Refined structure of a flexible heptasaccharide using 1H-13C and 1H-1H NMR residual dipolar couplings in concert with NOE and long range scalar coupling constants.

The heptasaccharide isolated from the cell wall polysaccharide of Streptococcus mitis J22 serves as an important model for the dynamics and conformation of complex polysaccharides, illustrating the nature of flexibility with rigid epitopes joined by flexible hinges. One-bond C-H residual dipolar couplings (1D(CH)) and long-range H-H residual dipolar couplings (nD(HH)) were measured for the heptasaccharide in a cetylpyridinium chloride/hexanol/brine lamellar liquid crystal medium. A method is proposed to determine the nD(HH) in natural abundance based on a 13C resolved 1H TOCSY pulse sequence previously published to determine the homonuclear scalar couplings. Different methods for interpretation of the 1D(CH) and the nD(HH) residual dipolar coupling data obtained were compared and combined with the NOE and long-range H,C and C,C scalar couplings available for this heptasaccharide. A flexible model of the heptasaccharide was determined in which two structurally well-defined regions involving four and two sugar residues, respectively are joined by a flexible hinge which involves two 1-->6 glycosidic linkages.