The Underrecognized Role of the Hydroperoxyl (HOO???) Radical in Chain Propagation of Lipids and its Implication in Antioxidant Activity

The hydroperoxyl, or perhydroxyl, radical (HOO•) is a minor albeit important player in the propagation and termination of lipid peroxidation in nonaqueous systems. The HOO• radical is mainly formed by an intramolecular 1,4-H-atom transfer from activated alkylperoxyl radicals (ROO•) deriving from amines, alcohols, substituted alkenes and pro-aromatic compounds, the most investigated being the natural essential oil component γ-terpinene. Thanks to its double-face oxidizing and reducing properties, HOO• radicals can provide an inhibition of the peroxidation by increasing the decay of ROO• radicals, or by interacting with antioxidants (AH). The reduction of the radical of the antioxidant by HOO• (A•+HOO• ➔ AH+O2) regenerates the active AH species and prolongs the inhibition period. In the case of quinones and nitroxides, H-atom donation from the HOO• radicals causes the formation of reduced species able to trap radicals by a catalytic mechanism. The intervention of HOO• radicals helps explaining some unusual effects observed during inhibited peroxidation. In this chapter, we review the main aspects of the HOO• radical chemistry related to the inhibition of lipid peroxidation and we propose possible future developments and applications.