Evidence of Conformational Heterogeneity for Carbohydrate Mimetics. NMR Study of Methyl β-C-Lactoside in Aqueous Solution

The structure and internal dynamics of methyl β- C -lactoside in aqueous solution has been derived by 1 H NMR and molecular modeling. A simulated annealing procedure under nuclear Overhauser effect (nOe) restraints produced a family of 50 structures. However, no single one of them was satisfactory; for each a rather important deviation between experimental and simulated data was found. Examination of the H−H correlation times given by off-resonance ROESY and comparison of the experimental and simulated coupling constants related to the interglycosidic dihedral angles indicated that the problem arose from internal mobility around the ψ angle. Using the result of a φ/ψ map, we have tried to find the best agreement between experimental and back-calculated nOe distances with two families having the same angle φ but different ψ values. The best fit is obtained for 40% of (φ A = 295°; ψ A = 300°) and 60% of (φ B = 295°; ψ B = 130°). This result represents the first step toward comparison of the structure and dynamics of C - and O -glycosides in aqueous solution.