Topographical requirements for delta opioid ligands: The synthesis and biological properties of a cyclic analogue of deltorphin I

A cyclic constrained analogue of deltorphin, {A figure is presented} has been proposed on the basis of an energetically favored model of a delta-selective conformation for deltorphin. The biological properties of this synthetic analogue demonstrate that incorporation of a disulfide bridge into Deltorphin I does not affect its high affinity for delta opioid receptors. The analogue shows low receptor selectivity as a result of a large increase in its affinity for mu receptors when compared to the parent deltorphin.