Conformational changes associated with complex formation between a mycobacterial polymethylpolysaccharide and palmitic acid.

The anomeric proton magnetic resonances of Mycobacterium smegmatis 3-O-methylmannose polysaccharide have chemical shifts intermediate betwen those of nonaglucoamylose and alpha-cyclodextrin, and on addition of palmitic acid most of these resonances are shifted upfield toward that of the cyclodextrin. This suggests that the methylated polysaccharide could have a conformation with some secondary structure intermediate between those of the two reference compounds, and that it forms a tightened coil upon addition of the lipid which yields an inclusion complex with the polysaccharide. The change in chemical shift is linear with lipid concentration, which indicates that the complex undergoes rapid exchange with free polysaccharide. The changes in proton chemical shifts of the polysaccharide and of the palmitic acid are consistent with the fatty acid being inserted in the coiled polysaccharide with its carboxyl group near the methyl aglycon.