Conformational analysis of a flexible oligosaccharide using residual dipolar couplings

We present a new approach to the analysis of the conformational and the mot ional properties of an oligosaccharide, methyl 3,6-di-O-(alpha -D-mannopyra nosyl)-alpha -D-mannpyranoside. The approach relies on an order matrix anal ysis of residual dipolar couplings in the solution state. By combining a nu mber of different types of couplings, D-I(CH), D-2(CH), and D-HH, an order matrix is solved for each ring of the trimannoside. The resulting order par ameters indicate the internal motion at the alpha (1,3) linkage to be limit ed, while significant motion is suggested at the alpha (1,6) linkage. Two s tructures for the trimannoside were determined by aligning the order tensor principal axes obtained from two different orienting media, bicelles and p hage. The very similar conformations at the alpha (1,3) linkage of these tw o structures confirm that the internal motion at the alpha (1,3) linkage is small and the conformation is a good representation of a single preferred structure. The different conformations at the alpha (1,6) linkage suggest t hat the motional amplitudes are large and the conformations must be viewed as virtual conformers. Compared with traditional NMR methods, data acquisit ion is easy and data analysis is straightforward.