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One-electron oxidation and reduction of vitamin C derivatives : 6-bromo-and 6-chloro-6-deoxy- ascorbic acid

6-Bromo- and 6-chloro-6-deoxy derivatives of ascorbate anion are able to transfer an electron to the oxidizing radicals ·OH, Br· −2 and RS· with the same rate constants as the ascorbate anion itself. The resulting radicals also show the same kinetic stabilities and optical absorption spectra as the well-characterized ascorbate radical anion (λmax = 360 nm ; ε360 = 330 m2 mol−1). This proves that there is no influence of the structural changes in the side chain on the antioxidant capacity of the ascorbate moiety. In contrast, measured reduction of the 6- halo-6-deoxy derivatives occurs significantly faster (up to one order of magnitude) than the reduction of unsubstituted ascorbate. For example, absolute rate constants of 4·6 × 109 and 2·2 × 107 dm3 mol−1 s−1 have been measured for the reactions of bromo- derivative with e−aq and (CH3)2ĊOH respectively. These radical-induced reductions proceed via dissociative electron capture and, under cleavage of the C-halogen bond, yield C-6 carbon-centered radicals. In the presence of oxygen the corresponding peroxyl radical is readily formed. This radical is again able to oxidize the ascorbate moiety (rate constant 2 × 107 dm3 mol−1 s−1). Results are discussed in terms of biological relevance of the investigated compounds regarding their ability to act as efficient antioxidants and bioreductive antitumour agents simultaneously.

vitamin C derivatives ; one electron oxidation/reduction ; antioxidant ; bioreductive antitumor agents

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