Preparation of completely C6-carboxylated curdlan by catalytic oxidation with 4-acetamido-TEMPO

► The C6-OH groups of curdlan were completely oxidized to C6-carboxyls using TEMPO. ► The pure β-(1,3)-polyglucuronic acid products were water-soluble. ► The β-(1,3)-polyglucuronic acids had weight-averaged DP values of 340–360. ► Pure β-(1,3)-polyglucuronic acid was used to assign the 1H and 13C NMR shift data. Pure (1→3)-β-polyglucuronic acid sodium salt was prepared from curdlan by oxidation with 4-acetamido-TEMPO/NaClO/NaClO2 in water at pH 4.7 and 35°C. The oxidation conditions, including the reaction time and amounts of reagents added, were optimized for the preparation of (1→3)-β-polyglucuronic acids with high molecular weights. The primary C6 hydroxyl groups of curdlan were completely oxidized to the corresponding C6-carboxylates using a one- or two-step reaction process by controlling the oxidation conditions, thus providing pure (1→3)-β-polyglucuronic acids consisting only of d-glucuronosyl units. Unfortunately, however, the increased amounts of reagents and long reaction time led to significant depolymerization of the curdlan during the oxidation process, and the resulting (1→3)-β-polyglucuronic acids had weight-average degrees of polymerization of 340–360. The 13C and 1H NMR chemical shifts of the products were successfully assigned using pure (1→3)-β-polyglucuronic acid.