Uric Acid Molecules Research Articles

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

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.

Atomic matching catalysis to realize a highly selective and sensitive biomimetic uric acid sensor

A main challenge for biomimetic non-enzyme biosensors is to achieve high selectivity. Herein, an innovative biomimetic non-enzyme sensor for electrochemical detection of uric acid (UA) with high selectivity and sensitivity is realized by growing Prussian blue (PB) nanoparticles on nitrogen-doped carbon nanotubes (N-doped CNTs). The enhancement mechanism of the biomimetic UA sensor is proposed to be atomically matched active sites between two reaction sites (oxygen atoms of 2, 8-trione, 6.9 Å) of UA molecule and two redox centers (FeII on the diagonal, 7.2 Å) of PB. Such an atomically matching manner not only promotes strong adsorption of UA on PB but also selectively enhances electron transfer between reaction sites of UA and active FeII centers of PB. This biomimetic UA sensor can offer great selectivity to avoid interferences from other oxidative and reductive species, showing excellent selectivity. An electrochemical biomimetic sensor based on PB/N-doped CNTs was applied to in situ detect UA in human serum, delivering a wide dynamic detection range (0.001–1 mM) and a low detection limit (0.26 μM). This work provides a high-performance UA sensor while shedding a scientific light on using atomic matching catalysis to fabricate highly sensitive and selective biomimetic sensors.

A novel sensor aluminum silicate modified carbon paste electrode for determination of anti-depressant dothiepin HCl in pharmaceutical formulation and biological fluids

A novel sensor aluminum silicate was used as a cheaper modifier for the rapid and sensitive voltammetric determination of dothiepin HCl (DTP). Cyclic voltammetry, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA) and square-wave voltammetry techniques were used to characterize the properties of the sensor.The best results were obtained when the measurements are carried at pH 7 using a Britton-Robinson buffer.Under optimum conditions, the linearity range was 2 × 10−7–1 × 10−4 M with a correlation coefficient equal to 0.9998. LOD and LOQ were determined to be 4 × 10−8, 12.1 × 10−8, M respectively. The obtained results were compared successfully with respect to those obtained using the official method. Moreover, this sensor is sensitive for the determination of DTP in the presence of ascorbic acid (AA), uric acid (UA), dopamine (DA) and common interference molecules. The high sensitivity, good reproducibility, and wide linear range make the modified electrode suitable for determination of DTP in pharmaceutical preparation, human serum and urine sample with excellent recoveries. The possible electro-oxidation pathway and the number of transferred electrons were also investigated.

Nanosized iron telluride for simultaneous nanomolar voltammetric determination of dopamine, uric acid, guanine and adenine

Herein, an efficient electrochemical sensor based on nano-sized iron telluride material (FeTe2) have been developed for the first time for simultaneous nanomolar determination of dopamine, uric acid, guanine and adenine molecules.

ZIF-67 derived porous Co3O4 hollow nanopolyhedron functionalized solution-gated graphene transistors for simultaneous detection of glucose and uric acid in tears

Biomarkers in tears have attracted much attention in daily healthcare sensing and monitoring. Here, highly sensitive sensors for simultaneous detection of glucose and uric acid are successfully constructed based on solution-gated graphene transistors (SGGTs) with two separate Au gate electrodes, modified with GOx-CHIT and BSA-CHIT respectively. The sensitivity of the SGGT is dramatically improved by co-modifying the Au gate with ZIF-67 derived porous Co3O4 hollow nanopolyhedrons. The sensing mechanism for glucose sensor is attributed to the reaction of H2O2 generated by the oxidation of glucose near the gate, while the sensing mechanism for uric acid is due to the direct electro-oxidation of uric acid molecules on the gate. The optimized glucose and uric acid sensors show the detection limits both down to 100nM, far beyond the sensitivity required for non-invasive detection of glucose and uric acid in tears. The glucose and uric acid in real tear samples was quantitatively detected at 323.2 ± 16.1μM and 98.5 ± 16.3μM by using the functionalized SGGT device. Due to the low-cost, high-biocompatibility and easy-fabrication features of the ZIF-67 derived porous Co3O4 hollow nanopolyhedron, they provide excellent electrocatalytic nanomaterials for enhancing sensitivity of SGGTs for a broad range of disease-related biomarkers.

