Structure-bioactivity relationships in lignins: A multivariate analysis

In this study, a diverse set of technical lignins derived from hardwood, softwood, and agricultural residues, isolated via Kraft, soda, organosolv, sulfite, hydrolysis, and laboratory-prepared copper-catalyzed alkaline hydrogen peroxide (Cu-AHP) processes, were comprehensively characterized to elucidate structure-bioactivity relationships. Molar mass distribution was determined by gel permeation chromatography (GPC); hydroxyl functional groups were quantified using quantitative phosphorus nuclear magnetic resonance (³¹P NMR) spectroscopy; glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC); thermal stability was evaluated by thermogravimetric analysis (TGA); and elemental composition was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES). Antioxidant activity was assessed using the DPPH radical scavenging assay, and antibacterial performance was evaluated by minimum inhibitory concentration (MIC) and agar diffusion tests against Staphylococcus aureus and Escherichia coli. Lignins enriched in syringyl units and free phenolic hydroxyl groups exhibited the strongest antioxidant and antibacterial performance, with MIC values against S. aureus as low as 1.56 mg mL^-1. Correlation analysis revealed that MIC decreased with increasing syringyl hydroxyl content (r = -0.73), while radical scavenging activity increased with decreasing aliphatic hydroxyl content (r = -0.94). Partial least squares (PLS) regression integrating structural descriptors and biological responses demonstrated strong predictive performance, confirming that lignin bioactivity is primarily governed by phenolic functionality and syringyl content. Bioactivity trends were further validated using representative lignin monomers, consistent with polymeric behavior. The superior performance of syringyl-rich lignins is attributed to their two methoxy groups, which enhance electron-donating capacity and stabilize phenoxy radicals, thereby improving antioxidant and antimicrobial activity.