Antioxidants and Free Radical Scavengers of Natural Origin

Natural compounds with antioxidative properties which have been reported from plant, microbial or animal sources are reviewed. Antioxidants have been found within most natural substance classes. Flavonoids are by far the most comprehensively investigated natural products. Reactivity towards various radicals has been studied under many experimental conditions and structure-activity relationships have been established. Tannins, coumarins and, more recently, xanthones have also been investigated in some detail. Studies on alkaloids have concentrated so far mostly on benzylisoquinoline alkaloids. At the same time, particular interest has been provided by some plant species or genera normally consumed as food or spices. Curcuminoids isolated from various Curcuma have shown potent antioxidant properties. Phenolic diterpenes from Rosmarinus officina/is possess very promising activity. The exceptional stability of sesame oil from Sesamum indicum led to the isolation of sesamol, together with several bisfuranyl lignans. The search for new natural antioxidants is not confined to higher plants. A great variety of metabolites including in particular phenazine and carbazole derivatives have been isolated from Streptomycetes or fungal strains. Several active compounds, among others porphyrins, hydroquinones, chromenes and complex indole alkaloids have been isolated from tunicates, bivalves and marine algae.