Use of model peptide reactions for the characterization of kinetically controlled ligation

Since the introduction of kinetically controlled ligation (KCL), a chemoselective reaction between a peptide-(α)thioarylester and a Cys-peptide-(α)thioalkylester, KCL has been utilized for the total chemical synthesis of large proteins (i.e., lysozyme and HIV-protease) by providing fully convergent synthetic routes. Although KCL has the potential to become an important chemistry for protein synthesis, the principle of KCL is not fully characterized. In particular, prior work on KCL has focused on the reactivity difference of the two different -(α)thioester forms-alkyl vs aryl. Another equally important feature of KCL, Xaa-Cys ligation sites, has not been investigated. The work reported here describes combinatorial KCL reactions using model peptides to dissect the interplay of the Xaa(1), Xaa(2), -(α)thioarylester, and -(α)thioalkylester. Results from these studies provide fundamental insights into the KCL reaction, and will lead to the optimal synthetic route for the routine chemical synthesis of large target protein molecules.