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The enantiomer of endomorphin-1 (Tyr-Pro-Trp-PheNH2) and the analogues containing (S)- or (R)-β-proline have been synthesized, and their affinities towards μ-opioid receptors have been measured. As expected, the incubations of the different peptides with some commercially available enzymes showed that the presence of D-residues gave strong resistance towards digestion. The presence of β-proline alone is sufficient to confer good resistance against the hydrolysis of the biologically strategic Pro–Trp bond.