Morphiceptin and β-casomorphin-5 analogues containing a reduced peptide bond: Selective μ-receptor agonists and a novel μ antagonist, H-Tyr-Proψ(CH₂-NH)Phe-Pro-Gly-OH

In order to prevent enzymatic degradation of β-casomorphin-5 (1) and morphiceptin, reduced peptide bonds were incorporated at the 2–3 and 3–4 bonds, respectively. The analogues were synthesized by a combination of solid phase methodology and reductive alkylation of resin-bound peptide amines with Boc-amino acid aldehydes (Boc: tert-butyloxycarbonyl) in the presence of NaBH₃CN. During reversed phase high pressure liquid chromatography purification, peak shape distortions could be observed. Epimerization was excluded, based on gas chromatography/mass spectroscopy analysis, which indicated acceptable levels of racemization (<3%) in the crude product. Instead, the phenomena could be attributed to slow cis/ trans isomerizations originating from the Xxx-Pro bonds in the sequence. The presence of different conformational isomers was also established by ¹H-nmr spectroscopy in DMSO-d₆. All analogues showed high stability in blood plasma, enhanced binding affinity for the μ receptor, and very low binding to the δ receptor. While the Phe³Ψ(CH₂-N) Pro⁴ analogues (3) and (5) displayed agonist activity, the Pro²Ψ(CH₂-NH) Phe³ modified analogue (2) showed antagonist activity comparable to D-Phe-Cys-Tyr-D -Trp-Arg-Thr-Pen-Thr-NH₂.