1,3,7-Trimethyl-2,4-dioxo-1,2,3,4-tetra-hydro-pteridine-6-carboxylic acid hemihydrate.

In the title compound, C(10)H(10)N(4)O(4)·0.5H(2)O, the two rings of the pteridine system are nearly coplanar [dihedral angle = 4.25†(9)°]. The atoms of the carboxyl group are also coplanar with the pteridine unit [r.m.s. deviation from the mean plane of the pteridine skeleton = 0.092†(2)†à ]. In the crystal, the presence of the water molecule of crystallization (O atom site symmetry 2) leads to a hydrogen-bonding pattern different from the one shown by many carboxylic acid compounds (dimers formed through O-H⋯O hydrogen bonds between neighbouring carboxyl groups): in the present structure, the water mol-ecule, which lies on a binary axis, acts as a bridge between two mol-ecules, forming a hydrogen-bonded dimer. In addition to the hydrogen bonds, there are π-π ring stacking inter-actions involving the pyrimidine and pyrazine rings [centroid-centroid distance = 3.689†(1)à ], and two different pyrazine rings [centroid-centroid distance = 3.470†(1)à ]. Finally, there is a C-O⋯π contact involving a carboxyl-ate C-O and the pyrimidine ring with a short O⋯Cg distance of 2.738†(2)†à .