ANTIOXIDANT ACTIVITIES OF DIHYDROLIPOIC ACID AND ITS STRUCTURAL HOMOLOGS

The relationships between structure and antioxidant activity of dihydr olipoic acid (DHLA) were studied using homologues of DHLA: bisonor-DHL A (a derivative which lacks two carbons in the hydrophobic tail), tetr anor-DHLA (which lacks four carbons) and a methyl ester derivative. It was observed that: i) DHLA homologues with shorter hydrocarbon tails (i.e., bisnor- and tetranor-DHLA) had greater ability to quench supero xide radicals (O2-); ii) no differences among homologues with differen t chain lengths were found for peroxyl radical (ROO.) scavenging in aq ueous solution, and iii) DHLA was the best membrane antioxidant in ter ms of ROO. scavening and lipid peroxidation inhibition. Differences am ong the DHLA homologues in their antioxidant properties in polar and a polar environments generally agreed with differences in their partitio n coefficients. The methyl ester was the least effective antioxidant b oth in aqueous phase and in membranes. Tetranor-DHLA was found not onl y to be less effective in preventing ROO.-induced lipid peroxidation, but also to induce lipid peroxidation in the presence of residual iron . Thus, the complexity of biological systems seems to complicate gener alizations on the correlation of molecular structure with antioxidant activity of DHLA.