The Structure of Uric Acid Dihydrate Crystals Revisited via First-Principle Methods

Abstract

The crystal structure of uric acid dihydrate (UAD) was reexamined using theoretical methods based on density functional theory. Different arrangements for the uric acid molecules in the structure were considered as a first attempt to investigate the disorder experimentally observed. The five possible periodic models obtained by allowing different orientations of the four molecules in the unit cell were generated and optimized; one of the monoclinic P21/c structures turns out to be the most stable and therefore the most likely, according to existing experimental evidence. The structural variations related to modifying the orientations of uric acid molecules are minor, indicating that molecules or groups of molecules could easily be incorporated into the most stable monoclinic P21/c phase in ‘flipped’ configurations without causing major distortion of the overall structure. These results contribute to explaining the reasons for the disorder observed in both natural and synthetic samples, while providing accurate data on realistic and hypothetical structures that can be used as a reference for developing new approximated models for predicting structural and crystal growth features of uric acid crystal phases.