The hydroperoxyl antiradical activity of Trolox in water: The effects of pH values on rate constants

Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid) is a highly hydrophilic α-tocopherol analog that is widely used as a standard against which the antioxidant ability of other chemicals is measured and represented in Trolox equivalents. However, the effect of pH values on the hydroxyl radical scavenging activity has not been fully studied yet. In this study, the HOO^• antiradical activity of Trolox in water was studied. It was found that the H-abstraction of the O1-H bond determined the activity of the neutral and monoanion states, whereas the electron transfer reaction of the hydroxyl anion state determined the activity of the dianion state. Although the total rate constant increased following the increase in pH levels, the overall rate constant of the Trolox + HOO^• reaction in water changed when pH levels rose due to the decrease in HOO^• molar fraction. The results also revealed that at pH < 2, the O1-radical was the main intermediate of the Trolox + HOO^• reaction in water, whereas, at pH ---gt--- 5, the anion-radical was the significant intermediate. Thus the rate constants and the reaction intermediates vary with the pH values.