The xanthine crystal structure contains a two-dimensional hydrogen bonded network (Figure ), with a mean deviation from the (101 ̅ ) plane of 0.0825 Å. There are six hydrogen bonds in total, which link the two independent molecules: there are no such interactions between symmetry-equivalent molecules. The hydrogen bonds can be sub-divided into four N-H … O interactions that link pyrimidinedione groups and two N-H … N contacts between imidazole rings. The hydrogen bonded layers are approximately parallel with the (101 ̅ ) planes of the unit cell (Figure ). The separation between the layers is approximately 3.2 Å, consistent with the presence of weak van der Waals' interactions. The hydrogen bonding in the xanthine structure bears a high resemblance to that observed in hypoxanthine crystals which also has two N-H … N contacts between imidazole rings . However, due to the lack of a 2 nd oxygen in hypoxanthine, there are only two N-H … O interactions that link pyrimidinedione groups.