A flexible model for the cell wall polysaccharide of Streptococcus mitis J22 determined by three-dimensional C-13 edited nuclear overhauser effect spectroscopy and C-13-H-1 long-range coupling constants combined with molecular modeling

We report on the conformation of a tetrasaccharide fragment in the repeating subunit of the cell wall polysaccharide of Streptococcus mitis J22, a receptor for the lectin of Actinomyces viscosus T14V in a bacterial coaggregation that is important in the ecological interactions of oral bacteria. Although there is considerable overlap of the H-1-nmr signals, some cross peaks can be extracted from conventional two-dimensonal nuclear Overhauser effect spectroscopy (NOESY) data on the polysaccharide. These data cannot befit to a single conformation of the tetrasaccharide fragment. Therefore we have prepared a polysaccharide sample fully enriched in C-13 from which we have determined accurate NOESY cross-peak volumes in a three-dimensional heteronuclear-resolved spectrum that allows accurate determination of many more NOESY cross peaks than does conventional two-dimensional spectroscopy. We have also used the C-13 enriched polysaccharide to measure accurate values of long-range C-13-H-1 coupling constants that can be correlated with glycosidic dihedral angles. Molecular modeling calculations on the polysaccharide fragment, including molecular dynamics simulations, identify multiple low-energy conformations This result is to be contrasted with previous calculations on blood group oligosaccharides in our laboratory using similar methods that showed relatively rigid conformations with little flexibility of the glycosidic linkages. The present NOESY and (3)J(CH), data can be reconciled with a model for the antigenic tetrasaccharide in which three distinct conformations are in fast exchange. We propose that some carbohydrate epitopes such as those of the blood group oligosaccharides are relatively rigid while others such as the tetrasaccharide fragment in these studies exhibit much greater flexibility. (C) 1996 John Wiley & Sons, Inc.