Hydroperoxyl radical (HOO•) as a reducing agent: unexpected synergy with antioxidants. A review

This review highlights the progress made in recent years in harnessing the peculiar chemistry of the hydroperoxyl, or perhydroxyl, radical (HOO^•) during lipid peroxidation, particularly with regard to its interaction with antioxidants. The HOO^• radical, the protonated form of superoxide, plays an important role in the propagation and termination of lipid peroxidation in nonaqueous systems. However, differently from alkylperoxyl (ROO^•) radicals that have only oxidizing ability, HOO^• has a two-faced oxidizing and reducing activity. The HOO^• radical can reduce the radical of the antioxidant (phenols and aromatic amines) by H-atom transfer (A^• + HOO^• ⟶ AH + O₂) thus increasing the length of the inhibition period and the effectiveness of the antioxidant. The simultaneous presence of HOO^• and ROO^• radicals triggers the catalytic antioxidant activity of quinones and nitroxides and explains the antioxidant activity of melanin-like polymers. The HOO^• radical can be formed by fragmentation of ROO^• radicals deriving from amines, alcohols, substituted alkenes and may be present at low concentrations in many oxidizing systems. Pro-aromatic compounds, like the natural essential oil component γ-terpinene, are the most effective sources of HOO^• and behave as co-antioxidants in the presence of nitroxides or quinones. The future developments and applications of HOO^• chemistry in the context of the inhibition of autoxidation are also discussed.