Capsular Polysaccharide
(Escherichia coli M41 )
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Introduction
The Escherichia coli M41 mutant capsular polysaccharide has a complex chemical sequence, which is made up of a tetrasaccharide in the main chain, flanked by a disaccharide in a the side chain.
Fig.1 Schematic representation of the repeat unit of E. coli M41
Diffraction data, to a resolution of 3 Å have been obtained from polycrystalline and oriented fibrillar samples. From this X-ray diffractogram, the structural features have been established as a left-handed 2-fold helix having a pitch of 30.4 Å.
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While the chain backbone is fairly close to the helix axis, the side chains are folding back toward the non-reducing end, shielding a major part of the backone from the surrounding backbone. As a result only one residue of the main chain appears available for any intermolecular interactions. There exists a significant amount of intrachain hydrogen bonds, either within the main chain, or within the side chain.
Within the orthorhombic unit cell, there are two antiparallel helices which are related by space group symmetry. Such an arrangement produces substantial inter-digitation between the helices, so that side chains and main chains are linked by hydrogen bonds in a complex scheme involving parallel and antiparallel strands. Despite the fairly good resolution ofthe diffractogram, it was not possible to locate such guest molecules as sodium ions and water molecules.
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