Influence of Stereochemistry on Relative Reactivities of Glucosyl and Mannosyl Residues in Konjac Glucomannan (KGM)

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.