The antioxidant activities of 18 typical phenolic acids were investigated using 2, 2′-diphenyl-1picrylhydrazyl (DPPH) and ferric ion reducing antioxidant power (FRAP) assays. Five thermodynamic parameters involving hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), and sequential proton-loss electron transfer (SPLET) mechanisms were calculated using density functional theory with the B3LYP/UB3LYP functional and 6-311++G (d, p) basis set and compared in the phenolic acids. Based on the same substituents on the benzene ring,-CH 2 cooH and-cH = CHCOOH can enhance the antioxidant activities of phenolic acids, compared with-COOH. Methoxyl (-ocH 3) and phenolic hydroxyl (-oH) groups can also promote the antioxidant activities of phenolic acids. These results relate to the O-H bond dissociation enthalpy of the phenolic hydroxyl group in phenolic acids and the values of proton affinity and electron transfer enthalpy (ETE) involved in the electron donation ability of functional groups. In addition, we speculated that HAT, SET-PT, and SPLET mechanisms may occur in the DPPH reaction system. Whereas SPLET was the main reaction mechanism in the FRAP system, because, except for 4-hydroxyphenyl acid, the ETE values of the phenolic acids in water were consistent with the experimental results. Phenolic acids, a class of compounds formed by the substitution of hydrogen atoms on benzene rings by a carboxylic acid group and at least one hydroxyl, are widely found in plants, plant foods, and human metabolites 1. Unlike flavonoids, free phenolic acids, such as benzoic, phenylacetic, and cinnamic acids, have high bioavailability and good water solubility 2. They can be absorbed in the stomach, whereas flavonoids cannot be absorbed, and only a small amount of flavonoids are transported passively through the intestinal wall into the blood 3-5. Most flavonoids are affected by pH, and digestive enzymes and intestinal microorganisms jointly affect C-ring cleavage, which breaks down into phenolic acids before being absorbed into the blood circulation system 6-8. Like flavonoids, phenolic acids are considered to be excellent antioxidants that can quench excessive free radical-induced body damage and chronic diseases 2. The antioxidant ability center of phenolic acids is phenolic hydroxyl, so the number and position of phenolic hydroxyls are directly related to their antioxidant activity 9. Moreover, the methoxy and carboxylic acid groups also have important effects on the antioxidant ability of phenolic acids 10,11. In recent years, with the development of computational chemistry based on density functional theory (DFT), theoretical results are often used to further explain the experimental results or predict the antioxidant activity of phenolic acids 12. Three key antioxidant mechanisms involved in the process of quenching free radicals are hydrogen atom transfer (HAT), single-electron transfer followed by proton transfer (SET-PT), and sequential proton-loss electron transfer (SPLET). HAT is a one-st...
