Metropolis Monte Carlo simulations have been performed on four substructures from the cell-wall polysaccharide antigen of Streptococcus group A to explore the conformational behaviour of these compounds. The compounds examined are the trisaccharide, propyl 3-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-O-(alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranoside, 1, the tetrasaccharide, propyl 3-O-(3-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-O-(alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranoside, 2, the hexasaccharide, propyl 3-O-(2-O-(3-O-(3-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranosyl)-3-O-(2-acetamido-2-deoxy-beta-glucopyranosyl)-alpha-L-rhamnopyranoside, 3, and the hexasaccharide, propyl 3-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-O-(3-O-(3-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-O-(alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranoside, 4. In general, the conformational flexibility of similar glycosidic linkages in different compounds is comparable. However, in a few cases, small differences in the conformations available to these linkages in different structural environments could be detected. Interestingly, a second conformation found for the beta-D-GlcNAc-(1-->3)-alpha-L-Rha linkage in three of the compounds was not populated in the hexasaccharide 4. Furthermore, a conformational locale of the alpha-L-Rha-(1-->3)-alpha-L-Rha linkage found to be populated in the trisaccharide 1, tetrasaccharide 2, and hexasaccharide 4 is negligibly populated in the hexasaccharide 3. Ensemble averaged proton-proton distances compare favourably with experimental average distances obtained from NMR spectroscopy. The trisaccharide branch point in the hexasaccharides is shown to be a highly defined conformational feature. The same unit has been found to be one of the crucial elements recognized by anti-Group A Streptococcus antibodies, a result that has implications for the design of improved immunodiagnostics and vaccines.
