| Abstract
| - Several cyclic lactam analogues of DynA-(1−13)NH2 were prepared in order to reducetheconformational flexibility in different regions of the native linearpeptide.Cyclo[d-Aspi,Dapi+3]DynA-(1−13)NH2 (Dap = α,β-diaminopropionic acid)analogues were designed on the basis of molecularmodeling using AMBER, which suggested that this constraint may becompatible with an α-helix.The cyclic portion of these constrained analogues spanned fromresidues 3 to 9, a region proposedby Schwyzer (Biochemistry 1986, 25,4281) to adopt a helical conformation at κ receptorsites.Analogues containing Dab (α,γ-diaminobutyric acid) or Orn inposition i + 3 were also synthesizedto examine the effects of larger ring size. The cyclic peptidesexhibited marked differences in bindingaffinities for κ, μ, and δ receptors and in opioid activity inthe guinea pig ileum (GPI). Cyclo[d-Asp6,Dap9]DynA-(1−13)NH2 showed both high κ receptor affinityand potent agonist activity inthe GPI, whilecyclo[d-Asp3,Dap6]DynA-(1−13)NH2 exhibited very weak binding affinity atall opioidreceptors as well as very weak opioid activity in the GPI.Cyclo[d-Asp5,Dap8]DynA-(1−13)NH2showed moderate binding affinity for κ receptors and was the most κselective ligand in this study,but this peptide exhibited very weak agonist activity in the GPI assay.Compared to thecorresponding linear peptides, all of the cyclic peptides exhibiteddecreased μ receptor affinity, whileκ receptor affinity was retained or improved. Therefore thecorresponding linear peptides weregenerally μ selective while the cyclic constrained peptidesdemonstrated slight selectivity for κ vs μreceptors or were nonselective. Increasing the ring size byincorporating Dab or Orn in positions6, 8, or 9 did not significantly affect the binding affinity for thethree opioid receptor types nor theopioid activity observed in the GPI. Circular dichroism spectra ofthecyclo[d-Aspi,Dapi+3]derivativesin 80% trifluoroethanol at 25 and 5 °C suggested differences in thestability of a helical structurewhen the constraint was incorporated near the N-terminus vs in themiddle of the peptide.
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