Article

Abstract

The antioxidant activities, kinh, of catechol, 1, 4-tert-butylcatechol, 2, and 3,5-di-tert-butylcatechol (DTBC), 3, determined by the inhibited oxygen-uptake method during peroxidation of styrene initiated by AIBN are 55.0 x 104, 88.4 x 104, and 149 x 104M-1s-1, respectively, and the stoichiometric factors (n) were 2.1-2.3. A decrease by 50-fold in kinhfor 3 and a drop of 1.1-1.4 in n observed during inhibited peroxidation of methyl linoleate in aqueous sodium dodecyl sulfate (SDS) initiated by di-tert-butylhyponitrile (DBHN) is attributed to hydrogen bonding by water on the antioxidant and on the intermediate di-tert-butylsemiquinone radical, 5, formed in the inhibition step. Combinations of ascorbyl palmitate with 3 exhibited cooperative (not synergistic) antioxidant effects during inhibited peroxidation of styrene in solution. Combinations of ascorbic acid with 3 exhibited synergistic effects during inhibited peroxidation of methyl linoleate initiated by DBHN in SDS micelles. A profile of the effect of concentration of ascorbate on this synergism indicates a mole of 3 is regenerated per mole of ascorbate. The thiols, homocysteine, and polyethylene glycol thiol (polythiol) also exhibited synergistic effects with DTBC in this inhibition. Either ascorbate or polythiol rapidly reduces di-tert-butyl-ortho-quinone (DTBQ), 4, to 3 in methanol or in SDS micelles, and the combination of 3 + ascorbate acted as an efficient inhibitor in this medium. The esr studies indicate the semiquinone radical, 5, produced photochemically from 3 or spontaneously from 3 + 4 in solution, to be very persistent at room temperature. A pathway, mediated by 5, is proposed to account for the cooperative and synergistic effects observed and for the additional combination effect discovered when the three inhibitors: 3, ascorbate, and a thiol are used in the SDS medium. Combinations of such antioxidants are expected to be useful for inhibition of yellowing of pulps and paper with high lignin content, and to be significant in the in vivo reductions of ortho-quinones and semiquinone radicals formed during oxidations of various biomolecules.Key words: catechols, ascorbate, thiols, radicals, antioxidants.