Effect of the substituents of the neighboring ring in the conformational equilibrium of iduronate in heparin-like trisaccharides

Based on the structure of the regular heparin, we have prepared a smart library of heparin-like trisaccharides by incorporating some sulfate groups in the sequence a-D-GlcNS- (1-4)-a-L-Ido2S-(1-4)-a-D-GlcN. According to the 3Dstructure of heparin, which features one helix turn every four residues, this fragment corresponds to the minimum binding motif. We have performed a complete NMR study and found that the trisaccharides have a similar 3Dstructure to regular heparin itself, but their spectral properties are such that allow to extract very detailed information about distances and coupling constants as they are isotropic molecules. The characteristic conformational equilibrium of the central iduronate ring has been analyzed combining NMR and molecular dynamics and the populations of the conformers of the central iduronate ring have been calculated. We have found that in those compounds lacking the sulfate group at position6 of the reducing end glucosamine, the population of S of the central iduronate residue is sensitive to the temperature decreasing to 19 % at 278K. On the contrary, the trisaccharides with 6-O-sulfate in the reducing end glucosamine keep the level of population constant with temperature circa 40% of S similar to that observed at room temperature. Another structural feature that has been revealed through this analysis is the larger flexibility of the L-IdoAS- D-GlcN glycosidic linkage, compared with the D-GlcNS-L-IdoA. We propose that this is the point where the heparin chain is bended to form structures far from the regular helix known as kink that have been proposed to play an important role in the specificity of the heparin-protein interaction. The sulfation pattern of the neighboring ring drives the conformational equilibrium of iduronate in heparin trisaccharides. A library of heparin-like trisaccharides incorporating sulfate groups in key positions has been prepared. The conformational equilibrium of the central iduronate ring has been analyzed and the populations were calculated by combining NMR and molecular dynamics (see figure).