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

Renal stones are heterogeneous composites of numerous microscopic crystals (e.g., calcium oxalate, calcium phosphate, uric acid, etc.) and 2–3 wt % amorphous organic “matrix”. Uric acid kidney stones are often red–orange–brown in color, though uric acid crystals are colorless. The stone color originates from a variety of components in the matrix, some of which are a broad range of urinary pigments or urochrome. Herein, we report the first definitive structure of one of these pigments, urorosein, and its ability to form intracrystalline inclusions in single crystals of both anhydrous uric acid and uric acid dihydrate. The preferred orientation of the included urorosein molecules in the uric acid crystals was determined through polarized light microspectroscopy. On the basis of these results, it seems likely that other urochrome pigments can locate in both intercrystalline and intracrystalline spaces in urinary precipitates. This expands the conventional picture of where “matrix” resides in these composite materials.

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