Absolute stereochemistry and preferred conformations of urate degradation intermediates from computed and experimental circular dichroism spectra

The enzymatic oxidation of urate leads to the sequential formation of optically active intermediates with unknown stereochemistry: (−)-5-hydroxyisourate (HIU) and (−)-2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). In accordance with the observation that a defect in HIU hydrolase causes hepatocarcinoma in mouse, a detoxification role has been proposed for the enzymes accelerating the conversion of HIU and OHCU into optically active (+)-allantoin. The enzymatic products of urate oxidation are normally not present in humans, but are formed in patients treated with urate oxidase. We used time-dependent density functional theory (TDDFT) to compute the electronic circular dichroism (ECD) spectra of the chiral compounds of urate degradation (HIU, OHCU, allantoin) and we compared the results with experimentally measured ECD spectra. The calculated ECD spectra for (S)-HIU and (S)-OHCU reproduced well the experimental spectra obtained through the enzymatic degradation of urate. Less conclusive results were obtained with allantoin, although the computed optical rotations in the transparent region supported the original assignment of the (+)-S configuration. These absolute configuration assignments can facilitate the study of the enzymes involved in urate metabolism and help us to understand the mechanism leading to the toxicity of urate oxidation products.