Various crystalline phases of uric acid are frequently identified components of human kidney stones, including anhydrous uric acid (UA) and uric acid dihydrate (UAD).
As
part of an ongoing study to investigate the diversity of uric
acid salts associated with crystal deposition diseases, calcium diurate
hexahydrate (CaUr2·6H2O) was prepared and
characterized by single crystal X-ray diffraction. The Ca2+-urate interactions observed are discussed in relation to previous
theoretical binding studies and other known urate salts.
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.
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