Conformer populations of L-iduronic acid residues in glycosaminoglycan sequences.

The 1H-n.m.r. 3J values for the L-iduronic acid (IdoA) residues for solutions in D2O of natural and synthetic oligosaccharides that represent the biologically important sequences of dermatan sulfate, heparan sulfate, and heparin have been rationalized by force-field calculations. The relative proportions of the low-energy conformers 1C4, 2S0, and 4C1 vary widely as a function of sequence and of pattern of sulfation. When IdoA or IdoA-2-sulfate units are present inside saccharide sequences, only 1C4 and 2S0 conformations contribute significantly to the equilibrium. This equilibrium is displaced towards the 2S0 form when IdoA-2-sulfate is preceded by a 3-O-sulfated amino sugar residue, and towards the 1C4 form when it is a non-reducing terminal. For terminal non-sulfated IdoA, the 4C1 form also contributes to the equilibrium. N.O.e. data confirm these conclusions. Possible biological implications of the conformational flexibility and the counter-ion induced changes in conformer populations are discussed.