Antioxidant activity of olive phenols: mechanistic investigation and characterization of oxidation products by mass spectrometry

In this work, the antioxidant activity of olive phenols is first characterized by their stoichiometries ntot (number of radicals trapped per antioxidant molecule) and their rate constants for the first H-atom abstraction k1 by the stable radical DPPH. It appears that oleuropein, hydroxytyrosol and caffeic acid have the largest k1 values, whereas dihydrocaffeic acid, an intestinal metabolite of caffeic acid, is the best antioxidant in terms of ntot. For phenols with a catechol moiety ntot is higher than two, implying an antioxidant effect of their primarily formed oxidation products. A HPLC–MS analysis of the main products formed in the AAPH-induced oxidation of olive phenols reveals the presence of dimers and trimers. With hydroxytyrosol and dihydrocaffeic acid, oligomerization can take place with the addition of water molecules. The antioxidant activity of olive phenols is then evaluated by their ability to inhibit the AAPH-induced peroxidation of linoleic acid in SDS micelles. It is shown that olive phenols and quercetin act as retardants rather than chain breakers like α-tocopherol. From a detailed mechanistic investigation, it appears that the inhibition of lipid peroxidation by olive phenols can be satisfactorily interpreted by assuming that they essentially reduce the AAPH-derived initiating radicals. Overall, olive phenols prove to be efficient scavengers of hydrophilic peroxyl radicals with a long lasting antioxidant effect owing to the residual activity of some of their oxidation products.