Methadone conformation and opioid activity

The inactive methadone analog threo-5-methylmethadone has a solid-state conformation in which the nitrogen is antiperiplanar to the tertiary carbon C(4). Since threo-5-methylmethadone exhibits no opioid agonism either in vivo or in vitro, methadone analogs probably do not have this conformation when bound to an opioid receptor. The potent agonist (-)-erythro-5-methylmethadone has a solid-state conformation in which the nitrogen atom is rotated back toward the phenyl rings on the quarternary carbon, suggesting that this unusual conformation is the active one.