1H-Benzo[g]pteridine-2,4-dione

The structure of the title compound, C(10)H(6)N(4)O(2), reported by Smalley et al. [(2021). Cryst. Growth Des. 22, 524–534] from powder diffraction data and (15)N NMR spectroscopy, is confirmed using low-temperature data from a twinned crystal. The tautomer in the solid state is alloxazine (1H-benzo[g]pteridine-2,4-dione) rather than isoalloxazine (10H-benzo[g]pteridine-2,4-dione). In the extended structure, the mol­ecules form hydrogen-bonded chains propagating in the [0 [Image: see text] 1] direction through alternating centrosymmetric R (2) (2)(8) rings with pairwise N—H⋯O inter­actions and centrosymmetric R (2) (2)(8) rings with pairwise N—H⋯N inter­actions. 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. [Image: see text]