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Influence of Stereochemistry on Relative Reactivities of Glucosyl and Mannosyl Residues in Konjac Glucomannan (KGM)
Author(s) -
Zhang Qimeng,
Mischnick Petra
Publication year - 2017
Publication title -
macromolecular chemistry and physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.57
H-Index - 112
eISSN - 1521-3935
pISSN - 1022-1352
DOI - 10.1002/macp.201700119
Subject(s) - chemistry , hydrolysis , thermogravimetric analysis , differential scanning calorimetry , glucomannan , reaction rate constant , thermal decomposition , mass spectrometry , electrospray ionization , chromatography , polymer chemistry , nuclear chemistry , stereochemistry , organic chemistry , kinetics , biochemistry , physics , quantum mechanics , thermodynamics
Methylation in water with NaOH/MeI is applied to study the influence of the stereochemistry on relative reactivities of d ‐mannosyl (M) compared to d ‐glucosyl (G) units in konjac glucomannan (KGM). The pH is kept constant at 13.6 over the course of the reaction and aliquots are removed after various time intervals. Methyl distribution in G and M residues is determined after perethylation, hydrolysis, and conversion to O ‐ethyl‐ O ‐methyl‐alditol acetates. The order of relative rate constants determined for the O ‐methyl Konjac glucomannans (M‐KGMs) in degree of substitution (DS) range 0.3–0.8 is G‐ k 6 > M‐ k 6 > G‐ k 2 ≈ M‐ k 2 > M‐ k 3 > G‐ k 3 . Oligosaccharides obtained by partial hydrolysis after full protection of M‐KGM with MeI‐ d 3 are labeled with m ‐amino‐benzoic acid and measured by liquid chromatography–electrospray ionization–mass spectrometry. DS/DP profiles are in full agreement with random distribution of methyl groups. Thermal properties of M‐KGMs are analyzed by differential scanning calorimetry and thermogravimetric analysis. Decomposition temperature increases with DS, while the temperature of an endothermic change decreases.