Unraveling the Structure-Antioxidant Property Relationship of Lignin: Insights from Botanical Sources and pH-Controlled Fractionation.

This study employs biomass source selection (hardwood, gramineae, softwood) and pH-controlled fractionation (pH 1, 7, 9) to analyze lignin structure and optimize antioxidant capacity. Acid-precipitated hardwood lignin exhibits higher antioxidant capacity (IC50 = 0.74 mg/mL, TEAC = 1.88 μmol/mg), attributed to higher Ph-OH (3.35 mmol/g, TPC = 0.31 GAE/g) and the possible synergistic effect of the S/G ratio (1.63). In the stepwise pH fractionation of aspen black liquor, the fraction at pH 1 (A1) exhibited the best antioxidant activity (IC50 = 0.64 mg/mL, TEAC = 2.01 μmol/mg) and the highest Ph-OH (3.68 mmol/g, TPC = 0.37 GAE/g), while fraction A9 exhibited the weakest antioxidant activity (Ph-OH = 2.49 mmol/g, TPC = 0.3 GAE/g). Ph-OH showed the strongest correlation with IC50 (|r| = 0.94), followed by Mw (|r| = 0.89) (Ph-OH > Mw > -COOH > Alk-OH). Adjusting Mw can complement the enhancement of Ph-OH, thereby optimizing the antioxidant function.