The "pseudo double-helical" structure of the gel-forming capsular polysaccharide from Rhizobium trifolii.

X-ray diffraction analysis of oriented fibers of the gel-forming, neutral, doubly branched, galactoserich capsular polysaccharide from Rhizobium trifolii has been used to determine and refine the molecular structure to a final R value of 0.28. The polysaccharide forms a 2-fold single helix of pitch 20.2 A, is stabilized by a series of hydrogen bonds that involve the side chains, and has the appearance of a double helix. Packing calculations reveal that the short-range ordering of these "pseudo double helices" is brought about by intermolecular hydrogen bonds that involve the side chains. Complete or partial removal of side chains will weaken the molecular association and is detrimental to the gelation process.