Aza-Amino Acid Scanning of Secondary Structure Suited for Solid-Phase Peptide Synthesis with Fmoc Chemistry and Aza-Amino Acids with Heteroatomic Side Chains

摘要

Aza-peptides,peptide analogues in which the alpha-carbon of one or more of the amino acid residues is replaced with a nitrogen atom,exhibit a propensity for adopting beta-turn conformations.A general Fmoc-protection protocol for the stepwise solid-phase synthesis of aza-peptides has now been developed based on the activation of N'-alkyl fluoren-9-ylmethyl carbazates with phosgene for coupling the aza-amino acid residues.This method has proven effective for introducing aza-amino acid residues with aliphatic (Ala,Leu,Val,and Gly) and aromatic (Phe,Tyr,and Trp) side chains.Acid promoted loss of aromatic side chains was noted with aza-Trp and aza-Tyr residues during peptide cleavage and suppressed by temperature control in the case of the latter.In addition,aza-peptides with heteroatomic side chain residues (Lys,Orn,Arg,and Asp) were conveniently synthesized using this protocol.Partial aza-amino acid scans were performed on three biologically active peptides:the potent tetrapeptide melanocortin receptor agonist,Ac-His-D-Phe-Arg-Trp-NH_2;the growth hormone secretagogue hexapeptide,GHRP-6,His-D-Trp-Ala-Trp-D-Phe-Lys-NH_2;and the human calcitonin gene-related peptide (hCGRP) antagonist,FVPTDVGPFAF-NH_2.This practical procedure for aza-amino acid scanning using Fmoc-based solid-phase synthesis should find general utility for probing the existence and importance of beta-turn conformations in bioactive peptides.