High-field NMR spectroscopy of synthetic heparin-like di- and trisaccharides has afforded a complete set of inter-proton-coupling data on α-L-idopyranosyluronic acid, which have been used for conformational analysis. These data lead to the conclusion that α-L-iduronic acid may display considerable conformational freedom including 1C4, 4C1 and 2S0 (e.g. compound 1) conformers.
The 2-O-sulphate substituent at iduronic acid tends to stabilize the 1C4 conformation (e.g. compounds 2 and 4). In addition carbohydrate substituents at position 4 hinder the 4C1 conformation (e.g. compound 3a,b versus 1).
Furthermore, the conformation of α-L-idopyranosyluronic acid 2-sulphate is affected seriously by the substituent at position 2 of the non-reducing glucosamine residue. Thus the presence of an ammonium group (i.e. compound 7) leads to unexpected deformation of the 1C4 form of iduronic acid, whereas a sulphamino substituent at this position (e.g. compound 5a,b) brings about the 2S0 skew boat conformation.
The iduronic acid moiety of a trisaccharide (compound 6), which represents a part of the anti-thrombin III-binding site of heparin, shows conformational preference for the 2S0 form in aqueous solution under low ionic conditions. However, increasing the ionic strength (e.g. 3 M NaCl) causes the conformational equilibrium to incline towards the 1C4 chair conformer. This feature may be indicative for the mechanism of conformational control as exerted by the AT-III protein upon heparin binding.
Several molecular models of preferred conformers have been constructed; Nuclear Overhauser Enhancement data, exo-anomeric effect and Van der Waals interactions have been taken into account.