Free energy landscapes of iduronic acid and related monosaccharides

The pyranose ring of l-iduronic acid (IdoA), a major constituent of the anticoagulant heparin, is an equilibrium of multiple ring puckers that have evaded quantification by experiment or computation. In order to resolve this enigma, we have calculated the free energy landscape of IdoA and two related monosaccharides from extensive microsecond simulations. After establishing that the simulated puckers had reached equilibrium, hypotheses were confirmed that (a) IdoA 1C4-and 4C1-chair conformations exchange on the microsecond time scale, (b) C5 epimerization leads to a 4C1-chair, and (c) IdoA 2-O-sulfation (IdoA2S) stabilizes the 1C4 conformer. The IdoA and IdoA2S 1C4 conformers were isoenergetic and computed to be 0.9 and 2.6 kcal mol-1 lower in free energy than their respective 4C1-chair conformations. The simulations also predicted that the IdoA 2SO-skew-boat was less populated than previously thought. Novel chemical synthesis and ultra-high-field NMR supported these observations, but slight discrepancies in observed and predicted NMR vicinal couplings implied that the simulation overestimated the population of the IdoA 4C1-chair with respect to 1C 4-chair due to small force field inaccuracies that only manifest in long simulations. These free-energy calculations drive improvements in computational methods and provide a novel route to carbohydrate mimetic biomaterials and pharmaceuticals. © 2010 American Chemical Society.