Structure/property relationships in bacterial polysaccharides

The three-dimensional structures of three bacterial polysaccharides, galactoglucan, Rhizobium trifolii capsular polysaccharide and welan have been determined by x-ray fiber diffraction analysis using the linked-atom least-squares technique. The structural details are discussed in relation to the observed physical properties in each case. Rhizobium meliloti mutants produce a simple, linear, acidic exopolysaccharide that has a disaccharide repeating unit. Both linkages in this galactoglucan with alternating glucopyranosyl and galactopyranosyl units are $1\to 3.$ It has 4,6-O-pyruvyl cyclic acetyl group on the $\alpha$-galactosyl unit and a 6-O-acetyl ester group on the $\beta$-glucosyl units. X-ray diffraction patterns of polycrystalline and well oriented specimens of the potassium salt of galactoglucan indicate that the polymer has two-fold helix symmetry and pitch 15.89 A. The molecular structure and packing arrangement of the two chains in an orthorhombic unit cell (a = 14.49 A, b = 9.79 A and c = 15.89 A) show that each disaccharide repeating unit is associated with one potassium and three water molecules. Both pyruvate and acetate groups are on the periphery of the helix, involved in the association of the polysaccharide chains and thus may be important in the nodule invasion process. Oriented fibers of the gel-forming, neutral, doubly branched, galactose-rich Rhizobium trifolii capsular polysaccharide diffract to give continuous intensities on layer lines. Packing calculations reveal that the short-range ordering of these pseudo double helices is brought by intermolecular hydrogen bonds that involve the side chains and this leads to gelation. Complete or partial removal of side chains will therefore weaken the molecular association and is detrimental to gel formation. Welan is branched, has a pentasaccharide repeating unit consisting of the same anionic tetrasaccharide in the main chain as the gel-forming linear polysaccharide gellan, and exhibits high viscosity. Analysis of x-ray data from polycrystalline and well oriented specimens of the calcium salt of welan reveal that the polymer forms a gellan-like three-fold, half-staggered, parallel double helix in which the side chains are on the periphery. Each carboxylate group is bound to a calcium ion which in turn, via water molecules, is involved in interactions responsible for the lateral organization of three double helices in a trigonal unit cell (a = b = 20.83 A and c = 28.69 A). Double helices I and II, arranged antiparallel similar to that of gellan, are not connected by calcium ions. The side chains have a dominant role in the intermolecular interactions among the welan double helices. (Abstract shortened by UMI.