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

Abstract

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. CsHnNaO3.2H20, orthorhombic, Pnab, a= 7.409(1), b = 17.549 (3), c = 6.332(1),&, 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.

Understanding geology through crystal engineering: coordination complexes, coordination polymers and metal–organic frameworks as minerals

“…As the dimensions and structure of uric acid ribbon motifs remain virtually unchanged between the two crystal structures, the unit-cell dimensions of the two forms of mineral uric acid are very close with the ribbons extending in the 210and ( 2 210) directions in the dihydrate and the directions (021) and (0 2 21) in the anhydrous structure. This is consistent with the observation that the anhydrous form of uric acid forms epitaxially on crystals of the dihydrate, with contact between the two phases being formed between the (010) planes of the previously considered orthorhombic structure of the dihydrate and the (100) planes of the anhydrous compound (Artioli et al, 1997). Interestingly, synthetic crystals of uric acid dihydrate undergo a rapid, irreversible dehydration at 298 K in air (Zellelow et al, 2010), or somewhat slower in aqueous solutions, but the natural occurring forms were observed to be more stable under the same conditions.…”

Solid-State Dehydration of Uric Acid Dihydrate

“…Effect of relative humidity (%RH) on the dehydration of UAD: 9 = 23%RH, red solid circles = 33%RH, green triangles = 43% RH, orange triangles = 55%RH, blue diamonds = 75%RH, and gray triangles = 98%RH. observed in the oriented epitaxial growth of UA on UAD surfaces in aqueous solutions, 16,36,37 further supporting a dissolution-recrystallization pathway.…”

X-ray and Neutron Diffraction in the Study of Organic Crystalline Hydrates

“…The authors describe the dehydration as reversible while it cannot be said whether the hydrate is stoichiometric or non-stoichiometric since the rehydration was carried out in a water saturated atmosphere-a condition in which both kinds of hydrates can be rehydrated. Some hydrates contain the water in discrete layers in the crystal structure, as shown by the example of uric acid dihydrate ( Figure 6) [46][47][48]. Although this feature is closely connected to higher guest mobility and thus could be assumed to be most common in non-stoichiometric hydrates, uric acid dihydrate in fact represents a stoichiometric hydrate which undergoes strong pseudomorphosis when dried.…”