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 O2, for example b-carotene and all carotenoids in general; (iii) co-oxidation processes characterized by fast cross-termination reactions, for example g-terpinene; and (iv) catalytic quenching of superoxide (O2-) with a superoxide dismutase-like mechanism, for example di-alkyl nitroxides and FeCl3. 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.