Structure−Antioxidant Activity Relationship of Ferulic Acid Derivatives:  Effect of Carbon Side Chain Characteristic Groups

Differences in the antioxidant activity of some biosynthetically related ferulic acid derivatives induced by the presence of characteristic groups (−COOH, −CHO, −CH2OH, −CH3, and −COOC2H5) at the end of their carbon side chain were investigated using both experimental and computational methods. The relative order of the scavenging activity toward the DPPH radical derived from kinetic studies was isoeugenol ∼ coniferyl alcohol ≫ ferulic acid ∼ coniferyl aldehyde ∼ ethyl ferulate. In bulk oil autoxidation (45 °C) the same order of activity was obtained. In the o/w emulsion autoxidation, lipophilicity of the phenols was the determining factor because the least polar compounds bearing −CH3 and −COOC2H5 were the most effective ones. The order of activity based on the O−H bond dissociation enthalpy (BDE) and ionization potential (IP) values, calculated by the density functional theory (DFT) method, was in accordance with the experimental radical scavenging order and with the electron-donating/withdrawing properties of the characteristic groups. Other molecular descriptors could not complement the experimental findings. Keywords: Ferulic acid derivatives; DFT; DPPH radical; bulk oil oxidation; emulsion oxidation; structure−antioxidant activity relationship