A force-field study of the conformational characteristics of the iduronate ring

Methods of molecular mechanics were applied to investigate the conformation of the (methyl 2-O-sulfate-4-methyl-α-L-idopyranose) uronic acid (DMIS), in order to correlate the peculiar vicinal proton coupling constants observed in polysaccharides containing the iduronate ring to the conformational characteristics of this sugar ring. We found three conformers with comparable energies, namely the two chair forms ¹ C₄ and ⁴ C₁ and the skew-boat form ² S₀ (L); the latter is separated from each chair form by a barrier of about 9 kcal/mol. Along the pseudorotational path three additional minima (³ S₁ , ¹ S₃ , and ¹ S₅ ) were found, yet at least 4 kcal/mol higher than ² S₀ . The results obtained for the relative energies of the three conformers and the conformation of the side groups were affected by the inclusion of the electrostatic term and, in particular, by the charge assigned to the ionic groups of DMIS. However, the conformational properties of the idopyranosidic ring in DMIS (and in related compounds) should still be interpreted in terms of equilibrium among these three conformers only.