Understanding the relationship between the structural properties of lignin and their biological activities.

Due to the antioxidant properties of lignin, it has been demonstrated as an active substance for treating oxidation-related and inflammatory diseases. However, how the structural properties of lignin affect its biological activities is still ambiguous. In this study, Kraft lignin from wheat straw (KL-A) was used as the raw material to fractionate into three fractions (e.g., KL-B, KL-C, and KL-D) with different molecular weight by ultrafiltration, which possessed different physicochemical properties. The biocompatibility, in vivo and in vitro scavenging abilities for reactive oxygen species (ROS), and anti-apoptotic abilities of the lignin fractions were evaluated using SW1353 chondrocyte cell lines and were quantitatively fitted to their physicochemical properties. The results showed that lignin fractions with lower molecular weights, lower G/S ratios, and higher non-condensed phenolic OH contents endowed lignin with stronger ROS scavenging ability in vivo and in vitro, but was accompanied by increased cytotoxicity to cells. The half maximal inhibitory concentration (IC50) of KL-A, KL-B, KL-C, and KL-D were separately determined as 44.02, 33.43, 32.41, and 18.40 μg/mL. Furthermore, KL-D, with the lowest molecular weight and highest number of functional groups, showed the best antioxidant ability, while it performed poorly in inhibiting cellular apoptosis of chondrocytes. Compared to KL-D, KL-C with inverse structural properties, performed better in anti-apoptosis of SW1353 cells, which is the optimum lignin as promising active substances to be applied in the treatment of osteoarthritis in biomedical engineering.