Conformational structure of some trimeric and pentameric structural units of heparin.

The molecular structure of trimeric units (D-E-F and F-G-H) and the pentamer D-E-F-G-H of heparin (sodium salts and their anionic forms) was studied using the B3LYP/6-31G(d) method. The equilibrium structure of the sodium salts of the trimers and pentamer investigated in the isolated state was determined by multidentate coordination of the sodium cations with oxygen atoms of the sulfate, carboxyl, and hydroxyl (hydroxymethyl) groups, respectively. The displacement of Na+ ions from the binding sites in the sodium salt of oligosaccharides studied resulted in the appreciable change of the overall conformation of the corresponding anion. Upon dissociation, a large change in both the position of the sulfate groups and the conformation across the glycosidic bonds was observed. The stable energy conformations around the glysosidic bonds found for the pentamer investigated are compared and discussed with the available experimental X-ray structural data for the structurally related heparin-derived pentasaccharides in cocrystals with proteins.