Redetermination of orotic acid monohydrate

The crystal structure of the title compound, which is also known as vitamin B13 (systematic name: 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid monohydrate), C5H4N2O4·H2O, was reported for the first time by Takusagawa & Shimada [Bull. Chem. Soc. Jpn (1973 ▶), 46, 2011–2019]. The present redetermination provides more precise values of the molecular geometry. The asymmetric unit comprises a planar diketo tautomer and a solvent water molecule. In the crystal structure, molecules are connected by O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds involving NH groups, two carbonyl O atoms and the solvent water molecule.

The crystal structure of the title compound, which is also known as vitamin B 13 (systematic name: 2,6-dioxo-1,2,3,6tetrahydropyrimidine-4-carboxylic acid monohydrate), C 5 H 4 N 2 O 4 ÁH 2 O, was reported for the first time by Takusagawa & Shimada [Bull. Chem. Soc. Jpn (1973), 46, 2011-2019. The present redetermination provides more precise values of the molecular geometry. The asymmetric unit comprises a planar diketo tautomer and a solvent water molecule. In the crystal structure, molecules are connected by O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds involving NH groups, two carbonyl O atoms and the solvent water molecule.
The author thanks MIUR (Rome) for financial support in 2006 of the project 'X-ray diffractometry and spectrometry'.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: KP2160).

Comment
Orotic acid monohydrate, the only effective precursor in the biosynthesis of pyrimidine nucleobases, was determined some 35 years ago (Takusagawa & Shimada, 1973). In this study, 1488 unique reflections were collected at an ambient temperature by photographic techniques using Cu Kα radiation. 1344 of these, having values significantly above background, were estimated visually and no absorption correction was applied [µ(Cu Kα) = 15.4 cm −1 ]. The final refinement led to R = 0.058 with standard deviations of 0.005Å in C-C bond lengths, As a part of a more general study of multiple hydrogen-bonded DNA/RNA-nucleobases as potential supramolecular reagents (Brunetti et al., 2000(Brunetti et al., , 2002Portalone et al., 1999;Portalone & Colapietro, 2007), this paper reports a redetermination of the crystal structure of the title compound, (I), with greater precision and accuracy. The asymmetric unit of (I) (Fig. 1) comprises a planar diketo tautomer and a crystal water molecule.

Experimental
The title compound (0.1 mmol, Sigma Aldrich of 98% purity) was dissolved in water (9 ml) and heated under reflux for 2 h. After cooling the solution to ambient temperature, crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation.

Refinement
All H atoms were found in a difference map and refined isotropically.

Special details
Experimental. Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.