Deltorphin II is a peptide ligand specific for the δ opioid receptor. In order to elucidate the role of Phe3 in binding to the δ opioid receptor, we synthesized a series of analogs in which Phe3 was replaced by various amino acids such as Ala, cyclohexylalanine (Cha), fluorophenylalanines, and other alkyl-side chain amino acids (Val, Leu, and norleucine (Nle)). It was found that [Cha3]deltorphin II and [Nle3]deltorphin II retain almost a full receptor binding affinity. The results indicated that the Phe3-phenyl group of deltorphin II can be substituted by the alkyl groups Such as cyclohexyl and propyl in the interaction with the δ receptor.
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