  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.
