Free Radical Scavenging and Antioxidative Activity of Caffeic Acid Amide and Ester Analogues:  Structure−Activity Relationship

The structure−activity relationships of synthetic caffeic acid amide and ester analogues as potential antioxidants and free radical scavengers have been investigated. The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH·) scavenging activity of the test compounds was N-trans-caffeoyl-l-cysteine methyl ester (5) > N-trans-caffeoyldopamine (4) > N-trans-caffeoyltyramine (3) > N-trans-caffeoyl-β-phenethylamine (2) > Trolox C (8) > caffeic acid phenethyl ester (1) > caffeic acid (6) > ferulic acid (7). This established that the radical scavenging activity of the compounds increased with increasing numbers of hydroxyl groups or catechol moieties and also with the presence of other hydrogen-donating groups (−NH, −SH). The antioxidative activity of the compounds was also investigated in an emulsified linoleic acid oxidation system accelerated by 2,2‘-azobis(2-amidinopropane) dihydrochloride. The order was 1 > 2 > 4 > 3 ≥ 5 > 6 > 8 > 7. Therefore, in the emulsion system, the antioxidative activity of the test compounds depends not only on the hydroxyl groups or catechol rings but also on the partition coefficient (log P) or hydrophobicity of the compounds. This supports the concept that hydrophobic antioxidants tend to exhibit better antioxidative activity in an emulsion system. Keywords: Antioxidants; antioxidative activity; free radical scavenging activity; caffeic acid analogues; ferulic acid; partition coefficient; structure−activity relationship