Enol form of uracil in the GU "wobble" pair. NMR evidence
  GU "wobble" pair may contain either the enol form of uracil and the keto form of guanine or the enol form of guanine and the keto form of uracil. A direct proof of uracil existence in enol form can be obtained by using NMR spectroscopy. In HNN-COSY experiments the GU pair imitates the GC pair due to the enolization of uracil. 
Denis A. Semenov, Vladimir A. Reznikov
physics.bio-ph q-bio.BM