Polyglucuronic acids prepared from α-(1 → 3)-glucan by TEMPO-catalytic oxidation

• Published in: Carbohydrate polymers 2024-04, Vol.330, p.121813-121813, Article 121813
• Main Authors: Chitbanyong, Korawit, Hou, Gaoyuan, Shibata, Izumi, Takeuchi, Miyuki, Kimura, Satoshi, Isogai, Akira
• Format: Article
• Language: English
• Subjects: 13C NMR spectroscopy; Methyl esterification; SEC/MALLS/RI; TEMPO-catalytic oxidation; α-(1 → 3)-Glucan
• ISSN: 0144-8617; 1879-1344
• DOI: 10.1016/j.carbpol.2024.121813

2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)-catalytic oxidation was applied to a water-insoluble α-(1 → 3)-glucan in water at pH 10 and room temperature (∼24 °C), with solid NaOCl·5H2O as the primary oxidant. Oxidation with NaOCl at 15 mmol/g gave a water-soluble TEMPO-oxidized product at a mass recovery ratio of 97 %. The carboxy content of the TEMPO-oxidized product was 5.3 mmol/g, which corresponds to a degree of C6-oxidation (DO) of 93 %. A new water-soluble α-(1 → 3)-polyglucuronic acid with a nearly homogeneous chemical structure was therefore quantitatively obtained. X-ray diffraction and solid-state 13C NMR spectroscopic analyses showed that the original α-(1 → 3)-glucan and its TEMPO-oxidized product with a carboxy content of 5.3 mmol/g had crystalline structures, whereas the oxidized products with DOs of 50 % and 66 % had almost disordered structures. The carboxy groups in the oxidized products were regioselectively methyl esterified with trimethylsilyl diazomethane, and analyzed by using size-exclusion chromatography with multi-angle laser-light scattering and refractive index detections. The results show that the original α-(1 → 3)-glucan and its oxidized products with DOs of 50 %, 66 %, and 93 % had weight-average degrees of polymerization of 671, 288, 54, and 45, respectively. Substantial depolymerization of the α-(1 → 3)-glucan molecules therefore occurred during catalytic oxidation, irrespective of the oxidation pH. [Display omitted]