Synthesis and activities of cyclic thrombin-receptor-derived peptide analogues of the Ser₄₂-Phe-Leu-Leu-Arg₄₆ motif sequence containing d-Phe and/or d-Arg

Summary

Cyclic analogues of the active thrombin receptor peptide SFLLR (TRP_42–46) containingd-Phe and/ord-Arg have been prepared by the solid-phase method, purified by reversed-phase HPLC and bioassayed in a rat smooth muscle contractile assay. Cyclization was achieved by forming an amide linkage between the-NH₂ and-COOH groups of the two leucine residues located at the N- and C-terminal positions of the linear protected precursor H₂N-Leu-Arg(Pmc)-Y-Phe-Leu-OH (Y=Gly,Acp) using 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluoroborate borate (HBTU) or 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) as coupling reagents andN,N′-diisopropylethylamine (DIPEA) in high dilution. Their structure was confirmed by fast by fast atom bombardment mass spectrometry and NMR methods. The cyclic peptides c-fLLrG, c-fLLRG, c-FLLrG and c-fLLrAcp, c-FLLrAcp so synthesized were assessed for their contractile activity in a rat gastric longitudinal muscle bioassay system which has been used previously to evaluate the biological activities of linear thrombin-receptor-derived polypeptides such as SFLLR (P5) and SFLLR-NH₂ (P5-NH₂).