Structure-antioxidant activity relationship of methoxy, phenolic hydroxyl, and carboxylic acid groups of phenolic acids

TLDR

The study speculates that HAT, SET‑PT, and SPLET mechanisms may operate in the DPPH reaction system. The authors measured antioxidant activities of 18 phenolic acids by DPPH and FRAP assays and calculated five thermodynamic parameters (HAT, SET‑PT, SPLET) using B3LYP/UB3LYP DFT with a 6‑311++G(d,p) basis set. The results show that –CH₂COOH and –CH=CHCOOH groups, as well as methoxyl and phenolic hydroxyl groups, enhance antioxidant activity, with SPLET being the dominant mechanism in the FRAP assay and the activity correlating with O‑H bond dissociation enthalpy, proton affinity, and electron transfer enthalpy.

Abstract

Abstract The antioxidant activities of 18 typical phenolic acids were investigated using 2, 2′-diphenyl-1-picrylhydrazyl (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.