The main hemicellulose in softwood, glucomannan (GM), structurally resembles cellulose, but shows quite different physical and chemical properties. GM and cellulose both are linear �-1, 4-linked glycans. The only difference is the configuration at C-2 in about 80% of the sugar residues. - In contrast to glucose, the 2-OH in mannose has an axial orientation. Due to this difference in stereochemistry, the relative reactivities of the free hydroxyl groups at positions 2, 3, and 6 of glucose and mannose, respectively, are expected to be different. Therefore, the relative reactivities of glucosyl compared to mannosyl units in methylation reactions are studied in this work.  GM isolated from spruce was methylated in DMSO/Li-dimsyl/MeI system and for comparison in an aqueous system. After dialysis and freeze drying, the alditol acetate method (AAM) was applied to the partially methylated GM to determine the substituent distribution in glucose and mannose by gas liquid chromatography (GLC). Peaks were assigned by GC/mass spectrometry (GC/MS) and comparison with standards prepared from �-1, 4-mannan and methylcellulose. Commercially available konjac glucomannan with a different sugar unit ratio was methylated in water/NaOH/MeI system and used as a reference for discussing the reactivity in this work.  For spruce GM, in the early stage of the reaction, glucose units were found to react faster than mannose for both DMSO and water as solvent. The order of reactivities in glucose was k2 > k3 > k6, and k3 >k2 > k6for mannose (in DMSO/Li-dimsyl/MeI). For KGM, the order in glucose was k2 > k6 > k3, in mannose all the OH groups are nearly evenly distributed. Comparing G and M in the GM, the following order was found: G-k2 > M-k3 > M-k2 > G-k3 > M-k6 >G-k6?.

Reactivity investigation of glucomannan from spruce

247th National Meeting (2014)

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