Abstract
The X-ray structure of the deep red crystalline complex lumiflavin 2,6-diamino-9-ethylpurine was determined. The flavin and adenine derivatives from H-bonded base pairs of the Watson-Crick type. The molecules in the crystal also associate via extensively overlapped flavin/adenine and flavin/flavin stacking interactions in which there are several contacts that are closer than van der Waals distances. This, and the red color of the crystals, is indicative of the formation of a charge-transfer complex.