The structure of an endomorphin analogue incorporating 1-aminocyclohexane-1-carboxlylic acid for proline is similar to the β-turn of Leu-enkephalin

Abstract Endomorphin (EM2, Tyr–Pro–Phe–Phe–NH 2 ) can assume various conformations related to cis / trans -rotamers of the amide linkage of Tyr–Pro. To control isomerization, restricted or flexible components have been introduced at the Pro position. We focused on [Chx 2 ]EM2, an EM2 analogue substituting 1-aminocyclohexane-1-carboxlylic acid (Chx) for Pro. X-ray diffraction analysis revealed that [Chx 2 ]EM2 is folded into the trans -form of Tyr–Chx. The manner of folding resembled that seen in D-TIPP, an EM analogue incorporating tetrahydroisoquinoline carboxylic acid, as well as the β-turn of Leu-enkephalin. Selectivity for the opioid μ-receptor was fairly well conserved by [Chx 2 ]EM, suggesting that the folded form is important for μ-selectivity.