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unpaired electrons at the ir antibonding level (31g). Thus, the
reaction of oxygen with molecules of singlet multiplicity such
as unsaturated fatty acids is spin forbidden. Singlet oxygen
(1Ag and 1lg) may add directly to unsaturated bonds to
produce allylic hydroperoxides; however, this reaction does
not initiate the abstraction of allylic hydrogens (6, 7). Furthermore,
there is insufficient singlet oxygen available under
normal, physiological conditions to initiate lipid peroxidation.
Significant amounts of the superoxide anion (O -) do
form in vivo (0.1-1.0 AM), but superoxide also is incapable of
abstracting bisallylic hydrogens from unsaturated bonds.
Under acidic conditions, superoxide may initiate oxidative
modification by forming perhydroxyl radicals (HOO ) or by
reacting with transition metals to form reactive hydroxyl
radicals (HO') (8, 9). Nevertheless, low trace metal concentrations,
the high availability of ligands that form tight
coordination complexes with metals, and the abundant antioxidant
capacity of plasma suggest that metal-catalyzed
autoxidation and reactive oxygen species play little, if any,
role in mediating lipid oxidation in vivo (10-12).