Phenolic acids and derivatives: Studies on the relationship among structure, radical scavenging activity, and physicochemical parameters

The antiradical activity of caffeic acid (1), dihydrocaffeic acid (5), and their corresponding n-alkyl esters was evaluated by using the 2,2-diphenyl- 1-picrylhydrazyl radical (DPPH.) method. Dihydrocaffeic acid (5) was the mo st potent compound, having an antiradical effect higher than that of (+/-)- alpha-tocopherol, whereas caffeic acid (1) was less efficient. Esterificati on of the carboxyl group of dihydrocaffeic acid (5) had a dramatic effect o n its antiradical potency, but similar effects were not observed for caffei c acid (1) derivatives. The n-alkyl esters of both phenolic series had simi lar potencies, and their antiradical activities were independent of the alk yl chain length, Dose-dependent scavenger effects were found in both series . Acid-base properties of the compounds, evaluated by using potentiometry a nd spectrophotometry, showed that the catechol moiety had pK(a2) and pK(a3) values of 9.24-9.02 and 11.38-10.99 in the dihydrocaffeic series and 8.48- 8.24 and 11.38-11.07 in the caffeic series, respectively. Antiradical activ ity and pK(a) values of the compounds were not related.