Structure and Radical Scavenging Activity Relationships of Pyrolytic Lignins

This work deals with antioxidant properties of pyrolytic lignins against two free radicals, 1,1-diphenyl-2-picrylhydrazyl and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid). Pyrolytic lignins produced by the thermal pyrolysis of the Etek lignin were extracted from the liquid pyrolysis product and fractionated using solvents of different polarities. The main functional groups linked to the lignin skeleton have been characterized by H-1 NMR and F-19 NMR upon acetylation and trifluoromethylation, respectively. Their radical scavenging activity against targeted free radicals was evaluated in vitro, and it was correlated to the content of studied functional groups. In contrast to the extracted Etek lignin, thermal pyrolysis produces lignin adducts that have higher scavenging efficacy than the nonthermally altered lignin and even higher than that of quercetin, a well-known phenolic antioxidant. The phenyl hydroxyl and methoxyl groups appear to be the main lignin structural factors contributing to the overall scavenging properties against the DPPH and ABTS free radicals. Also, these results show that there is no correlation between the content of aliphatic hydroxyl and carbonyl groups and the antioxidant activity.