Peroxyl

Peroxyl is a term used for peroxyl radicals, especially the chemical species ROO•, in which a peroxide linkage carries an unpaired electron on one terminal oxygen. Peroxyl radicals are reactive intermediates in many oxidation processes, including organic autoxidation, lipid peroxidation in biological membranes, and atmospheric chemistry. The hydroperoxyl radical HO2• is related but distinct and is sometimes discussed separately from ROO•.

Structure and stability: A peroxyl radical has the general formula R–O–O•. Its stability is influenced by the substituent R; benzylic, allylic, tertiary, or conjugated R groups can stabilize ROO• by resonance or hyperconjugation. In solution and in membranes, ROO• species are generally short-lived, with lifetimes that depend on the surrounding medium, temperature, and presence of radical traps or antioxidants.

Formation: In organic chemistry, ROO• forms by reaction of an alkyl radical with molecular oxygen (R• + O2 → ROO•). In biological systems, lipid hydroperoxides formed by lipoxygenases or other oxidative pathways can participate in reactions that generate ROO• during radical chain processes. Autoxidation of lipids in membranes is another common source of peroxyl radicals, supporting propagation of oxidative chain reactions.

Reactivity: ROO• are strong hydrogen-atom abstractors and can propagate lipid peroxidation by abstracting a hydrogen from neighboring lipids to form ROOH and lipid radicals. They can react with double bonds, form additional hydroperoxides, or terminate chain reactions by combining with another radical or via antioxidant-mediated reactions. Antioxidants such as tocopherols (vitamin E) and ascorbate can quench ROO• by donating electrons or hydrogen atoms, converting them to non-radical products.

Significance and detection: Peroxyl radicals are central to oxidative stress, aging, and inflammatory processes due to their role in damaging lipids, proteins, and DNA. In chemistry, they influence polymer degradation and atmospheric oxidation. Methods to study ROO• include electron paramagnetic resonance (EPR) with spin traps, spectrophotometric assays for lipid hydroperoxides, and kinetic analyses of autoxidation reactions.