Effects of urea and sodium hydroxide on the molecular weight and conformation of α-(1→3)-D-glucan from Lentinus edodes in aqueous solution

The weight-average molecular weight (M-w) and intrinsic viscosity ([eta]) of the alpha-(1 --> 3)-D-glucan (L-FV-II) from Lentinus edodes in 0.5 and 1.0 M NaOH aqueous solution containing urea, were studied by light scattering and viscometry. The M-w value of the glucan decreased with increase of the urea and NaOH concentration. A strong intermolecular hydrogen bonding confers water-insolubility on the glucan, but NaOH and especially urea, broke this hydrogen bonding leading to enhanced water-solubility. Use of 1.0 M urea-1.0 M NaOH as solvent broke not only intermolecular hydrogen bonds but also partial covalent bonds of the a-glucan in aqueous solution, resulting in a decrease of M-w and [eta]. The urea and NaOH concentrations, storage time with stirring, and mode of preparation of the polysaccharide in aqueous solution significantly affected the determination of M-w and [eta]. The dependences of specific rotation and fluorescence emission ratio of a probe on urea concentration showed that a change in the molecular conformation of the alpha-glucan in 0.5 M NaOH aqueous solution containing urea occurred in the range 0.3-0.6 M urea. The 0.5 M urea-0.5 M NaOH aqueous solution is a suitable solvent for the glucan, and the M-w and [eta] values obtained were 5.21 x 10(5) and 148 cm(3) g(-1), respectively. Degradation of the glucan was obvious after storage for 15 months. (C) 2000 Elsevier Science Ltd. All rights reserved.