Peptide oxo-esters for ligation of peptide hydrazide at Gln and Asn sites

Abstract

We describe a practical strategy for ligation of peptide hydrazide at Gln and Asn sites. Peptide hydrazides with Gln, Asn and Asp at the C-terminus would form by-products during TFA-based peptide cleavage due to the intramolecular cyclization of the side chain functional groups with the hydrazide moiety. Therefore, the ligation junctions of Gln-Cys, Asn-Cys and Asp-Cys are rarely chosen for hydrazide-based native chemical ligation. To solve this problem, we propose a strategy for the preparation of peptide hydrazides with C-termnal Gln, Asn and Asp, through the hydrazinolysis of peptide oxo-esters. This strategy avoids the trifluoroacetic acid treatment of peptide hydrazides and can be readily used for the synthesis of peptide hydrazides with C-terminal Gln, Asn and Asp. Through oxidative NaNO2 treatment of peptide hydrazides, we successfully obtain C-terminal thioester peptides of Gln and Asn. The utility of the oxo-ester based strategy has been demonstrated in the chemical synthesis of SHK toxin and is expected to expand the scope of hydrazide-based native chemical ligation in chemical protein synthesis.