One-electron Oxidation and Reduction Reactions of Vitamin C Derivatives: 6-bromo- and 6-chloro-6-deoxy-ascorbic Acid

Abstract

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 (λ_max = 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.