title: Critical role of deep hydrogen tunneling to accelerate the antioxidant reaction of ubiquinol and vitamin E.
abstract: In biomembranes a variety of antioxidants work to suppress oxidative damage. Vitamin E and ubiquinol are among the most important lipid-soluble antioxidants, which trap lipid peroxyl radicals directly or work cooperatively in the regeneration of vitamin E radicals by ubiquinol. Here, we investigate the latter regeneration reaction by using variational transition-state theory with multidimensional tunneling corrections. The result shows that the system forms a compact H-bonded complex by significantly rearranging the donor and acceptor moieties, which leads to a rather narrow potential barrier for H transfer and a very large tunneling effect with a transmission coefficient >4000. In accord with experiment, the Arrhenius activation energy is found to be very small (∼1 kcal/mol), which is interpreted here in terms of mean tunneling energy through the barrier. Regarding the electronic structure, we demonstrate that the present reaction proceeds via a proton-coupled electron transfer (PCET) mechanism and suggest that the PCET character also contributes to the large tunneling effect by sharpening the potential barrier. Finally, a systematic comparison is made among relevant reactions and it is indicated that the antioxidant defense of biomembranes may benefit rather significantly from quantum tunneling to enhance the reaction efficiency.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University , Kyoto 606-8502, Japan.
authors.forename: Taichi
authors.initials: T
authors.lastname: Inagaki
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yamamoto
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000737:chemistry; D014810:Vitamin E / Q000737:chemistry*
chemical_list: D000975:Antioxidants; D014451:Ubiquinone; D014810:Vitamin E; D006859:Hydrogen; C003741:ubiquinol