Conformational Properties of the CalliFMRF-Amide Series Neuropeptides

Conformational properties of five neuropeptides belonging to the calliFMRF-amide series with the Xaa-Pro-Yaa-Gln-Asp-Phe-Met-Arg-Phe-NH₂ homologous sequences were studied by the method of theoretical conformational analysis. Three members of these group [(1) (Xaa = Thr, Yaa = Gln), (2) (Xaa = Thr, Yaa = Ser), and (3) (Xaa = Yaa = Ser)] can stimulate the saliva secretion from the separated salivary gland of the Calliphora vomitoria fly, whereas two other calliFMRF-amides [(4) (Xaa = Lys, Yaa = Asn) and (5) (Xaa = Ala, Yaa = Gly)] are inactive in this biological test. Low-energy spatial structures of the studied compounds were determined by a conformational analysis. A comparison of the stable structures of the biologically active and inactive neuropeptides revealed a similarity in their conformational properties and allowed determination of the role of separate residues in the peptide folding. The calculations demonstrated that the C-terminal hexapeptide fragment identical in all the five peptides tends to form -helical structure, whereas the variable N-terminal tripeptide regions of calliFMRF-amides (1)–(5) form more conformationally flexible structures.