Structure−Activity Study on the Phe Side Chain Arrangement of Endomorphins Using Conformationally Constrained Analogues

Endomorphins-1 and -2 were substituted with all the β-MePhe stereoisomers in their Phe residues to generate a conformationally constrained peptide set. This series of molecules was subjected to biological assays, and for β-MePhe<sup>4</sup>-endomorphins-2, a conformational analysis was performed. Incorporation of (2<i>S</i>,3<i>S</i>)-β-MePhe<sup>4</sup> resulted in the most potent analogues of both endomorphins with enhanced enzymatic stability. Their μ opioid affinities were 4-times higher than the parent peptides, they stimulated [<sup>35</sup>S]GTPγS binding, and they were found to be full agonists. NMR experiments revealed that C-terminal (2<i>S</i>,3<i>S</i>)-β-MePhe in endomorphin-2 strongly favored the gauche (−) spatial orientation which implies the presence of the χ = −60° rotamer of Phe<sup>4</sup> in the binding conformer of endomorphins. Our results emphasize that the appropriate orientation of the C-terminal aromatic side chain of endomorphins is substantial for binding to the μ opioid receptor.