Characterization of the radical scavenging activity of lignins--natural antioxidants.

The present work is devoted to studies of the radical scavenging properties of lignins, which are recognized as efficient antioxidants of natural origin. Radical scavenging efficiency of a series of lignins isolated from deciduous and coniferous wood species and 10 lignin related monomeric compounds were examined against 1,1-diphenyl-2-picrylhydrazyl (DPPH*) radical in homogeneous conditions using ESR and spectrophotometry methods. Some structure-activity relationships are proposed, pointing out the importance of the non-etherified OH phenolic groups, ortho-methoxy groups, hydroxyl groups and the double bond between the outermost carbon atoms in the side chain for increasing scavenger activity. Analysis of rate constants for the lignins-DPPH* interaction revealed the contribution of polymer molecular weight and pi-polyconjugation systems. The pi-conjugation systems of lignins operate as catalysts/activators of the interaction with DPPH*. Heterogeneity in terms of component composition (carbohydrate admixtures) and polydispersity is the factor which can decrease drastically the antioxidant efficiency of isolated lignins. The connection of the antibacterial effect of kraft lignin with radical scavenging activity of its soluble fraction was assumed.