title: Reconciling disparate models of the involvement of vinyl radicals in cobalamin-mediated dechlorination reactions.
abstract: Inner-sphere (nonradical) and outer-sphere (radical-based) mechanisms have been suggested for cobalamin-mediated dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). In this study, the role of free vinyl radicals was investigated using deuterated radical traps: d(8)-isopropanol and d(8)-tetrahydrofuran. For both substrates, addition of trap resulted in production of deuterated dechlorination products, and higher concentrations of trap resulted in increased amounts of deuterated products. However, only a finite proportion of the products were trappable: 8% of the PCE-derived products and 86% of the TCE-derived products result from free radicals. The data show that the reaction does not proceed solely by either an inner-sphere or an outer-sphere mechanism and led to the hypothesis that caged radical intermediates were involved in the mechanism. The untrappable fraction of products are hypothesized to result from in-cage reactions. This hypothesis was investigated using d(5)-glycerol as a radical trap and viscogen. Although increased viscosity resulted in decreased formation of free-radical-derived products, consistent with the cage hypothesis, these data were inconclusive. The role of d(8)-isopropanol in enhancing the production of radicals in this system via an acetone ketyl radical chain mechanism was also investigated, and no evidence for such an effect was found.
authors: Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
authors: Sarah
authors: S
authors: Kliegman
authors: Kristopher
authors: K
authors: McNeill
mesh_terms: D005609:Free Radicals / Q000737:chemistry*; D054879:Halogenation; D007700:Kinetics; D008956:Models, Chemical; D013237:Stereoisomerism; D013750:Tetrachloroethylene / Q000737:chemistry; D014241:Trichloroethylene / Q000737:chemistry; D014753:Vinyl Compounds / Q000737:chemistry*; D014783:Viscosity; D014805:Vitamin B 12 / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D014753:Vinyl Compounds; D014241:Trichloroethylene; D014805:Vitamin B 12; D013750:Tetrachloroethylene