Bioactive lignin from maleic acid hydrotropic fractionation: Revealing the structural-bioactivity relationship

Lignin possesses diverse bioactivities due to its unique physicochemical structure. This study investigates the structural-bioactivity relationships of lignin derived from maleic acid hydrotropic fractionation (MAHF) of two types of herbaceous biomass. The results indicated that lignin with higher phenolic hydroxyl (-OH) content (up to 2.0 mmol/g) and carboxyl (-COOH) groups (up to 0.89 mmol/g) exhibited significantly enhanced antioxidant activity. The highest antioxidant of MAHF lignin (MAHL) reached 98 % for scavenging DPPH (2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl) at 0.56 mg/mL. Antibacterial tests revealed that MAHLs demonstrated inhibition rates of 66 % against E. coli and 54 % against S. aureus at 10 mg/mL. MAHLs at 1 mg/mL concentration blocked >98 % of UV radiation. Furthermore, the study demonstrated that lignin with higher phenolic hydroxyl (-OH), carboxyl (-COOH), and syringyl (S) units and conjugated double bonds exhibit enhanced bioactive properties. Lignin with lower Mw and PDI also tends to possess good bioactivities. The findings from the study can facilitate the application of lignin as an efficient, cost-effective, and renewable biopolymer additive in various industries.