Structural Characterization of the Bark and Core Lignins from Kenaf (Hibiscus cannabinus)

Dioxane and milled wood lignins were isolated from the core and bark of kenaf ( Hibiscus cannabinus ), variety Salvador. These lignins were characterized by 13 C and 1 H NMR, FTIR, and UV spectroscopies, permanganate oxidation, and alkaline hydrolysis followed by GC and GC-MS analysis of the released products and by functional groups analysis. The permanganate oxidation and alkaline hydrolysis was also applied to “in situ” lignins. Isolated and “in situ” lignins showed significant differences in composition and structure. Strong structural differences were observed between bark and core lignins, suggesting their different biosynthetic routes. The β-O-4 type linkages are the main interunit linkages and are more abundant in bark than in core lignin. The core lignin is more “condensed” and shows higher contents of β-β plus β-5 linkages than those in the bark lignin. Permanganate oxidation showed that both core and bark “in situ” lignins are HGS-type lignins with HGS proportions of 15:66:19 and 12:56:32, respectively (H including coumarate structures in the case of core lignin). Coumarates represent about 50% of the H units of core lignin and are absent in bark lignins. The presence of suberin-like aliphatic chains covalently bound to lignin was suggested for bark lignin.