Theoretical Elucidation of the Radical-Scavenging-Activity Difference of Hydroxycinnamic Acid Derivatives

To elucidate the nitrogen dioxide radical‐scavenging‐activity difference of three hydroxycinnamic acid derivatives (HCAs), i.e., caffeic acid (=3‐(3,4‐dihydroxyphenyl)prop‐2‐enoic acid; 1), ferulic acid (=3‐(4‐hydroxy‐3‐methoxyphenyl)prop‐2‐enoic acid; 2), and sinapic acid (=3‐(4‐hydroxy‐3,5‐dimethoxyphenyl)prop‐2‐enoic acid; 3), a combined density‐functional‐theory (DFT) method, labeled as B3LYP/6‐31G(d)//AM1, was employed to calculate adiabatic ionization potentials (IPs) for parent molecules and anions derived by proton dissociation. It was found that not the IPs of the parent HCAs but those of the anions account well for the experimentally observed higher activity of caffeic acid as compared to those of the other two HCAs, suggesting that the antioxidant anion should be taken into consideration in the selection or rational design of novel antioxidants; this, however, was neglected in previous studies.