Environmental and metabolic parameters affecting the uric acid production of Arxula adeninivorans

The yeast Arxula adeninivorans has previously been shown to naturally secrete the redox molecule uric acid (UA). This property suggested that A. adeninivorans may be capable of functioning as the catalyst for a mediator-less yeast-based microbial fuel cell (MFC) if the level of UA it secretes could be increased. We investigated the effects of a number of parameters on the level of UA produced by A. adeninivorans. The concentration of UA accumulated in a dense cell suspension of A. adeninivorans after 20h incubation was shown to be significantly lower in aerated suspensions compared with that in anaerobic conditions due to UA being rapidly oxidised by dissolved oxygen. The presence of carbon sources, glucose and glycerol, both caused a reduction in UA production compared with that in starvation conditions. The transgenic A. adeninivorans strain, G1221 (auox), showed higher UA production at 37°C, but at 47°C, the wild-type LS3 accumulated higher concentrations; however, elevated temperatures also resulted in very high cell mortality rates. An initial buffer pH of 8 caused a higher concentration of UA to accumulate, but high pH is detrimental to cell metabolism and the cells actively work to lower the pH of their environment. It appears that most parameters which increase the amount of UA produced by A. adeninivorans have concomitant disadvantages for cell metabolism, and as such, its potential as a self-mediating MFC catalyst seems doubtful.

Ab initio structure determination of kidney stone potassium quadriurate from synchrotron powder diffraction data, a 150 year problem solved

Abstract The structure of potassium urate, co-crystallized with uric acid K(C5H3N4O3)(C5H4N4O3), from human kidney stones, is determined ab initio from synchrotron powder diffraction data, space group P21/c, a = 3.60504(9) A, b = 19.9293(7) A, c = 18.4841(6) A, β = 100.503(2)°, Z = 4. Each uric acid molecule is connected to a urate anion through three hydrogen bonds, both molecules are related by a pseudo-inversion center and are randomly distributed in the crystal. Two of the oxygen atoms of the uric acid molecule and two of the urate anions are involved in the potassium coordination forming KO7 monocapped trigonal prisms sharing their triangular bases, building infinite chains extending along the short a axis. Such a formulation, though controversial, has been claimed to exist since 1862 and named potassium quadriurate. The crystal structure is optimized by energy minimization (DFT) in the solid state, using a hybrid PBE0 functional. The capacity of the urate anion to connect by strong hydrogen bonds to a uric acid molecule has implications concerning the uric acid transport in biological media.

Uric Acid Spherulites in the Reflector Layer of Firefly Light Organ

BackgroundIn firefly light organs, reflector layer is a specialized tissue which is believed to play a key role for increasing the bioluminescence intensity through reflection. However, the nature of this unique tissue remains elusive. In this report, we investigated the role, fine structure and nature of the reflector layer in the light organ of adult Luciola cerata.Principal FindingsOur results indicated that the reflector layer is capable of reflecting bioluminescence, and contains abundant uric acid. Electron microscopy (EM) demonstrated that the cytosol of the reflector layer's cells is filled with densely packed spherical granules, which should be the uric acid granules. These granules are highly regular in size (∼700 nm in diameter), and exhibit a radial internal structure. X-ray diffraction (XRD) analyses revealed that an intense single peak pattern with a d-spacing value of 0.320 nm is specifically detected in the light organ, and is highly similar to the diffraction peak pattern and d-spacing value of needle-formed crystals of monosodium urate monohydrate. However, the molar ratio evaluation of uric acid to various cations (K+, Na+, Ca2+ and Mg2+) in the light organ deduced that only a few uric acid molecules were in the form of urate salts. Thus, non-salt uric acid should be the source of the diffraction signal detected in the light organ.ConclusionsIn the light organ, the intense single peak diffraction signal might come from a unique needle-like uric acid form, which is different from other known structures of non-salt uric acid form. The finding of a radial structure in the granules of reflector layer implies that the spherical uric acid granules might be formed by the radial arrangement of needle-formed packing matter.

Adhesion Properties of Uric Acid Crystal Surfaces

Two key steps in kidney stone formation--crystal aggregation and attachment to renal tissues--depend on the surface adhesion properties of the crystalline components. Anhydrous uric acid (UA) is the most common organic crystalline phase found in human kidney stones. Using chemical force microscopy, the adhesion force between various functional groups and the largest (100) surface of UA single crystals was measured in both aqueous solution and model urine. Adhesion trends in the two solutions were identical, but were consistently lower in the latter. Changes in the solution ionic strength and pH were also found to affect the magnitude of the adhesion. UA surfaces showed the strongest adhesion to cationic functionalities, which is consistent with ionization of some surface uric acid molecules to urate. Although hydrogen-bonding and van der Waals interactions are usually considered to be dominant forces in the association between neutral organic compounds, this work demonstrates that electrostatic interactions can be important, particularly when dealing with weak acids under certain solution conditions.

