The structure of the extra-cellular polysaccharide of Beijerinckia mobilis

Abstract

Structural studies on the extracellular polysaccharides elaborated by Beijerinckia mobilis, a tropical soil species of the group Azotobacteriacae, have been made and the component monosaccharides characterised. Fractionation of the total extracellular mucilage, after removal of contaminating protein, was effected by the precipitation of an acid fraction with quaternary ammonium salts. The uncomplexed neutral fraction (Mw = 2,000) was further subdivided into a neutral polysaccharide (N₁) and a low molecular weight fraction (N₂) consisting of glucose, mannose and related oligosaccharides. The origin of this fraction is unknown but it is thought to arise either from incomplete metabolism by the organism at the time of harvesting, or from the presence of an enzyme in the medium which breaks down the polysaccharide.

The neutral polysaccharide (N₂) is composed of glucose, mannose, and arabinose (1:1:1), and was found to have a low molecular weight and to be highly branched.

The acid fraction is a high molecular weight polymer (Mw = 20 x 10 6) composed of L-guluronic acid (55%) D-glucose (15%) and D-glycero-D-manno-heptose (20%), together with acetic and pyruvic acids. From the results of methylation, periodate oxidation and partial hydrolysis experiments the following repeating unit is proposed as a backbone for the polysaccharide: →3)L-GulUA(1→3) L-GulUA(1→2) D-gly-D-Manhept(1-. The remaining glucose and uronic acid residues were also found to be 1,3-linked and are thought to form separate side chains linked to C-4 of the uronic acid residues in the backbone. Combined pyruvic acid was found to be in ketal linkage to C-4 and C-7 of about 25% of the D-glycero-D-manno-heptose units.

The similarities between this polysaccharide and that from the related species Azobacter indicum are discussed.

In order to establish the linkages of the guluronic acid units it was necessary to prepare and characterise the methylated derivatives of gulose. These experiments are detailed in a reprint to be found at the end of the thesis.

Also included are reports on two six week projects carried out at Alginate Industries, Girvan: Appendix IV. Studies on gel formation with Alginates. Appendix V. Studies on the viscosity of solutions of the extracellular polysaccharide of B. mobilis under different conditions.