Measurement of long-range2 13C-1H coupling constants of 95% uniformly 13C-labeled polysaccharide from Streptococcus mitis J22.

The coaggregation of Streptococcus mitis strain J22 in the early stages of dental plaque formation has been shown to result from interaction of cell wall polysaccharides with lectins on the surface of other oral bacterial species. This bacterium was grown in a medium containing 13C as the sole carbon source. We have isolated the lectin receptor polysaccharide from this strain with full enrichment in 13C and have determined a number of two-bond and three-bond 13C-1H coupling constants from measurements of the offsets in two-dimensional homonuclear nmr spectra [exclusive correlated spectroscopy (E-COSY) method]. A scheme for reliable extraction of these coupling constants from homonuclear Hartmann-Hahn and nuclear Overhauser effect spectroscopy spectra is tested in model compounds. We interpret the three-bond coupling across the glycosidic linkage in terms of dihedral angles in order to provide conformational information to supplement molecular modeling and nuclear Overhauser effect data. We show that the E-COSY method works well even for coupling constants smaller than the nmr line width and that a number of the 3JCH across the glycosidic linkage are in the range of 1-2 Hz, which is much smaller than many previously reported values.