A Uric Acid Polymorph Generated via Thermal Decomposition

Uric acid is a metabolic product of purine metabolism and a normal constituent of urine. It can exist in multiple solid forms, e.g. hydrates and salts, though solution crystallization methods have to date yielded only one anhydrous polymorph (UA, Form I). Here, we report a novel anhydrate (UA, Form III), which is generated via the solid-state decomposition of ammonium urate hydrate at temperatures above 200 °C. UA Forms I and III are topologically similar, but have b-axes that differ by ∼0.2 Å. The distinct a/b ratios create features in the X-ray powder diffraction patterns that unambiguously distinguish the two forms. A structure model for Form III was developed using Rietveld refinement methods. Differences in the electron density delocalization in the two forms suggest that Form III consists of a mix of tautomers, whereas Form I consists of molecules exclusively in the triketo form. The compositions are rationalized based on DFT calculations, which predict a significant population of higher-energy tautomers is present under the thermal conditions used to prepare Form III. By contrast, Form I crystallizes from aqueous solutions where the lowest-energy tautomer has an abundance of >99%.