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

1998; American Chemical Society; Volume: 46; Issue: 8 Linguagem: Inglês

10.1021/jf9801320

1520-5118

Ana M. L. Seca, José A. S. Cavaleiro, Fernando M. J. Domingues, Armando J. D. Silvestre, Dmitry V. Evtuguin, Carlos Pascoal Neto,

Polymer composites and self-healing

Dioxane and milled wood lignins were isolated from the core and bark of kenaf (Hibiscus cannabinus), variety Salvador. These lignins were characterized by 13C and 1H 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. Keywords: Lignin; kenaf; Hibiscus cannabinus; 13C NMR; 1H NMR; UV spectroscopy; FTIR spectroscopy; permanganate oxidation; phenolic acids