Comprehensive NMR spectroscopic characterization of water-insoluble and water-soluble α-glucans

Dextrans and water-insoluble α-glucans are versatile polysaccharides with a complex structural composition. Most commonly, ¹H NMR spectroscopy in D₂O is used for their structural characterization. This approach provides basic information on the glycosidic linkages and the degree of branching, but some important signals are overlapping. Furthermore, information on elongated side chains or highly branched segments are not obtained and water-insoluble α-glucans cannot be analyzed. Therefore, the aim of this study was to establish conditions for a comprehensive NMR analysis of dextrans and water-insoluble α-glucans. A pretreatment by sonication and the use of 5 % tetrabutylphosphonium acetate [P₄₄₄₄][OAc] in dimethylsulfoxide-d₆ (DMSO‑d₆) at 40 °C proved to be well suited to dissolve different α-glucans and analyze them by one- and two-dimensional NMR spectroscopy. Various α-glucans containing all common structural elements were synthesized and characterized by using the established conditions. In contrast to ¹H NMR spectroscopy in D₂O, the different backbone linkages and dextran side chains yielded characteristic signals in [P₄₄₄₄][OAc]/DMSO‑d₆. Furthermore, signals characteristic for elongated side chains and highly branched segments could be identified. Therefore, the established conditions and marker signals allow for a very detailed characterization of dextrans as well as water-insoluble α-glucans with minimal sample workup.