The Elusive Crystal Structure of Uric Acid Dihydrate: Implication for Epitaxial Growth during Biomineralization

The crystal structure of the elusive uric acid dihydrate phase, a known component of human pathological biomineralizations, has been investigated by a combination of synchrotron and conventional X-ray diffraction experiments. C5H4N4O3.2H2O, orthorhombic, Pnab, a = 7.409(1), b = 17.549(3), c = 6.332(1) Å, Z = 4, wR = 0.030 and 0.041 for two independently measured datasets. The molecular packing, encompassing hydrogen-bonded layers of water molecules and statistically disordered organic moieties, fully clarifies on a structural basis the observed epitaxial growth of uric acid dihydrate with associated phases in human stones, such as anhydrous uric acid and whewellite. Synchrotron data confirmed the existence of weak reflections violating the extinction conditions and are interpreted by the presence of short-range ordering of the uric acid molecules at distances of the order of a few adjacent cells.