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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) Å]. 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)Å], and two different pyrazine rings [centroid-centroid distance = 3.470 (1)Å]. 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) Å.

1,3,7-Trimethyl-2,4-dioxo-1,2,3,4-tetrahydropteridine-6-carboxylic acid hemihydrate