Reactivity of chlorogenic acid toward hydroxyl and methyl peroxy radicals relative to trolox in nonpolar media

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.

Comparison of the scavenging capacities of phloroglucinol and 2,4,6-trihydroxypyridine towards HO˙ radical: a computational study

In this work the scavenging capacities of biologically active phloroglucinol (1,3,5-trihydroxybenzene, THB–OH) and structurally similar 2,4,6-trihydroxypyridine (THP–OH) towards HO˙ were examined. This task was realized by means of density functional theory, through investigation of all favorable antioxidative pathways in two solvents of different polarity: benzene and water. It was found that in benzene both compounds conform to the hydrogen atom transfer (HAT) and radical adduct formation (RAF) mechanisms. In water, the mechanisms of antioxidative action of the investigated compounds are far more complex, especially those of THB–OH. This compound and HO˙ undergo all four investigated mechanisms: HAT, RAF, sequential proton loss electron transfer (SPLET), and single electron transfer-proton transfer (SET-PT). HAT, RAF and SPLET are operative mechanisms in the case of THP–OH. Independently of solvent polarity, both investigated compounds are more reactive towards HO˙ in comparison to Trolox. Our final remark is as follows: the electron-withdrawing effect of the nitrogen is stronger than the electron-donating effect of the OH groups in the molecule of THP–OH. As a consequence, THB–OH is more powerful antioxidant than THP–OH, thus implying that the presence of nitrogen decreases the scavenging capacity of the respective compound.

The scavenging capacities of biologically active phloroglucinol (1,3,5-trihydroxybenzene, THB–OH) and structurally similar 2,4,6-trihydroxypyridine (THP–OH) towards HO˙ were elucidated.

Antioxidative Action of Ellagic Acid-A Kinetic DFT Study

Although one can find numerous studies devoted to the investigation of antioxidative activity of ellagic acid (EA) in the scientific literature, the mechanisms of its action have not yet been fully clarified. Therefore, further kinetic studies are needed to understand its antioxidative capacity completely. This work aims to reveal the underlying molecular mechanisms responsible for the antioxidative action of EA. For this purpose, its reactions with HO^• and CCl₃OO^• radicals were simulated at physiological conditions using the quantum mechanics-based test for overall free-radical scavenging activity. The density functional theory in combination with the conductor-like polarizable continuum solvation model was utilized. With HO^• radical EA conforms to the hydrogen atom transfer and radical adduct formation mechanisms, whereas sequential proton loss electron transfer mechanism is responsible for scavenging of CCl₃OO^• radical. In addition, compared to trolox, EA was found more reactive toward HO^•, but less reactive toward CCl₃OO^•. The calculated rate constants for the reactions of EA with both free radicals are in a very good agreement with the corresponding experimental values.