Proteins play critical roles in the structure, function and regulation of everyday biological functions. Isolation of discrete proteins from natural sources or recombinant DNA methods can be tedious and low yielding creating a need for synthetic approaches to overcome these difficulties.1 Native chemical ligation (NCL) has become the most common method for accessing large proteins using chemical synthesis. Classical NCL occurs between a peptide possessing of an N-terminal cysteine and a second peptide possessing a C-terminal thioester. This method is best suited for cysteine ligation sites or alanine ligation sites following desulfurization.2-5
Recently the Hutton group developed a silver(I)-promoted coupling of peptide carboxylic acids with amino ester thioamides. The reaction proceeds through an isoimide intermediate to yield the imide following O->N acyl transfer, with no epimerisation. Regioselective hydrolysis of peptide imides results in a native amide bond rendering the method useful for N->C direction peptide synthesis.6,7
This work focuses on exploiting the reactivity of peptide isoimides, such as by ‘trapping’ with an endogenous amine. Using this reactivity, we have been able to ligate various amino acids and peptides possessing a C-terminal carboxylate to peptides possessing a thioamide at the N-terminal peptide bond using silver carbonate as the only additive. This ligation occurs under mild conditions and proceeds in good yields. Preliminary results show this reaction is independent of the N-terminal amino acid residue and is therefore a robust method for ligating two peptide fragments at any amide bond location.