STRUCTURAL CHARACTERIZATION OF ALKALI-EXTRACTABLE BAMBOO LIGNIN: AN NMR PRESPECTIVE

摘要

This research investigated the physicochemical properties and structural characterizations of seven alkali-extractable lignin preparations by a two-step isolation method from bamboo. The lignin preparations were analyzed by GPC to determine average molecular weights, and further characterized by 13C and HSQC NMR spectroscopy. The most striking characteristics of these lignin preparations are their low average molecular weights (770-1890 g mol-1). It also revealed that these lignin preparations 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 β-O-4′ linkages (about 70%) and resinol substructures (β-β′, 19%) together with small amounts of phenylcoumaran substructures (β-5′) were identified to be involved in the lignin preparation (NaOH extractable). Furthermore, the results revealed that these lignin preparations 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.