Thermal Behavior and Phase Transition of Uric Acid and Its Dihydrate Form, the Common Biominerals Uricite and Tinnunculite

Abstract

Single crystals and powder samples of uric acid and uric acid dihydrate, known as uricite and tinnunculite biominerals, were extracted from renal stones and studied using single-crystal and powder X-ray diffraction (SC and PXRD) at various temperatures, as well as IR spectroscopy. The results of high-temperature PXRD experiments revealed that the structure of uricite is stable up to 380 °C, and then it loses crystallinity. The crystal structure of tinnunculite is relatively stable up to 40 °C, whereas above this temperature, rapid release of H2O molecules occurs followed by the direct transition to uricite phase without intermediate hydration states. SCXRD studies and IR spectroscopy data confirmed the similarity of uricite and tinnunculite crystal structures. SCXRD at low temperatures allowed us to determine the dynamics of the unit cells induced by temperature variations. The thermal behavior of uricite and tinnunculite is essentially anisotropic; the structures not only expand, but also contract with temperature increase. The maximal expansion occurs along the unit cell parameter of 7 Å (b in uricite and a in tinnunculite) as a result of the shifts of chains of H-bonded uric acid molecules and relaxation of the π-stacking forces, the weakest intermolecular interactions in these structures. The strongest contraction in the structure of uricite occurs perpendicular to the (101) plane, which is due to the orthogonalization of the monoclinic angle. The structure of tinnunculite also contracts along the [010] direction, which is mostly due to the stretching mechanism of the uric acid chains. These phase transitions that occur within the range of physiological temperatures emphasize the particular importance of the structural studies within the urate system, due to their importance in terms of human health. The removal of supersaturation in uric acid in urine at the initial stages of stone formation can occur due to the formation of metastable uric acid dihydrate in accordance with the Ostwald rule, which would serve as a nucleus for the subsequent growth of the stone at further formation stages; afterward, it irreversibly dehydrates into anhydrous uric acid.

Dmisteinbergite, CaAl2Si2O8, a Metastable Polymorph of Anorthite: Crystal-Structure and Raman Spectroscopic Study of the Holotype Specimen

“…The calculations of the structural complexity parameters for different anorthite polymorphs were performed using the TOPOSPro software package [58] and the values are given in Table 6. It can be seen that the crystal structures of the metastable CaAl2Si2O8 polymorphs dmisteinbergite and svyatoslavite are much simpler than that of anorthite, in good agreement with Goldsmith's principle and similar previous observations [22][23][24][25][26][27][28]59,60].…”

Accordion‐Like Carbon with High Nitrogen Doping for Fast and Stable K Ion Storage

“…UA has a melting point > 300 °C, while it has a decomposition temperature around 400 °C, which indicates that the decomposition of UA is accompanied by melting. [ 30 ] We tracked the pyrolysis behavior of UA by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) ( Figure 1 a). UA experienced an endothermic process from 380 to 520 °C with a peak centering at 440 °C.…”

“…UA sample treated with an annealing temperature of X °C is labeled as UA‐X. The XRD pattern of UA‐380 showed a strong peak at 27.7 o (Figure S5a, Supporting Information) assigned to the (120) crystalline plane of UA (Figure S6, Supporting Information), [ 30 ] while the other diffraction peaks of UA disappeared. This could be due to the reason that the parallel π‐π stacking of UA molecules is more stable and could be maintained to higher temperatures.…”

Thermal behavior of uranyl selenite minerals derriksite and demesmaekerite

“…Although this direction corresponds to the space between the uranyl selenite chains with the deficiency of strong bonds, the contraction of the structure and rotation of the figure of the TEC should be mainly attributed to the orthogonalization of the oblique triclinic angles of the unit cell. This effect was described by Filatov (2008) and recently observed in (Izatulina et al 2018(Izatulina et al , 2019. Along the [010], the linkage between the U-bearing chains and interlayer Cu-Pb complex occurs via sharing O atoms of the selenite groups, which explains the insignificant expansion of the structure in this direction.…”