Structural Characterization of Alkali-Extractable Lignin Fractions from Bamboo

This research investigated structural characterization of seven alkali-extractable lignin fractions by a two-step isolation method from bamboo (Bambusa rigida sp.). Different wet chemistry (sugar analysis of their associated hemicelluloses, composition of phenolic acids and aldehydes) and spectroscopy methods (UV, FT-IR, ¹H, ¹³C and HSQC NMR spectroscopy) as well as average molecular weight have been employed to assess the major structural features. The most striking characteristics of these lignin fractions are the almost absence of neutral sugars (1.12–2.21%) and their low average molecular weights (770–1890 g/mol). It also revealed that these lignin fractions contained considerable amounts of esterified p-coumaric acid, which cannot be completely hydrolyzed under the conditions given. Moreover, semi-quantitative HSQC-NMR showed that a dominant amount of -O-4′ linkages (about 82%) and resinol substructures (-′, 9%) together with small amounts of phenylcoumaran substructures (-5′) were identified to be involved in the LiOH lignin fraction. However, a decrease of -O-4′ (about 70%) was found in NaOH lignin fraction. This indicated that -O-4′ units were subjected to more degradation under aqueous NaOH treatment. Furthermore, the results confirmed that these lignin fractions belonged to HGS-type lignin, and contained slightly more amounts of syringyl units than guaiacyl units. From S/G ratios, it also implied that non-condensed syringyl units were more liable to release than the guaiacyl units from the cell wall under the mildly alkaline condition.