Critical Role of Deep Hydrogen Tunneling to Accelerate the Antioxidant Reaction of Ubiquinol and Vitamin E

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.