Effects of Chinese Liquors on Cardiovascular Disease Risk Factors in Healthy Young Humans

Objectives. To elucidate whether consumption of two Chinese liquors, tea-flavor liquor (TFL) and traditional Chinese liquor (TCL) have protective effects on cardiovascular disease (CVD) risk factors in healthy human subjects. Methods. Forty-five healthy subjects (23 men, 22 women), aged 23–28, were recruited and randomized into two groups: TFL and TCL, and consumed 30 mL/day (45% (v/v) alcohol) of either liquor for 28 days. Results. Serum high-density lipoprotein cholesterol/low-density lipoprotein cholesterol (HDL-C/LDL-C) and apolipoprotein A1 were significantly increased, and total cholesterol (TC) and TC/HDL-C were significantly decreased after the intervention in both groups (P < 0.05). Serum uric acid (P = 0.004 for TFL, P = 0.001 for TCL), glucose (P < 0.001 for TFL, P < 0.001 for TCL) and endothelial adhesion molecules (P < 0.05) were significantly decreased after the intervention. ADP-induced whole blood platelet aggregation was also significantly decreased after the intervention in both TFL and TCL groups (P < 0.05). Conclusions. TFL and TCL consumption had protective effects on CVD risk factors in young humans. However, the results were valid only for 28 days, and that the possibility of adverse effect (liver, kidney) of chronic alcohol consumption should be considered.

Increased concentrations of IL-18 and uric acid in sickle cell anemia: Contribution of hemolysis, endothelial activation and the inflammasome

Sickle cell anemia (SCA) is a common, severe monogenetic disorder characterized by chronic hemolysis, frequent infections, a chronic inflammatory state and recurrent occlusions of the microcirculation, resulting in painful crises, organ damage and premature death. This study evaluated associations between serum levels of IL-18, uric acid, hemolytic markers, and inflammatory molecules in SCA patients. A cross-sectional study was performed including 45 SCA patients (median age of 20.5years) without general symptoms and who had not undergone blood transfusions. Inclusion criteria for the steady-state SCA patients were the absence of hospitalization and the absence of infections. Interleukin-18 and uric acid levels were correlated closely with markers of hemolysis, endothelial dysfunction and others cytokines levels. These findings suggest probable influences of IL-18 and uric acid in the pathophysiology of vascular occlusion in SCA. Additional studies should be performed to characterize similar prognosis markers and possible therapeutic targets.

The comparison of different gold nanoparticles/graphene nanosheets hybrid nanocomposites in electrochemical performance and the construction of a sensitive uric acid electrochemical sensor with novel hybrid nanocomposites

In this paper, water soluble poly(diallyldimethylammonium chloride)–graphene nanosheets (PDDA–GNs) were synthesized and characterized by UV–visible absorption spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the basis of PDDA–GNs, three different types of gold nanoparticles/graphene nanosheets (AuNPs/GNs) hybrid nanocomposites were obtained by one-pot synthesis, in situ reduction and adsorption methods, respectively. These nanocomposites were used as electrode materials for electrochemical determination of uric acid (UA). The results indicated adsorption to be the best method to synthesize hybrid nanocomposites from the electrochemical point of view. Given the fact positively charged PDDA–AuNPs could interact with negatively charged UA molecules, we then synthesized PDDA–protected gold nanoparticles/graphene nanosheets (PDDA–AuNPs/GNs) hybrid nanocomposites by adsorption method, for the first time. As were expected, PDDA–AuNPs/GNs gave better performance for UA than AuNPs/GNs obtained by adsorption, and the anodic peak current of UA obtained by cyclic voltammetry (CV) increased 102.1-fold in comparison to bare GCE under optimizing conditions. Differential pulse voltammetry (DPV) was used to quantitatively determine UA. The linear range of UA was from 0.5μM to 20μM and the detection limit was 0.1μM (S/N=3) with a high sensitivity of 103.08μAμM−1cm−2. The assay results of urine sample provided satisfying recoveries by standard addition method. In addition, the anodic peaks of adrenaline (AD) and UA were well resolved at PDDA–AuNPs/GNs modified electrode, while they were too overlapped to separate at bare electrode, as a result of that UA was successfully detected in the presence of AD. In conclusion, rapid synthesis of PDDA–AuNPs/GNs were realized and applied as an advanced hybrid electrode material for UA determination.

