We report data on nuclear polarization formed in photoreaction of intramolecular electron transfer from adenine to flavin that was revealed several years ago by R. Kaptein by CIDNP. The question of whether one or two types of biradicals are formed stays unresolved. This work clarified the problem and confirmed that single common emissive maximum in magnetic field dependence of CIDNP may originate from single level crossing of short-lived biradical and that the FAD molecule has a compact shape.