SYNTHESIS AND OPIOID ACTIVITY OF CONFORMATIONALLY CONSTRAINED DYNORPHIN-A ANALOGS .2. CONFORMATIONAL CONSTRAINT IN THE ADDRESS SEQUENCE

Several cyclic lactam analogues of Dyn A-(1-13)NH2 were prepared in or der to reduce the conformational flexibility in different regions of t he native linear peptide. Cyclo[D-Asp(i),Dap(i+3)]Dyn A-(1-13)NH2 (Dap = alpha,beta-diaminopropionic acid) analogues were designed on the ba sis of molecular modeling using AMBER, which suggested that this const raint may be compatible with an alpha-helix. The cyclic portion of the se constrained analogues spanned from residues 3 to 9, a region propos ed by Schwyzer (Biochemistry 1986, 25, 4281) to adopt a helical confor mation at kappa receptor sites. Analogues containing Dab (alpha,gamma- diaminobutyric acid) or Orn in position i + 3 were also synthesized to examine the effects of larger ring size. The cyclic peptides exhibite d marked differences in binding affinities for kappa, mu, and delta re ceptors and in opioid activity in the guinea pig ileum (GPI); Cyclo[D- Asp(6),Dap(9)]Dyn A-(1-13)NH2 showed both high kappa receptor affinity and potent agonist activity in the GPI, while cyclo[D-Asp(3),Dap(6)]D yn A-(1-13)NH2 exhibited very weak binding affinity at all opioid rece ptors as well as very weak opioid activity in the GPI. Cyclo[D-Asp(5), Dap(8)]Dyn A-(1-13)NH2 showed moderate binding affinity for kappa rece ptors and was the most kappa selective ligand in this study, but this peptide exhibited very weak agonist activity in the GPI assay. Compare d to the corresponding linear peptides, all of the cyclic peptides exh ibited decreased mu receptor affinity, while kappa receptor affinity w as retained or improved. Therefore the corresponding linear peptides w ere generally mu selective while the cyclic constrained peptides demon strated slight selectivity for kappa vs mu receptors or were nonselect ive. 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 GP I, Circular dichroism spectra of the cyclo[D-Asp(i),Dap(i+3)] derivati ves in 80% trifluoroethanol at 25 and 5 degrees C suggested difference s in the stability of a helical structure when the constraint was inco rporated near the N-terminus vs in the middle of the peptide.