Preparation and recognition performance of uric acid-imprinted material prepared with novel surface imprinting technique

Acrylonitrile (AN) was first graft-polymerized on the surfaces of crosslinked polyvinyl alcohol (CPVA) microspheres by initiating of cerium salt, and then the grafted polyacrylonitrile (PAN) was transformed to polyamidoxime (PAO) via amidoximation transform reaction, resulting in the functional microspheres PAO/CPVA. By adopting the novel surface-molecular imprinting technique put forward by us, uric acid molecule -imprinted material MIP -PAO/CPVA was prepared with glutaraldehyde as crosslinking agent The binding character of MIP -PAO/CPVA towards uric acid was investigated in depth with both batch and column methods and using guanine as a contrast substance whose chemical structure is similar to uric acid to a certain extent. The experimental results show that the surface imprinted material MIP -PAO/CPVA has excellent binding affinity (a great binding capacity of 104 mg/g) and high recognition selectivity for the template molecule, uric acid. The selectivity coefficient of PAO/CPVA microspheres (non-imprinted material) for uric acid relative to guanine is only 1.273, displaying no recognition selectivity for uric acid. However, after imprinting, the selectivity coefficient of MIP -PAO/CPVA for uric acid in respect to guanine is remarkably enhanced to 14.00, displaying the excellent recognition selectivity and binding affinity towards uric acid molecules.

Course of matrix metalloproteinase-9 isoforms after the administration of uric acid in patients with acute stroke

Oxidative stress as well as expression and activity of matrix metalloproteinase 9 (MMP-9) are rapidly enhanced after cerebral ischemia. The magnitude of these effects is related to stroke outcome. In human stroke, the extent of oxidative stress correlates well with increased MMP-9 expression. The aim of this study was to evaluate whether treatment with the antioxidant molecule uric acid (UA) decreased the levels of MMP-9 in stroke patients treated with rtPA. The patients were part of a pilot, double-blind, randomized, vehicle-controlled study of patients with acute stroke treated with rtPA (< 3 h) and randomized to receive an intravenous infusion of UA (n = 16) or vehicle (n = 8). Total matrix metalloproteinase (tMMP)-9 and active (aMMP-9) levels were measured in serum at baseline (< 3 h), at the end of study treatment infusion (< 5.5 h), and at 48 hours. Total MMP-9 and aMMP-9 increased very early after stroke onset in patients allocated vehicle after rtPA therapy. Lower increments of aMMP-9 were associated with better outcome at 3 months. UA treatment was associated with reduced levels of aMMP-9 at T1 (p < 0.02) in multivariate models adjusted for age, NIHSS score, and baseline aMMP-9 levels. The decline of aMMP-9 attained after UA administration supports further clinical assessment of UA therapy in patients with acute stroke.

Cerebrospinal fluid and serum uric acid levels in patients with multiple sclerosis

Peroxynitrite was hypothesized to be involved in the pathogenesis of multiple sclerosis (MS) through its various neurotoxic effects. Uric acid (UA) was shown to be a strong peroxynitrite scavenger. We analyzed cerebrospinal fluid (CSF) and serum UA concentrations in 30 MS patients and 20 controls with non-inflammatory neurological diseases (NIND) and correlated these findings with demographic and clinical characteristics of MS patients. Disease activity was assessed by brain magnetic resonance imaging (MRI) and the CSF/serum albumin quotient as an indicator of the state of blood-brain-barrier (BBB). Serum UA concentrations were found to be significantly lower in MS patients compared with controls (p=0.019). CSF UA concentrations were lower in MS patients as compared to controls, as well as in patients with active MS (clinical and/or MRI activity) in comparison to patients with inactive MS or controls, but these differences were not statistically significant. Significant correlation was found between CSF and serum UA concentrations (p=0.016) in MS patients, but not in controls; and between CSF UA concentrations and the CSF/serum albumin quotient in MS patients (p=0.043), but not in controls. Our results support the significance of UA in the pathogenesis of MS. Decreased serum UA concentrations in MS patients might be due to both intrinsically reduced antioxidant capacity and increased UA consumption in MS. CSF UA concentrations may not be a reliable marker of disease activity in MS since its concentration is dependent on leakage of UA molecules from serum through the damaged BBB and the balance between consumption/production within the central nervous system (CNS).

