Isomerization of 6-O-substituted glucose and fructose under neutral pH conditions and subsequent ?-elimination reactions

Isomaltose (6-O-alpha-D-glucopyranosyl-D-glucose) and isomaltulose (palatinose; 6-O-beta-D-glucopyranosyl-D-fructose) were heated to 90 degrees C in 100 mM sodium phosphate buffer (pH 7.5). Aldose-ketose isomerization between isomaltose, isomaltulose, and epi-isomaltulose was observed in the early stage of the reaction, alongside the release of a small amount of glucose. The total concentration of these disaccharides gradually decreased as the heating time increased. However, this decrease did not correlate with the amount of glucose or fructose released, suggesting that the releases of these monosaccharides were not caused by the hydrolysis of glycosidic linkages. A slight decrease in the pH of the reaction solution was attributed to the formation of two organic acids, 6-O-beta-Dglucopyranosyl-3-deoxy-D-arabino-hexonic acid (1) and 6-O-beta-D-glucopyranosyl-3-deoxy-D-ribo-hexonic acid (2). These compounds were formed from the beta-elimination of the hydroxyl group at the C-3 of fructose, leaving a substituted glucose residue at the C-6 position, followed by keto-enol tautomerization and benzilic acid rearrangement. Although approximately 30% of 1 and 2 were degraded after 360 min of heating at 90 degrees C in 100 mM sodium phosphate, a little release of glucose was observed, indicating no hydrolysis of the glucoside bond at C-6. Besides 1 and 2, time-dependent changes in the NMR spectra of the reaction mixture in water indicated the formation of formic acid and the presence of species possibly resulting from the beta-elimination of the hydroxyl group from 3- and 4-ulose. The glucose released by heating isomaltose and isomaltulose may be generated via tautomerizations of keto-enols between the C-4 and C-5 positions and cleavage of 6-O-glycosidic linkage via beta-elimination.

Automated summary

Isomaltose and isomaltulose undergo isomerization and release of glucose upon heating in sodium phosphate buffer. The concentration of these disaccharides decreases with heating time, but this decrease is not directly correlated with the release of glucose or fructose. The reaction also leads to a slight decrease in pH and the formation of organic acids, along with the presence of formic acid and species resulting from hydroxyl group elimination. The released glucose is likely generated through tautomerization and cleavage of the glycosidic linkage.