Extraordinary kinetic behavior of the α-tocopheroxyl (vitamin E) radical

  1. Get@NRC: Extraordinary kinetic behavior of the α-tocopheroxyl (vitamin E) radical (Opens in a new window)
DOIResolve DOI: http://doi.org/10.1021/jo00122a026
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Journal titleThe Journal of Organic Chemistry
Pages54565467; # of pages: 12
Subjectalpha tocopherol; radical; article; autooxidation; lipid peroxidation; reaction analysis
AbstractRate constants which have been reported for the bimolecular self-reaction of α-tocopheroxyl radicals vary by about 5 orders of magnitude. We have found that the observed bimolecular rate constant can vary by about a factor of 7 during a single, but typical experiment, e.g., in chlorobenzene at 37 °C from ca. 7 x 103 M-1 s-1 initially to ca. 1 x 103 M-1 s-1 finally. The overall reaction involves a disproportionation with the transfer of a hydrogen atom from the 5-methyl group of one radical to the phenoxyl oxygen atom of the other radical forming α-tocopherol and an o-quinone methide. In the slow regime (which corresponds to the true reaction of two α-tocopheroxyl radicals) this disproportionation has a deuterium kinetic isotope effect of 3.7. The bizarre kinetic behavior observed with α-tocopheroxyl radicals has been traced to a very minor impurity which will be present in any normal sample of α-tocopherol. The impurity in question is a bisphenol in which two α-tocopherol moieties have become linked through their 5-methyl carbon atoms. This bisphenol is a "natural" impurity in α-tocopherol since it will be formed upon exposure of α-tocopherol to air. The coupling of two o-quinone methide molecules yields a spiro-dimer which is then reduced to the bisphenol, probably by unoxidized α-tocopherol. © 1995 American Chemical Society.
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AffiliationNational Research Council Canada (NRC-CNRC); NRC Steacie Institute for Molecular Sciences (SIMS-ISSM)
Peer reviewedYes
NPARC number21276588
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Record identifierf3fd8f98-d046-410e-b73a-fdbb456970d9
Record created2015-10-13
Record modified2016-05-09
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