Non‐phenolic radical‐trapping antioxidants

Objectives The aim of this review article is to introduce the reader to the mechanisms, rates and thermodynamic aspects of the processes involving the most biologically relevant non‐phenolic radical‐trapping antioxidants. Key findings Antioxidant defences in living organisms rely on a complex interplay between small molecules and enzymes, which cooperate in regulating the concentrations of potentially harmful oxidizing species within physiological limits. The noxious effects of an uncontrolled production of oxygen‐ and nitrogen‐centered radicals are amplified by chain reactions (autoxidations), sustained mainly by peroxyl radicals (ROO • ), that oxidize and alter essential biomolecules such as lipids, lipoproteins, proteins and nucleic acids. Summary Non‐phenolic antioxidants represent an important and abundant class of radical scavengers in living organisms. These compounds react with peroxyl radicals through various mechanisms: (i) formal H‐atom donation from weak X‐H bonds (X = O, N, S), as in the case of ascorbic acid (vitamin C), uric acid, bilirubin and thiols; (ii) addition reactions to polyunsaturated systems with formation of C‐radicals poorly reactive towards O 2 , for example β‐carotene and all carotenoids in general; (iii) co‐oxidation processes characterized by fast cross‐termination reactions, for example γ‐terpinene; and (iv) catalytic quenching of superoxide (O 2 •− ) with a superoxide dismutase‐like mechanism, for example di‐alkyl nitroxides and FeCl 3 . Kinetic data necessary to evaluate and rationalize the effects of these processes are reported. The mechanisms underlying the pro‐oxidant effects of ascorbate and other reducing agents are also discussed.