NMR solution conformation of heparin-derived tetrasaccharide

The solution conformation of the homogeneous, heparin-derived tetrasaccharide ΔUA2S(1 → 4)-α-D-GlcNpS6S(1 → 4)-α-L-IdoAp2S(1 → 4)-α-D-GlcNpS6S (residues A, B, C and D respectively, where IdoA is iduronic acid) has been investigated by using ¹H- and ¹³C-NMR. Ring conformations have been defined by J-coupling constants and inter-proton nuclear Overhauser effects (NOEs), and the orientation of one ring with respect to the other has been defined by inter-ring NOEs. NOE-based conformational modelling has been done by using the iterative relaxation matrix approach (IRMA), restrained molecular dynamics simulations and energy minimization to refine structures and to distinguish between minor structural differences and equilibria between various ring forms. Both glucosamine residues B and D are in the ⁴C₁ chair conformation. The 6-O-sulphate group is oriented in the gauche-trans configuration in the D ring, whereas in the B ring the gauche-gauche rotomer predominates. Uronate (A) and iduronate (C) residues are mostly represented by ¹H₂ and ²S₀ twisted boat forms, respectively, with small deviations in expected coupling constants and NOEs suggesting minor contributions from other A and C ring conformations.