Studies on bio-antioxidants--micellar effects on the reduction of nitroxides by vitamin C

The kinetics of reduction of nitroxides including 4-hydroxy-TEMPO, 4-methoxy-TEMPO and 4-hexanoyloxy-TEMPO, which are of different lipophilicities, by vitamin C in cationic, non-ionic and anionic micelles, i.e. CTAB, Triton X-100 and SDS, respectively, have been studied by FSR spectroscopy by a stopped-flow technique. A mechanism for the reaction conducted in micelles is proposed and the rate constants for the elementary reactions are evaluated. It is found that the rates of single electron transfer reactions involving the nitroxides are dependent on the nature of the micelle and the lipophilicity of the nitroxide. The rates are increased in CTAB, decreased in SDS, whereas unaffected in Triton X-100. And the greater the lipophilicity of the nitroxide, the more pronounced the rate variation. As high as a 3600-fold increase in the rate was observed for 4-hexanoyloxy-TEMPO in CTAB over that in SDS. The micellar effects are rationalized on the basis of analysis of parameters and line shape of the ESR spectra for the nitroxides in the micelles.