BU72 is a μ opioid of exceptionally high affinity and potency (Figure ) . Its dissociation constant (Ki) for μOR ranges from 0.15 nM in crude brain membranes , to lower values in transfected cell membranes , and as low as 0.01 nM for purified μOR with Gi protein . Very few ligands for any protein exceed this extraordinary affinity, which is considered an effective upper bound on the strength of non-covalent binding .