STRUCTURAL CHARACTERIZATION OF THE BARK AND CORE LIGNINS FROM KENAF (HIBISCUS-CANNABINUS)

Dioxane and milled wood Lignins were isolated from the core and bark o f kenaf (Hibiscus cannabinus), variety Salvador. These lignins were ch aracterized by C-13 and H-1 NMR, FTIR, and UV spectroscopies, permanga nate oxidation, and alkaline hydrolysis followed by GC and GC-MS analy sis of the released products and by functional groups analysis. The pe rmanganate oxidation and alkaline hydrolysis was also applied to ''in situ'' lignins. Isolated and ''in situ'' lignins showed significant di fferences in composition and structure. Strong structural differences were observed between bark and core lignins, suggesting their differen t biosynthetic routes. The beta-O-4 type linkages are the main interun it linkages and are more abundant in bark than in core lignin. The cor e lignin is more ''condensed'' and shows higher contents of beta-beta plus beta-5 Linkages than those in the bark lignin. Permanganate oxida tion showed that both core and bark ''in situ'' lignins are HGS-type l ignins 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 b ark lignins. The presence of suberin-like aliphatic chains covalently bound to lignin was suggested for bark lignin.