Sialochemical and oxidative analyses in radioactive I131-treated patients with thyroid carcinoma

I131 in relatively high doses has been shown in the past to cause damaging salivary effects and oral discomfort in patients. Although lower dosage is now widely accepted, I131 may still be the source of salivary damage over the long-term and subsequent harmful effects on both the oral cavity and the gastrointestinal tract, into which the saliva is swallowed. This study examined the effects of radioactive I131 on salivary gland activity, saliva composition and oxidative profile, and related oral discomfort complaints following thyroidectomy due to carcinoma of thyroid gland. Out of 40 consenting female post-thyroidectomy patients, 23 (mean age 50+/-4 years old) were treated with I131 while 17 (mean age 46+/-4) were not. Whole saliva from all subjects was analyzed for antioxidant and biochemical composition and flow rate. The salivary flow rates of both groups were similar but their composition differed considerably. Salivary superoxide dismutase enzyme (SOD), total protein, and albumin concentrations were significantly reduced in the treated patients by 40, 25, and 18% respectively (P<0.05), as were all other salivary antioxidants. Oral discomfort complaints were far more prevalent in the I131-treated patients. I131-dependent damage to the salivary glands was evidenced by a broad spectrum of compositional alterations and oral complaints. Reduction in salivary antioxidant status, SOD enzyme, and the uric acid molecule leaves the oral cavity less protected against oxidative stress. This is the first report of radioactive I131 treatment being harmful to salivary glands due to compromised salivary compositional and oxidative profile and oral discomfort complaints.

Biotransformation of nitric oxide in the cerebrospinal fluid of amyotrophic lateral sclerosis patients

Recent findings indicate that nitric oxide (NO•) over-production might be an important factor in the pathogenesis of sporadic amyotrophic lateral sclerosis (SALS). We measured significantly higher concentrations of uric acid and thiol group-containing molecules (R–SH groups) in the cerebrospinal fluid (CSF) from SALS patients compared to controls. The above factors, together with a slightly increased free iron concentration found in the CSF, favour conditions necessary for the formation of the dinitrosyl iron complex, capable of NO• bio-transformation. Thus, we performed ex vivo saturation of CSF (from both SALS patients and controls) with NO•. A decrease in the level of R–SH was found. This was more pronounced in the CSF from SALS patients. In the CSF from SALS patients the production of nitrite and hydroxylamine was greater than that observed in the CSF from controls. Moreover, we also found increased Cu,Zn-SOD activity in the CSF from SALS patients (when compared to control subjects) but no activity corresponding to Mn-SOD in any CSF samples. As Cu,Zn-SOD can react with nitroxyl forming NO•, the conditions for a closed, but continuous, loop of NO• biotransformation are present in the CSF of ALS patients.

Uric acid monohydrate—a new urinary calculus phase

In our laboratory more than 100,000 urinary calculi have been analysed since 1972. Amongst this huge sample, 15 specimens originating from a total of eight patients were observed showing similar characteristics but escaping unambiguous identification with any of the substances that have been described so far in urinary concrements. Therefore, the unknown substance was submitted to a more extended analytical regimen. Structural analysis by x-ray crystallography turned out to be most successful, identifying the unknown material as uric acid monohydrate. Uric acid monohydrate crystallizes in the monocline space group P2(1)/c. Within the crystal, uric acid and water molecules form continuous layers by hydrogen bonds. This is in contrast to uric acid in its water free and its dihydrate forms, which both crystallize by forming 3-dimensional networks To the best of our knowledge , the existence of a monohydrate form of uric acid has not been reported so far. Accordingly, this is the first report on uric acid monohydrate as a urinary stone component. The frequency of only 0.015% in our survey indicates that uric acid monohydrate is rarely the main component in concrements, in contrast to uric acid and uric acid dihydrate with frequencies of 10% and 6%, respectively. The infrared spectrum of uric acid monohydrate is very similar to that of the other crystal forms of uric acid. Because of this similarity and its low frequency, uric acid monohydrate may have been overlooked as a component of urinary concrements. X-ray diffraction allows for better differentiation in routine stone analysis. All samples of uric acid monohydrate were found by solid state NMR spectroscopy to be highly contaminated by amorphous material. This material consisted of long aliphatic chains reminiscent of lipids and fatty acids, respectively. Concrements consisting of other forms of uric acid or urate lacked this amorphous component. Therefore, a role of this aliphatic material has to be taken into consideration when discussing the conditions that may favour the rare formation of concrements from uric acid monohydrate. As for as the metabolic situation of the affected patients is concerned, no common peculiarities became evident by a retrospective survey.

Quantum chemical studies on tautomerization of uric acid

The preferred tautomeric form of biologically active uric acid molecule was predicted using the gas phase semi-empirically calculated relative stabilities, aqueous phase calculated acidity constants and the other molecular properties. The obtained results had indicated that the trioxo, dioxo or monooxo forms are favored over the dihydroxy, monohydroxy or trihydroxy forms tautomerically, successively for ring-chain tautomerizm. The same trend, however, was not observed for annular the tautomerizm.

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 epitaxic 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.