Synthetic Gallic Acid Derivatives as Models for a Comprehensive Study of Antioxidant Activity

The synthesis and antioxidant efficiencies of amphiphilic gallic acid derivatives are reported. To specify the impact of chemical structure on the antioxidant efficiency, several structural modifications of gallic acid were performed. The following structural features were chosen: i)introduction of hydrophobic or hydrophilic residues on the gallic acid and the type of their linkage, ii)the hydrophilic and/or lipophilic character of the whole molecule. The physico-chemical studies of the different series prepared revealed that the antioxidant efficiency of this polyphenol depends clearly on the nature of the linkage with both hydrophilic and hydrophobic parts. A push-pull effect is always necessary, and ester or amide bonds seem well adapted to increase the antioxidant efficiency. Second, under the oxidation conditions applied, it was observed that the hydrophilic and/ or lipophilic character affects drastically the antioxidant activity of gallic acid derivatives. The results obtained are in accordance with the polar paradox, hydrophobic derivatives inhibit oxidation in an aqueous phase, whereas hydrophilic products are not efficient.