Abstract
Seven lignin fractions were isolated from dewaxed sweet sorghum stem and their physicochemical features were investigated. The dewaxed sweet sorghum stem was extracted sequentially with hot water, 50% dioxane and dimethyl sulfoxide (DMSO), and various concentrations of NaOH. The treatments resulted in dissolution of 0.5, 0.7, 0.2, 1.3, 1.6, 0.8, and 0.5% lignin, corresponding to release of 8.9, 12.5, 3.6, 23.2, 28.6, 14.3, and 8.9% of the total solubilized lignin, respectively. The seven lignin fractions were comparatively characterized by chemical composition, structural features, and molecular weights using alkaline nitrobenzene oxidation, acid hydrolysis, UV, FT-IR, 1H and 13C NMR spectroscopy, and GPC. The purity of the lignin fractions isolated by 50% dioxane and alkali was higher than that of the lignin preparations obtained by hot water and DMSO, in which the content of associated polysaccharides in the former lignin fractions (0.16–2.12%) was much lower than those of the latter lignin preparations (10.39 and 37.03%). It was found that the seven lignin fractions contained substantial amounts of non-condensed guaiacyl and syringyl units with fewer p-hydroxyphenyl units, and had weight-average molecular weights between 1270 and 3590 g mol −1. No substantial differences in the main structural features between the lignin preparations isolated with 50% dioxane and alkali were found and these treatments did not result in significant changes in lignin composition and its structure.
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