Noncatalytic Hydrothermal Elimination of the Terminal D-Glucose Unit from Malto- and Cello-Oligosaccharides through Transformation to D-Fructose

Noncatalytic hydrothermolyses of malto- and cello-oligosaccharides (di-, tri-, tetraose), linked by alpha- and beta-1, 4-glycosidic bonds, respectively, were investigated at 100-140 degrees C. In situ C-13 NMR spectroscopy was applied to elucidate the position and pathways of the glycosidic bond breakage and the dependence of the hydrolysis rate on the bond type. Spectral analysis was carried out quantitatively as a function of time with the mass balance confirmed, and it was shown for both the malto- and the cello-oligosaccharides that the terminal D-glucose unit with a free anomeric carbon is selectively eliminated after transformation to D-fructose. Site-selective breakage of the glycosidic bonds proceeded on the order of hours. The initial apparent rates for terminal hydrolysis were found to be independent of the degree of oligomerization but dependent on the type of glycosidic bond. Rate constants were larger for the alpha-1,4-linked malto-oligosaccharides by a factor of 3-4 than for the beta-1, 4-linked cello-oligosaccharides. The pathways and mechanisms for the malto- and cello-oligosaccharide hydrothermolyses are common and can be understood in terms of the elementary reactions of the di- and monosaccharides.