Comparison of the conformation and dynamics of a polysaccharide and of its isolated heptasaccharide repeating unit on the basis of nuclear Overhauser effect, long-range C-C and C-H coupling constants, and NMR relaxation data.

A comparison of the conformation and dynamics of the cell wall polysaccharide of S. mitis J22 and the heptasaccharide repeating unit made from this polysaccharide was performed on the basis on nmr data. We have previously reported a model for this highly flexible polysaccharide in which four residues of the antigenic epitope adopt a defined conformation as do the two residues of the lectin-binding epitope. These domains are connected by a 6-substituted galactofuranoside residue that acts as a flexible hinge and the repeating subunits are joined by phosphodiester linkages that provide further flexibility. Homonuclear nuclear Overhauser effect (NOE) and long-range C-C and C-H scalar coupling constants measured in uniform (13)C-labeled samples of the polysaccharide and heptasaccharide were very similar, indicating a similar conformational average in solution. Significant differences in the solution dynamics were found from the heteronuclear relaxation data, T(1), T(1 rho), and NOE, which reflect the faster molecular tumbling of the heptasaccharide. Internal motions occurring on a picosecond time scale are relatively uniform along the polymer while dynamics on the time scale longer than a few nanoseconds is characteristic of hinge motion.