New strategy for the conformational analysis of carbohydrates based on NOEand C-13 NMR coupling constants. Application to the flexible polysaccharide of Streptococcus mitis J22

For complex oligosaccharides, which are relatively rigid with modest excurs ions from a single minimum energy conformation, it is straightforward to bu ild conformational models from NOE data. Other oligosaccharides are more fl exible with transitions between distinct minima separated by substantial en ergy barriers. We show that modeling based on scalar coupling data is super ior to NOE-based modeling for the latter case. Long range C-13-C-13 and C-1 3-H-1 coupling constants measured for the heptasaccharide repeating subunit of the cell wall polysaccharide from Streptococcus mitis J22 are correlate d with individual glycosidic dihedral angles, effectively uncoupling the de grees of freedom of the oligosaccharide and allowing a search for combinati ons of dihedral angles which are energetically reasonable, i.e., with no ba d van der Waals contacts, and which can be combined to satisfy all the meas ured J values. Allowed values of the individual angles can then be combined to search for overall oligosaccharide conformations which contribute to th e ensemble. We show that while the polysaccharide from S. mitis J22 is flex ible, requiring multiple conformations, most of the flexibility is localize d to a few bonds and only a rather small number of conformations is require d to reproduce the experimental NOE and scalar coupling data.