Conformation of crystalline type III pneumococcal polysaccharide

The conformation of crystalline Type III pneumococcal polysaccharide, poly[(1 yields 3)-beta-D-GlcpA-(1 yields 4)-beta-D-Glcp], has been studied by X-ray diffraction and stereochemical analysis. The X-ray pattern, recorded at 80% relative humidity, led to a trigonal unit-cell, with alpha = beta = 1.028 nm, c = 2.77 nm, and gamma = 120 degrees. Meridional reflections on the third and the sixth layer-lines show that the chain consists of a three-fold helix, with a repeat of 0.923 nm. Conformational analysis, by the virtual-bond method, shows that one right-handed and two left-handed helices are equally favorable, on the basis of energies of isolated chains. Packing of the three helix models in the unit cell, by contact-distance criteria, ruled out the possibility of the right-handed helical conformation. One of the left-handed helices has intramolecular hydrogen-bonds O-3'...O-5 and O-2...O-6' between the residues in the (1 yields 4) linkage, and O-4'...O-5 between those in the (1 yields 3) linkage. In the other left-handed helix, the O-4'...O-5 hydrogen bond is replaced by an O-2...O-2' hydrogen bond. The choice between the two left-handed helix models is inhibited by the paucity of X-ray intensity data.