Comparative structural characterization of milled wood lignin and lignin carbohydrate complex fractions from larch tissues with representative oxidative transformation of sapwood LCC

Lignin–carbohydrate complexes (LCCs) are covalent assemblies that contribute substantially to biomass recalcitrance, yet their tissue-dependent structural diversity in softwood and their fate during oxidative depolymerization remain insufficiently understood. In this exploratory study based on one larch tree, MWL and LCC-enriched fractions were sequentially isolated from the heartwood, sapwood, and bark of larch and characterized by FT-IR, UV–vis, GPC, TG/DTG, and 2D HSQC NMR. Clear tissue specificity was observed. Among the MWL fractions, HMWL showed the highest β-O-4 content (77.5%), whereas among the LCC fractions, BLCC exhibited the highest phenyl glycoside (PhGlc) content (5.4%). The LCC fractions possessed larger molecular weights than the corresponding MWLs, and an apparent positive relationship was observed between β-O-4 abundance and molecular weight across tissues, with HLCC showing the highest apparent molecular weight among the isolated LCC-rich fractions. To probe oxidative structural evolution, SLCC was selected as a representative substrate because of its relatively abundant LCC linkages and typical lignin interunit distribution. Under optimized conditions over H₃PMo₁₂O₄₀ in methanol/water (8:2, v/v) at 140 °C for 2 h, the monomer yield reached 9.1 wt%. HSQC analysis of residual solids after mild and severe treatments indicated progressive loss of carbohydrate-associated signals and LCC-related correlations, together with substantial depletion of lignin side-chain signals. These observations provide an exploratory, spectroscopically based description of the transformation of a representative sapwood LCC fraction under oxidative conditions. • Larch LCCs exhibited distinct tissue-dependent structural features. • Heartwood MWL showed the highest β-O-4 content. • Bark LCC showed the highest phenyl glycoside content. • SLCC underwent stepwise oxidative disassembly under H 3 PMo 12 O 40 /O 2 .