The crystallization of uric acid dihydrate (UAD) from model supersaturated aqueous solutions containing 21 different molecular dye probes and 11 inorganic salts was investigated. UAD crystals grown at room temperature were found to include low concentrations (0.01-0.42%) of cationic and neutral dyes but not anionic dyes. From dilute dye solutions, a preference for inclusion in either {011} or {102} growth sectors was exhibited for some dyes while others showed little specificity. At higher dye concentrations, inclusion generally increased but specificity tended to diminish. Crysoidine G was exceptional in that over a broad range of solution concentrations, it was consistently and exclusively included on growth hillocks located in {011} sectors. Most dye-doped UAD crystals also exhibited morphological changes, namely the development of {210} faces, which are typically absent in pure UAD crystals. UAD grown from salt solutions showed inclusion of Na þ , K þ , and Mg 2þ ions in concentrations similar to dyes; however, none affected the macroscopic morphology. While the literature has numerous examples showing that anionic and polyanionic species can assert dramatic effects on the mineralization of inorganic phases, this study suggests that cationic species may play a more pronounced role in the crystallization of other types of biomaterials.
Uric acid dihydrate (UAD) is a crystalline constituent present in a significant fraction of human renal precipitates. Using a combination of techniques including powder X-ray diffraction, hot-stage light microscopy, differential scanning calorimetry, and thermogravimetric analysis, the mechanism and kinetics of its irreversible dehydration to polycrystalline anhydrous uric acid (UA) is analyzed as a function of intrinsic sample parameters (e.g., crystal size, shape, structure) and environmental conditions (e.g., temperature and humidity). The highly anisotropic dehydration of UAD to UA is rationalized on the basis of its crystal structure and morphology, and appears to involve no other crystalline intermediate phases.Mechanistic models derived from both isothermal and nonisothermal kinetic data support a one-dimensional-phase boundary mechanism. Increasing relative humidity conditions were found to decrease the dehydration kinetics up to a point, after which a dissolution-recrystallization mechanism is enabled, and the conversion of UAD to UA is significantly accelerated.
Anhydrous uric acid (UA) was crystallized from supersaturated aqueous solutions at 37 °C in the presence of 21 different molecular dye probes and 11 inorganic salts. Growth from dilute solutions of all cationic and neutral dyes resulted in hourglass inclusions in {201} and {001} growth sectors. For most dyes, growth at increasingly higher [dye] sol'n led to the appearance of {121} side faces coincident with dye inclusion in {121} growth sectors. Inclusions in UA-dye crystals grown from concentrated dye solutions showed a loss of selectivity in most cases, but four dyes acridine orange, methyl violet, neutral red, and Bismarck brown; exhibited unique inclusion behavior under these conditions. Parallel studies showed Na þ , K þ , and Mg 2þ ions were also included in UA grown from salt solutions, though the presence of salts did not alter the macroscopic morphology. In all cases, the concentration of dopant included in the UA matrix ranged from 0.01 to 0.35%. The growth modification and inclusion behavior observed in UA-dye and UA-salt are compared to that of uric acid dihydrate (UAD), discussed in the preceding paper in this issue
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