Antioxidant properties of flaxseed lignans using in vitro model systems

The major objectives of this study were to investigate the antioxidant properties of flaxseed lignans secoisolariciresinol (SECO 2) and secoisolariciresinol diglycoside (SDG 1) and their major oxidative compounds using 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH 47) in an in vitro model of lipid peroxidation. This investigation was facilitated by the structural elucidation of the major oxidative compounds and the ability of flaxseed lignans to delay the onset of oxidation in two model systems. This study showed that SECO 2 oxidation occurs at the aromatic (4-OH) and aliphatic (9-OH) hydroxyl groups. Conversely for SDG 1, only compounds derived from the oxidation of aromatic hydroxyl groups were obtained because the 9-OH position is glucosylated. SECO 2 oxidation with AAPH 47 showed that the intermediate 2a is most likely involved in the generation of early-forming (48 and 52) and 2c for the formation of late-forming (49, 50 and 51) oxidation compounds. Compound 48 is formed from dimerization of 2a that is converted to 52 and then to 51. Compound 50 was formed by the addition of a carbon-centre free radical of AAPH (AP radical) to 2c. Compounds 50 and 51 trap carbon-centered AP radicals supporting SECO 2 as a chain-breaking antioxidant and AAPH 47 as a proper model for study of SECO 2 oxidation in vitro. SDG 1 oxidation with AAPH 47 indicated that intermediates 1b and 1c are most likely involved for the formation of early forming compounds (55 and 58) and 1a leads to the late forming compounds (56 and 57). Compound 55 is a result of dimerization. Compound 56 may be directly formed via intermediate radical 1a by adding AP free radicals. Compound 56 was a stable non-radical compound that could trap AP free radicals, thereby supporting SDG 1 as a chain-breaking antioxidant. Hydrogen abstraction from 4-hydroxyl yielded the radical 1a and hydroxyl radical addition to 1a yielded 57. Compound 58 formed from the addition of •OH or H2O to 1c. This study demonstrated that AAPH 47 produces carbon-centred AP radicals upon thermal decomposition and mimics the formation of lipid peroxyl radicals. Interaction of carbon-centred AP radicals with SECO 2 and SDG 1 provides a good model to study the antioxidant reactions of SECO 2 in vitro. p*The relative antioxidant capacity of the flaxseed lignans versus BHT 17, in two model systems, was determined. The stoichiometric ratio for SECO 2 and SDG 1 were 1.5 and 1.1-1.2, respectively, compared to BHT 17 (2.0). The induction time by Rancimat analyzer measured inhibition of autoxidation mediated by flaxseed lignans SECO, SDG and SDG polymer in comparison with BHT 17. The induction time data demonstrated that SECO 2 protected canola oil better than either SDG 1 or SDG polymer 3. These results are important for better understanding about the chemistry behind flaxseed lignan antioxidant activities. This study provided useful evidence that flaxseed lignans can be used as natural antioxidants