L-Iduronic acid (IdoA) is the major uronic acid component of the glycosaminoglycans (GAGs) dermatan sulfate, and heparin. It is also present in heparan sulfate although here in a minor amount relative to its carbon-5 epimer glucuronic acid. IdoA is a hexapyranose sugar. Most hexapyranoses are stable in one of two chair conformations ¹C₄ or ⁴C₁. L-iduronate is different and adopts more than one solution conformation, with an equilibrium existing between three low-energy conformers. These are the ¹C₄ and ⁴C₁ chair forms and an additional ²S₀ skew-boat conformation.