Carbanilation of cereal β-glucans for molecular weight determination and conformational studies

Cereal β-glucans can form aggregates in aqueous solution. The presence of aggregates in cereal β-glucan solutions led to inaccurate determination of molecular weights and it was believed that intermolecular hydrogen bonding caused the aggregation. To eliminate aggregates, a carbanilation method for molecular weight determination of cereal β-glucans was developed. Wheat β-glucan samples were selected for investigation. The carbanilation method can prevent intermolecular hydrogen bonding by blocking hydroxyl groups with phenyl carbamate groups. The carbanilates of cereal β-glucans were prepared by the reaction of cereal β-glucans with phenylisocyanate catalyzed by DMSO and pyridine. To avoid degradation during the carbanilation reaction, relatively mild conditions were used, which led to incomplete substitution (DS: ∼2). However, after the carbanilation reaction, the carbanilates dissolved completely in 1,4-dioxane solution without any detectable aggregates, which allowed accurate molecular weight determination. The degree of substitution (DS) of carbanilates was determined by both a nitrogen content method and an FT-IR method. The FT-IR method proved to be the more effective for DS estimation. Using this method, the converted molecular weights of cereal β-glucans were in good agreement with the results measured in 0.5 M NaOH solution, which previously was shown to be a good solvent for cereal β-glucans. After the carbanilation reaction, conformational changes of carbanilates were studied by static and dynamic light scattering techniques. The fractal dimension (df = 2.27) and the structure sensitive parameters (ρ >2) suggested a porous globular structure for partially carbanilated β-glucans.

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