COOPERATIVE ANTIOXIDANT EFFECTS OF ASCORBATE AND THIOLS WITH DI-TERT-BUTYLCATECHOL DURING INHIBITED PEROXIDATION IN SOLUTION AND IN SODIUM DODECYL-SULFATE (SDS) MICELLES

The antioxidant activities, k(inh), of catechol, I, 4-tert-butylcatech ol, 2, and 3,5-di-tert-butylcatechol (DTBC), 3, determined by the inhi bited oxygen-uptake method during peroxidation of styrene initiated by AIBN are 55.0 x 10(4), 88.4 x 10(4), and 149 x 10(4) M(-1)s(-1), resp ectively, and the stoichiometric factors (n) were 2.1-2.3. A decrease by 50-fold in k(inh) for 3 and a drop of 1.1-1.4 in n observed during inhibited peroxidation of methyl linoleate in aqueous sodium dodecyl s ulfate (SDS) initiated by di-tert-butylhyponitrile (DBHN) is attribute d to hydrogen bonding by water on the antioxidant and on the intermedi ate 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 sty rene in solution. Combinations of ascorbic acid with 3 exhibited syner gistic effects during inhibited peroxidation of methyl linoleate initi ated by DBHN in SDS micelles. A profile of the effect of concentration of ascorbate on this synergism indicates a mole of 3 is regenerated p er mole of ascorbate. The thiols, homocysteine, and polyethylene glyco l thiol (polythiol) also exhibited synergistic effects with DTBC in th is inhibition. Either ascorbate or polythiol rapidly reduces di-tert-b utyl-ortho-quinone (DTBQ), 4, to 3 in methanol or in SDS micelles, and the combination of 3 + ascorbate acted as an efficient inhibitor in t his medium. The esr studies indicate the semiquinone radical, 5, produ ced 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 observ ed and for the additional combination effect discovered when the three inhibitors: 3, ascorbate, and a thiol are used in the SDS medium. Com binations of such antioxidants are expected to be useful for inhibitio n of yellowing of pulps and paper with high lignin content, and to be significant in the in vivo reductions of ortho-quinones and semiquinon e radicals formed during oxidations of various biomolecules.