Oxidation of curdlan and other polysaccharides by 4-acetamide-TEMPO/NaClO/NaClO2 under acid conditions

Regioselective oxidation of C6 primary hydroxyls to carboxylate groups was applied to curdlan to prepare water-soluble oxidized products with less depolymerization by 4-acetamide-TEMPO/NaClO/NaClO2 treatment in water at pH 4.7 and 35 °C. Approximately 40–95% of the C6-OH groups of curdlan were converted to C6-carboxylate groups by the oxidation under the above conditions for 4–24 h, and these C6-oxidized curdlans became water-soluble. Although partial depolymerization was inevitable during the oxidation, the oxidized curdlan with 95% C6-carboxylation had a weight-average degree of polymerization of more than 1000. This value was much higher than that prepared by the conventional TEMPO/NaBr/NaClO oxidation at pH 10. When the 4-acetamide-TEMPO/NaClO/NaClO2 oxidation at pH 4.7 and 35 °C for 24 h was applied to amylose, starches and pullulan, all the oxidized products except amylose became water-soluble. Carboxylate contents of the oxidized products were, however, lower than that for curdlan. Thus, the C6-OH groups of (1 → 3)-β-glucan are more susceptible to the oxidation than those of glucans with (1 → 4)-α- and (1 → 6)-α-glycoside bonds.