Studies on Galactomannan in Seeds of Astragalus sinicus L.

Abstract

A hot water-soluble polysaccharide was isolated from the endosperm of the ripe seed of Astragalus sinicus L. and fractionated with ethanol to obtain purified galactomannan, whose homogeneity was ascertained by DEAF-cellulose column chromatography and sedimentation analysis. The specific rotation of the galactomannan was [α]¹⁸D+61.5 (c=1.1, H₂O), and its molar ratio of D-galactose to D-mannose was 1:2.3. The molecular weight was estimated to be 15, 900 by hypoiodometry and gel filtration on Sephadex G-75. On oxidation of the galactomannan 1.18 moles of periodate per mole of hexose residue were consumed, accompanying the formation of 0.31 moles of formic acid. The products of this galactomannan by Smith degradation were glycerol, erythritol and D-mannose in a molar ratio of 2.5:4.5:1.
The specific rotation of fully methylated galactomannan was [α]¹⁸D+48 (c=0.8, CHCl₃). The methylated galactomannan gave the following sugars by hydrolysis: 2, 3, 4, 6-tetra-O-methyl-D-mannose, 2, 3, 4, 6-tetra-O-methyl-D-galactose, 2, 4, 6-tri-O-, 2, 3, 6-tri-O- and 2, 3-di-O-methyl-D-mannose, in an approximate molar ratio of 1:30:12:24:30. The results were obtained from gas chromatography of alditol acetate derivatives of methylated sugars, of hydrolysates. From these results it is suggested that the chemical structure of galactomannan from seeds of Astragalus sinicus L. may be composed of a main chain consisting of (1→4) and (1→3)-linked β-D-mannose residues with side chains consisting of single β-D-galactose residues linked to the main chain through (1→6) bonds.