Aml. Seca et al., STRUCTURAL CHARACTERIZATION OF THE BARK AND CORE LIGNINS FROM KENAF (HIBISCUS-CANNABINUS), Journal of agricultural and food chemistry, 46(8), 1998, pp. 3100-3108
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.