ONE-ELECTRON OXIDATION AND REDUCTION REACTIONS 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)-O-., 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 (lambda(max)=360 nm; epsilon(360) = 330 m(2) 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 x 10(9) and 2.2 x 10(7) dm(3) mol(-1) s(-1) have been measured for the reactions of bromo-derivative with e, and (CH,),COH 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 x 10(7) dm(3) mol(-1) s(-1)). Results are discussed in terms of biologial relevance of the investigated compounds regarding their ability to act as efficient antioxidants and bioreductive antitumour agents simultaneously.