Theoretical and Kinetic Tools for Selecting Effective Antioxidants: Application to the Protection of Omega-3 Oils with Natural and Synthetic Phenols.

Radical-scavenging antioxidants play crucial roles in the protection of unsaturated oils against autoxidation and, especially, edible oils rich in omega-3 because of their high sensitivity to oxygen. Two complementary tools are employed to select, among a large set of natural and synthetic phenols, the most promising antioxidants. On the one hand, density functional theory (DFT) calculations provide bond dissociation enthalpies (BDEs) of 70 natural (i.e., tocopherols, hydroxybenzoic and cinnamic acids, flavonoids, stilbenes, lignans, and coumarins) and synthetic (i.e., 2,6-di-tert-butyl-4-methylphenol (BHT), 3-tert-butyl-4-hydroxyanisol (BHA), and tert-butylhydroquinone (TBHQ)) phenols. These BDEs are discussed on the basis of structure-activity relationships with regard to their potential antioxidant activities. On the other hand, the kinetic rate constants and number of hydrogen atoms released per phenol molecule are measured by monitoring the reaction of phenols with 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical. The comparison of the results obtained with these two complementary methods allows highlighting the most promising antioxidants. Finally, the antioxidant effectiveness of the best candidates is assessed by following the absorption of oxygen by methyl esters of linseed oil containing 0.5 mmol L(-1) of antioxidant and warmed at 90 °C under oxygen atmosphere. Under these conditions, some natural phenols namely epigallocatechin gallate, myricetin, rosmarinic and carnosic acids were found to be more effective antioxidants than α-tocopherol.

Myricetin, rosmarinic and carnosic acids as superior natural antioxidant alternatives to α-tocopherol for the preservation of omega-3 oils.

22 natural polyphenols are compared to 7 synthetic antioxidants including BHT, BHA, TBHQ and PG with regard to their ability to protect omega-3 oils from autoxidation. The antioxidant efficiency of phenols is assessed using the DPPH test and the measurement of oxygen consumption during the autoxidation of oils rich in omega-3 fatty acids. Also, the bond dissociation enthalpies (BDE) of the Ar-OH bonds are calculated and excellent correlations between thermodynamic, kinetic and oxidation data are obtained. It is shown that kinetic rates of hydrogen transfer, number of radicals scavenged per antioxidant molecule, BDE and formation of antioxidant dimers from the primary radicals play an important role regarding the antioxidant activity of phenols. Based on this, it is finally shown that myricetin, rosmarinic and carnosic acids are more efficient than α-tocopherol and synthetic antioxidants for the preservation of omega-3 oils.

Boosting effect of ortho-propenyl substituent on the antioxidant activity of natural phenols.

Seven new antioxidants derived from natural or synthetic phenols have been designed as alternatives to BHT and BHA antioxidants. Influence of various substituents at the ortho, meta and para positions of the aromatic core of phenols on the bond dissociation enthalpy of the ArO-H bond was evaluated using a DFT method B3LYP/6-311++G(2d,2p)//B3LYP/6-311G(d,p). This prediction highlighted the ortho-propenyl group as the best substituent to decrease the bond dissociation enthalpy (BDE) value. The rate constants of hydrogen transfer from these phenols to DPPH radical in a non-polar and non-protic solvent have been measured and were found to be in agreement with the BDE calculations. For o-propenyl derivatives from 2-tert-butyl-4-methylphenol, BHA, creosol, isoeugenol and di-o-propenyl p-cresol, fewer radicals were trapped by a single phenol molecule, i.e. a lower stoichiometric number. Reaction mechanisms involving the evolution of the primary phenoxyl radical ArO are proposed to rationalise these effects.