1-Benzo[]pteridine-2,4-dione.

The structure of the title compound, CHNO, reported by Smalley [(2021). , 524-534] from powder diffraction data and N NMR spectroscopy, is confirmed using low-temperature data from a twinned crystal. The tautomer in the solid state is alloxazine (1-benzo[]pteridine-2,4-dione) rather than isoalloxazine (10-benzo[]pteridine-2,4-dione). In the extended structure, the mol-ecules form hydrogen-bonded chains propagating in the [01] direction through alternating centrosymmetric (8) rings with pairwise N-H⋯O inter-actions and centrosymmetric (8) rings with pairwise N-H⋯N inter-actions. The crystal chosen for data collection was found to be a non-merohedral twin (180° rotation about [001]) in a 0.446 (4):0.554 (6) domain ratio.