Phenolic antioxidants - radical-scavenging and chain-breaking activity: A comparative study

Fifty phenolic antioxidants (AH) (42 individual compounds and 8 binary mixtures of two antioxidants) were chosen for a comparative analysis of their radical-scavenging (H-donating) and chain-breaking (antioxidant) activity. Correlations between experimental (antiradical and antioxidant) and predictable (theoretical) activities of 15 flavonoids, 15 hydroxy cinnamic acid derivatives, 5 hydroxy chalcones, 4 dihydroxy coumarins and 3 standard antioxidants (butylated hydroxytoluene, hydroquinone, DL-alpha-tocopherol) were summarized and discussed. The following models were applied to explain the structure-activity relationships of phenolic antioxidants of natural origin: (a) model 1, a DPPH assay used for the determination of the radical-scavenging capacity (AH + DPPH center dot --> A center dot + DPPH-H); (b) model 2, chemiluminescence of a model substrate RH (cumene or diphenylmethane) used for the determination of the rate constant of a reaction with model peroxyl radicals (AH + RO2 center dot --> ROOH + A center dot); (c) model 3, lipid autoxidation used for the determination of the chain-breaking antioxidant efficiency and reactivity (AH + LO2 center dot --> LOOH + A center dot; A center dot + LH (+O-2) --> AH + LO2 center dot); and (d) model 4, theoretical methods used for predicting the activity (predictable activity). The highest lipid oxidation stability was found for antioxidants with a catecholic structure and for their binary mixtures with DL-alpha-tocopherol, as a result of synergism between them.