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 center dot) during lipid peroxidation, particularly with regard to its interaction with antioxidants. The HOO center dot 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 center dot) radicals that have only oxidizing ability, HOO center dot has a two-faced oxidizing and reducing activity. The HOO center dot radical can reduce the radical of the antioxidant (phenols and aromatic amines) by H-atom transfer (A(center dot) + HOO center dot -> AH + O-2) thus increasing the length of the inhibition period and the effectiveness of the antioxidant. The simultaneous presence of HOO center dot and ROO center dot radicals triggers the catalytic antioxidant activity of quinones and nitroxides and explains the antioxidant activity of melanin-like polymers. The HOO center dot radical can be formed by fragmentation of ROO center dot 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 gamma-terpinene, are the most effective sources of HOO center dot and behave as co-antioxidants in the presence of nitroxides or quinones. The future developments and applications of HOO center dot chemistry in the context of the inhibition of autoxidation are also discussed.