chelate-Catalyzed Autoxidation of Ascorbic Acid... 

Since transition metal salts catalyze the autoxidation of organic compounds, the various deactivators are used to decrease catalyzed hydroperoxide decomposition to free radicals. Different chelate-forming compounds are used as such deactivators [6]. For example, for... 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

Transition-metal-ion-free solutions of ascorbate autoxidize (i.e., react with 02) only slowly (e.g., carefully demetalized with a chelating resin such as Chelex 100 1.25 x 10-4mol dm"3 solutions lose only 0.05% ascorbate/15min (Buettner 1988 for a review giving valuable information how to deal with ascorbate solutions, see Buettner and Jurkiewicz 1995). The stability of ascorbate solutions is dramatically reduced in the presence of EDTA which apparently catalyzes the degradation by chelating adventitious iron ions (Buettner and Jurkiewicz 1995). 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

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