Conformational equilibria of alpha-L-iduronate residues in disaccharides derived from heparin

The disaccharides IdoA(2SO3)-anManOH(6SO3) and IdoA-anManOH (where IdoA represents alpha-L-iduronate, anManOH represents 2,5-anhydro-D-mannitol and SO3 represents sulphate ester) were prepared from bovine lung heparin using HNO2 depolymerization, borohydride reduction and desulphation, and were examined by 400 MHz 1H-n.m.r. spectroscopy. Three-bond proton-proton coupling constants around the IdoA ring were determined under a range of experimental conditions. For unsulphated IdoA all four proton-proton coupling constants varied markedly as a function of temperature, pH and solvent, providing clear evidence for a rapid conformational equilibrium. These data were analysed in terms of the three most energetically stable IdoA conformers: 1C4, 4C1, and 2S0. Predicted coupling constants for these conformers were determined using a modified Karplus-type relationship. For unsulphated IdoA in dimethyl sulphoxide the equilibrium was provoked strongly in favour of a slightly distorted 4C1 'chair' IdoA conformer for which coupling constants have not previously been reported. For sulphated IdoA in aqueous conditions and at low pH the equilibrium is strongly in favour of the alternative 1C4 chair conformer. Under many conditions, however, significant contributions from all three conformers occur for the non-reducing terminal IdoA in these disaccharides.