Polysaccharides of tropical pasture herbage. V. A galactoglucomannan from Townsville lucerne (Stylosanthes humilis)

Abstract

After removal of pectic substances, lignin, and hemicelluloses from Townsville lucerne stems, the residual ?α-cellulose? contained significant amounts of mannose. Further extraction with a mixture of sodium hydroxide and sodium borate gave a galactoglucomannan which was partly purified by precipitation with barium hydroxide to give a crude yield of 0.35% based on the α-cellulose or 0.12% based on the original plant dry weight. Further purification by chromatography on QAE- Sephadex gave a polymer which showed only one component on free boundary electrophoresis in borate buffer. Its structure was determined by periodate oxidation, Smith degradation, and methylation-hydrolysis. The backbone of the polymer consists of glucose and mannose units, linked 1,4, but their order within the chain is not known. The non- reducing end-groups are glucopyranose and galactopyranose units in the ratio 1 : 4. The galactose may be present as single unit substituents at a frequency of 1 galactose per 14 main-chain units and all such substitution occurs at mannose units with none at glucose units. The similarity of the optical rotation to that of galactoglucomannans previously isolated from wood suggests that the majority of the glycosidic linkages are β-oriented.