ANTIOXIDANT PROPERTIES OF 6-O-PALMITOIL-L-ASCORBIC ACID
Ascorbic acid (vitamin C) is a biological cofactor that plays an important role in numerous biological processes that are fundamental to cellular functions. Despite the fact that the chemistry of ascorbic acid and ion ascorbate in aqueous solutions was being studied for a long time, many reactions with their participation in vivo occur in the active sites of enzymes or at the interfaces of membranes that have a hydrophobic local environment, therefore knowledge of ascorbic acids chemistry or its derivatives in non-aqueous media are important for a comprehensive understanding of their role. Styrene is a good substrate for the testing of chain-breaking antioxidants and the determination of antioxidant reactivity. The antioxidant activity of 6-O-palmitoil-L-ascorbic acid (PAA) in the model reaction of autoxidation of styrene initiated by azobis(isobutyronitrile) (AIBN) in chlorobenzene was studied. This reaction initiated by the thermal decomposition of AIBN at 37 °C under air atmosphere was followed by monitoring the oxygen consumption with a recording gas-absorption apparatus, which uses differential pressure transducer as a detector. The rate of initiation was measured in a preliminary set of experiments from the length of the induction period, using alpha-tocopherol as a reference antioxidant. The effects of pyridine and trichloroacetic acid additives were also studied and discussed. The rate constant of PAA with peroxyl radicals at 37 °C is 1.4 x 105 L/(M s). However, the stoichiometric inhibition coefficient is much lower than in polar solvents. The rate constant of the reaction with peroxyl radicals in chlorobenzene is higher than in acetonitrile; however, the inhibition efficiency decreases with increasing concentration, which is apparently due to a reaction of the oxyl-radical of PAA in the chain-propagation.