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

The heptasaccharide isolated from the cell wall polysaccharide of Streptoco ccus mitis J22 serves as an important model for the dynamics and conformati on of complex polysaccharides, illustrating the nature of flexibility with rigid epitopes joined by flexible hinges. One-bond C-H residual dipolar cou plings (D-1(CH)) and long-range H-H residual dipolar couplings (D-n(HH)) we re measured for the heptasaccharide in a cetylpyridinium chloride/hexanol/b rine lamellar liquid crystal medium. A method is proposed to determine the D-n(HH) in natural abundance based on a C-13 resolved H-1 TOCSY pulse seque nce previously published to determine the homonuclear scalar couplings. Dif ferent methods for interpretation of the D-1(CH) and the D-n(HH) residual d ipolar coupling data obtained were compared and combined with the NOE and l ong-range H,C and C,C scalar couplings available for this heptasaccharide. A flexible model of the heptasaccharide was determined in which two structu rally well-defined regions involving four and two sugar residues, respectiv ely are joined by a flexible hinge which involves two 1 -->6 glycosidic lin kages.