Synthesis, pharmacological evaluation and conformational investigation of endomorphin-2 hybrid analogues

This study reports on new pharmacologically active endomorphin-2 analogues, incorporating beta (2)-hPhe, beta (3)-hPhe and beta (3)-hTic unnatural amino acids in the place of the Phe(3)-Phe(4)residues. Such alpha, beta-hybrid analogues were designed to exploit the great potential of beta-amino acids in generating conformational variation at the key positions 3 and 4, with the aim of evaluating the effect on the opioid binding affinity. Ligand-stimulated binding assays indicated that some analogues retained a significant affinity, especially for the delta receptor. H-1 NMR and molecular modelling suggested the predominance of bent structures for all compounds. The molecular docking with the mu-opioid receptor model was also performed, highlighting a common binding mode for active compounds and helping to rationalize the observed structure-activity data.