Gas-Phase and Solution Conformations of the α-L-Iduronic Acid Structural Unit of Heparin

The IdoA2S structural unit of heparin (subunit G) may oscillate among the three conformations (4C1, 1C4, and 2So). Only the twisted boat conformation allowed the biologically active pentasaccharide unit of heparin (DEFGH) to bind to antithrombin. Our work reports, in detail, the results of systematic large-scale theoretical investigations of the three basic conformations (4C1, 1C4, and 2So) of the IdoA2S structural unit of heparin, its anionic forms, and its sodium salt using the B3LYP/6-311++G(d, p) and B3LYP/6-31+G(d) model chemistries. According to our calculations, the most stable structure of these molecules corresponds to the 2So skew-boat conformation. This form is also the most stable in a water solution. The 2So conformation of neutral molecules is not maintained in the anionic species. With anions, both 1C4 and 4C1 conformations are present. The relative stability of individual species of the substituted iduronic acid affects extra stabilization by means of intramolecular hydrogen bonds. The calculated macroscopic pK a of 1,4-DiOMe IdoA2S are as follows: pK a = 0.25 for the terminal C(2)−OSO3H group, pK a = 3.67 for the terminal C(5)−CO2H group, and pK a = 14.00 for the C(3)−OH hydroxyl group. The computed Gibbs interaction energies, ΔG°, for the reaction 1,4-DiOMe IdoA2S2- + 2Na+ ⇌ 1,4-DiOMe IdoA2SNa2 (4C1, 1C4, and 2So conformations) are negative and span a rather small energy interval (from −1244 to −1290 kJ mol-1).