Effects of solvents and media on the antioxidant activity of alpha-tocopherol

The effects of solvents and media on the antioxidant activity of alpha-tocopherol were studied. The antioxidant activities of alpha-tocopherol in different solvents decreased in the order of acetonitrile = hexane > ethanol = methanol, which indicates that the antioxidant activity of alpha-tocopherol is smaller in protic solvent than in aprotic solvent. The antioxidant activity of 2-(4,8,12-trimethyltridecyl)-5-hydroxy-2,4,6,7-tetramethylindan , which has similar structure to alpha-tocopherol but does not have ether oxygen, was also measured in protic and aprotic solvents. Its antioxidant activity was smaller than that of alpha-tocopherol in every solvent, but interestingly, substantially the same solvent effects were observed. These results show that the hydrogen bonding between the protic solvent and either oxygen is not important but that the hydrogen binding between protic solvent and phenolic group reduces the activity of alpha-tocopherol. Antioxidant activities of alpha-tocopherol in micelle system and liposomal membrane were markedly reduced compared with that in homogeneous solution. Solvent effect on the alpha-tocopheroxyl radical was also studied by using electron spin resonance. The hyperfine splitting constants of a5CH3H and a7CH3H were smaller in protic solvent than in aprotic solvent, which shows that lone-pair orbital energy on 5-CH3 and 7-CH3 is smaller in protic solvent. The ESR spectra of alpha-tocopheroxyl radical in liposomal membrane and micelle were similar to those observed in aprotic solvent and in protic solvent, respectively, suggesting that alpha-tocopheroxyl radical is located predominantly in the lipophilic domain of the liposomal membrane but in or closer to water phase of micelle aqueous suspensions.