Capsular Polysaccharide

(Rhizobium trifolii)

 

• Introduction

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Introduction

The Rhizobium trifolii capsular polysaccharide has a complex chemical sequence, which consists of hexamer repeat composed of a trisaccharide in the main chain, and two side chains, i.e. a monosaccharide and a monosaccharide. Despite the poor quality of the X-ray diffractogram (continuous intensities on layer lines and very few Bragg reflections), a structural model was proposed suggesting the occurrence of a right-handed 2-fold helix of pitch 20.2 Å .

Fig.1 Schematic representation of the repeat unit of Rhizobium trifolii

The structure is very sinous and it is stabilized by eight hydrogen bonds per repeat. The mono- and di-saccharide side chains are in the periphery, and the latter display inter-residue hydrogen bonding. As a result, the extensively hydrogen-bonded polymer chain has the appearance of a double helix.

 

Fig.2 Schematic representation of the expanded form of Rhizobium trifollii capsular polysaccharide

The quality of the diffractogram is not sufficient to establish unambiguously the packing features of the structure.  There are no direct interactions among the main chains of adjacent helices. It is speculated that the formation of the stable branched polysaccharide helices is promoted by the presence of the side chains.