Structure–Activity Study on the Spatial Arrangement of the Third Aromatic Ring of Endomorphins 1 and 2 Using an Atypical Constrained C Terminus

Abstract The discovery of endomorphins (EMs) has opened the possibility of searching for new analgesics. However, the design of peptide analgesics has proven to be very difficult as a result of their conformational flexibility and a lack of clarity in structure–activity relationships (SAR). In EMs, the amino acid side chains exhibit considerable conformational flexibility, especially in the third aromatic ring, which is free to adopt a bioactive conformation. To resolve these problems, a series of C terminus EM analogues, [Xaa 4 ‐R]EMs, modified through the substitution of Phe 4 with nonaromatic residues and termination with benzyl groups, were designed to generate conformational constrains of the third aromatic ring by amide bond and torsion angles ( ϕ 4 and ψ 4 ) of Xaa 4 . Introduction of ( S )‐α‐methyl or ( S )/( R )‐α‐carboxamide on the methylene unit of the benzyl group was designed to produce an atypical topographical constraint ( ϕ 5 ) of the third aromatic ring rotation. Interestingly, some EM derivatives, with elimination of the C‐terminal carboxamide group and significant changes in the address sequence (Phe 4 ‐NH 2 ), still exhibited higher μ‐opioid receptor (MOR) affinity than unmodified EMs. In contrast, some analogues with incorrectly constrained C termini displayed very low affinity and pharmacological activities. Thus, our results indicate that these EM analogues, with atypical constrained C termini, provide model compounds with potent MOR agonism. They also give evidence that the proper spatial orientation and conformational restriction of the third aromatic ring are crucial for the interaction of EMs with MOR.