Synthesis and opioid activity of conformationally constrained dynorphin A analogues. 2. Conformational constraint in the 'address' sequence

Several cyclic lactam analogues of Dyn A-(1-13)NH₂ were prepared in order to reduce the conformational flexibility in different regions of the native linear peptide. Cyclo[D-Asp(i),Dap(i+3)]Dyn A-(1-13)NH₂ (Dap = α,β- diaminopropionic acid) analogues were designed on the basis of molecular modeling using AMBER, which suggested that this constraint may be compatible with an α-helix. The cyclic portion of these constrained analogues spanned from residues 3 to 9, a region proposed by Schwyzer (Biochemistry 1986, 25, 4281) to adopt a helical conformation at K receptor sites. Analogues containing Dab (α,γ-diaminobutyric acid) or Orn in position i + 3 were also synthesized to examine the effects of larger ring size. The cyclic peptides exhibited marked differences in binding affinities for κ, μ, and δ receptors and in opioid activity in the guinea pig ileum (GPI). Cyclo[D- Asp⁶,Dap⁹]Dyn A-(1-13)NH₂ showed both high κ receptor affinity and potent agonist activity in the GPI, while cyclo[D-Asp³,Dap⁶]Dyn A-(1-13)NH₂ exhibited very weak binding affinity at all opioid receptors as well as very weak opioid activity in the GPI. Cyclo[D-Asp⁵,Dap⁸]Dyn A-(1-13)NH₂ showed moderate binding affinity for κ receptors and was the most κ selective ligand in this study, but this peptide exhibited very weak agonist activity in the GPI assay. Compared to the corresponding linear peptides, all of the cyclic peptides exhibited decreased μ receptor affinity, while κ receptor affinity was retained or improved. Therefore the corresponding linear peptides were generally μ selective while the cyclic constrained peptides demonstrated slight selectivity for κ vs μ receptors or were nonselective. Increasing the ring size by incorporating Dab or Orn in positions 6, 8, or 9 did not significantly affect the binding affinity for the three opioid receptor types nor the opioid activity observed in the GPI. Circular dichroism spectra of the cyclo[D-Asp(i),Dap(i+3)] derivatives in 80% trifluoroethanol at 25 and 5 °C suggested differences in the stability of a helical structure when the constraint was incorporated near the N-terminus vs in the middle of the peptide.