Stability of (1→3)-β-polyglucuronic acid under various pH and temperature conditions

Regioselective oxidation of C6 primary hydroxyls to carboxyls was applied to curdlan, to prepare a water-soluble (1→3)-β-polyglucuronic acid Na salt [(1,3)-β-PGluA], using a 4-acetamido-TEMPO/NaClO/NaClO₂ oxidation system at pH 4.7. Changes in the chemical structure and degree of polymerization of (1,3)-β-PGluA treated in water at various temperatures and pHs were studied to evaluate the stability of (1,3)-β-PGluA to these treatments. This polyuronic acid was found to be stable, without any depolymerization, to treatment in water at pHs 1-9 and room temperature for up to 128 h; slight depolymerization was observed at pHs 11 and 13. When heated in water at pH 1 and high temperatures (1,3)-β-PGluA molecules were randomly depolymerized by hydrolysis, primarily forming glucuronic acid. In contrast, dicarboxylic-acid-type monomers containing ethylene carbons were formed from the C1-carboxyl or C1 reducing ends of (1,3)-β-PGluA molecules by treatment under alkaline conditions; this was initiated by β-alkoxy elimination, similar to the peeling-off reaction of cellulose.