Kinetic investigation of linoleic acid peroxidation catalyzed by the copper (II) in the presence of hydrophilic and lipophilic antioxidants

Antioxidants are compounds that can delay or inhibit lipid oxidation. Ascorbic acid and ?-tocopherol can act both as the debatable prooxidant activity and a potent antioxidants. There are very few data in literature regarding the activities of hydrophilic or lipophilic antioxidants and polar paradox.In the first part of this study, the peroxidation of linoleic acid (LA) in the presence of Cu(II) ions alone, and Cu(II)-ascorbate , Cu(II)-?-tocopherol and Cu(II)-H2O2 combinations with the potent antioxidant, ?-tocopherol (TocH) and quercetin (QR), was investigated in aerated and incubated emulsions at 37 oC and pH 7. LA peroxidation induced by copper(II) or copper (II)-different systems followed pseudo-first order kinetics with respect to hydroperoxides concentration and and secondary (aldehydes? and ketones?like) oxidation products, detected by ferric-thiocyanate and TBARS tests, respectively. The differences between the rate constants for primary and secondary product formation were dependent on ?-tocopherol concentrations. ?-Tocopherol showed both antioxidant and prooxidant effects depending on concentration and also on the simultaneous presence of Cu(II) and ascorbate. Copper(II)-ascorbate combinations generally led to distinct antioxidant behavior at low concentrations of ?-tocopherol and slight prooxidant behavior at high concentrations of ?-tocopherol, probably associated with the recycling of tocopherol by ascorbate through reaction with tocopheroxyl radical, while the scavenging effect of ?-tocopherol on lipid peroxidation was maintained as long as ascorbate was present. On the other hand, in Cu(II) solutions without ascorbate, the antioxidant behavior of tocopherol required higher concentrations of this compound because there was no ascorbate to regenerate it.In the second part of this study, the peroxidation of linoleic acid (LA) in the presence of either Cu(II) ions alone or Cu(II) ? ions with Trolox (TR), ascorbic acid (AA), hydroquinone (HQ), gallic acid (GA) as hydrophilic antioxidants, or with ?-tocopherol (TocH), ascorbyl palmitate (AP), tert- butyl hydroquinone (TBHQ), propyl gallate (PG), as their lipophilic homologues, was investigated in aerated and incubated emulsions at 37oC and pH 7. LA peroxidation induced by copper(II) system followed pseudo first order kinetics with respect to primary (hydroperoxides) and secondary (aldehydes? and ketones?like) oxidation products. The extent of copper?initiated peroxide and aldehyde production in a LA system assayed by ferric thiocyanate and thiobarbituric acid reactive substances (TBARS) methods were used to determine possible antioxidant and prooxidant activities of the added hydrophilic and lipophilic antioxidants. Except some concentrations of a-Tocopherol, the others showed antioxidant effects depending on their polarities. The results were evaluated in the light of structural requirements and polar paradox for hydrophilic and lipophilic antioxidants.In conclusion, the results from this study partly confirm the hypothesis that polar paradox experiences limitations in oil- in-water and its validity is also dependent on the concentration of the antioxidants employed. The results in this study are based on primer and secondary oxidation productsKey words: Linoleic acid peroxidation , copper?initiated prooxidant activity, hydroperoxides , hydrophilic and lipophilic antioxidant protection, polar paradox, TBARS method.