New endomorphin analogues containing alicyclic β-amino acids:: Influence on bioactive conformation and pharmacological profile

Endomorphins were subjected to a number of structural modifications in a search for their bioactive conformations. The alicyclic beta-amino acids cis-(1S,2R)ACPC/ACHC, cis-(IR,2S)ACPC/ACHC, trans(IS,2S)ACPC/ACHC, and trans-(1R,2R)ACPC/ACHC were introduced into endomorphins to examine the conformational effects on the bioactivity. Use of a combination of receptor binding techniques, H-1 NMR, and molecular modeling allowed the conclusion that Pro(2) substitution by these residues causes changes in structure, proteolytic stability, and pharmacological activity. It seems that the size of the alicyclic beta-amino acids does not have marked influence on the receptor binding affinities and/or selectivities. Among the new analogues, the cis-(1S,2R)ACPC(2) and cis-(1S,2R)ACHC(2)-containing derivatives displayed the highest binding potencies and efficacies in receptor binding and ligand-stimulated [S-35]GTP gamma S functional experiments. Molecular dynamic simulations and H-1 NMR studies of the cis-ACPC/ACHC-containing analogues revealed that many conformations are accessible, though it is most likely that these peptides bind to the mu-opioid receptor in a compact, folded structure rather than extended.