Isolation and Purification of Polysaccharides

In the last few decades, the novel functions of polysaccharides have provided a major impetus for increasing scientific attention. Among the most promising aspects are their immunomodulatory and antitumor effects, thickening agents and stabilizer effects.

It is well known that the molecular weight, conformation, chemical modification, and solubility of the polysaccharides significantly affect their antitumor and immunomodulatory activities. However, the studies on molecular weight, solution property, and chain conformation of these polymer solutions have seldom been published. The scant evidence available inspired us to embark on the present study: the isolation and purifi cation of polysaccharide fractions from an edible fungus, Lentinus edodes, the determination of its chemical structure and molecular weight; the assay of antitumor activity of polysaccharides and their sulfated derivatives in vivo and in vitro.

Figure 1

The light-scattering intensities were measured with a DAWN® multi-angle light scattering instrument (MALS) equipped with a He–Ne laser at 25°C. The refractive index of 0.25 M LiCl–Me2SO, measured by an Abbe refractometer, was 1.4795. The refractive index increments (dn/dc) were measured with a double-beam differential refractometer at 633 nm and 25°C. The polysaccharide solutions were dialyzed against solvent for 72 h. The values of specific refractive index increment dn/dc in 0.25 M LiCl– Me2SO and in aq 0.5 M NaCl were determined to be 0.058 and 0.133 cm³ /g, respectively. ASTRA software was utilized for data acquisition and analysis.

Figure 2

Two kind of α- and β- glucans co-exist in the extract from fruiting bodies of Lentinus edodes. The water-insoluble fraction was identified to be α-(1, 3)-D-glucan with a few (1, 6) branch linkages, and the most abundant polysaccharide component. The water-soluble fraction was β-(1, 3)-D-glucan with (1, 6) branches. Strong intermolecular hydrogen bonds exist in the α-glucan, resulting in its water-insolubility, but there are not any aggregates or multiple-helix structures for the α-glucan in aqueous 0.5 M urea/0.5 M NaOH or DMSO/0.25 M LiCl. The M_w values for α-glucan in the above solvents were 24.1 × 10⁴ and 10.9 × 10⁴ , respectively. [η]- M_w relationship of the β-glucan in 0.5 M NaCl solution at 25°C was [η]=7.69 × 10^-6 M_w 1.32 at M_w from 1.87 × 10⁵ to 1.2 × 10⁶ . Predominant species of the β-glucan exist as triple-stranded helical chains in 0.5 M NaCl solution, and as single-flexible chains in DMSO.

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