Quantitative structural characterization of the lignins from the stem and pith of bamboo (Phyllostachys pubescens)

Abstract

Milled wood lignins (MWL) were isolated from the stem (MWLS) and pith (MWLP) of bamboo (Phyllostachys pubescens). The nonacetylated and acetylated bamboo MWLs were investigated by Fourier transform infrared, quantitative 13C-nuclear magnetic resonance (NMR), 2D heteronuclear single quantum coherence (HSQC) NMR, and 31P-NMR spectroscopy. The MWL consists of p-hydroxyphenyl (1-2%), guaiacyl (21-31%), and syringyl (67-78%) units associated with p-coumarates and ferulates. A modified quantitative 13C-NMR and 2D-HSQC analysis has demonstrated that the predominant intermonomeric linkages are of the type β-O-4 (45-49 per 100 C9units, i.e., per C900) along with small amounts of other structural units such as resinols (3.6-7.4 per C900), tetrahydrofuran (2.0-2.3 per C900), phenylcoumaran (2.8-4.5 per C900), spirodienones (1.3-2.3 per C900), and α,β-diaryl ethers (2.8-2.9 per C900). MWLPcontained more p-coumarates than MWLS. The various degrees of y-acylation (17-27%) were positively associated with S/G ratios in the lignins; however, y-acylation was inversely correlated to the ratio between β-β and β-O-4 side chains in these lignin fractions. Moreover, a flavonoid compound (tricin) was also detected in the MWLSbut not in MWLP. The two MWLs are very similar in terms of molecular weights and the contents of OHphenand OHaliph.