Comparative Antioxidant Activity and Structural Feature of Protocatechuic Acid and Phenolic Acid Derivatives by DPPH and Intracellular ROS

Background: Antioxidants are compounds that inhibit oxidation in biological systems and in the natural environment as free radical scavengers, reactive oxygen scavengers, or reducing agents. Phenolic acid compounds, such as Protocatechuic Acid (PCA), are one of the most important antioxidants occurring in the natural environment. Methods: In this study, the relationships between structure and antioxidant activities of the phenolic acids including PCA were investigated systematically with a particular focus on electronic substituent effects using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. The antioxidant activities of the compounds were further investigated at the intracellular level with the DCF-DA assay using a fluorescence microplate reader and a flow cytometry. Results: Electron-donating substituents in an ortho- or para-position to a hydrogen donor in the aromatic structure stabilized the resulting antioxidant-derived radical by delocalization across the entire molecule. A hydrogen-bond donating substituent increased antioxidant activity by stabilizing the incipient phenolic radical. Ascorbic acid and the o-substituted phenolic acids with hydrogenbonding donors and p-substituted 2,5-dihydroxybenzoic acid significantly reduced intracellular ROS levels. Conclusion: The study of relationship between antioxidant properties and hydrogen-bonding ability/ electronic effect of various substituents in the phenolic acids provides a promising strategy to develop sophisticated antioxidant foods and drugs.