Highly efficient one-pot ligation and desulfurization

The combination of native chemical ligation and desulfurization is considered a powerful strategy in protein synthesis. Homogeneous desulfurization conditions based on a radical induced reaction have been widely used in the syntheses of various challenging proteins and their analogues. However, the presence of aryl thiols in the reaction mixture as a ligation catalyst hampers one-pot ligation/desulfurization, hence mandating additional purification/lyophilization steps prior to desulfurization. This significantly reduces the yield and prolongs the ligation process. Here we report that the use of preformed peptide-aryl thioester allows for efficient one-pot ligation and desulfurization. This approach was tested successfully for various model peptides including the synthesis of ubiquitin from two fragments. However, in the case of the synthesis of di-ubiquitin chains, where the ligation is mediated by δ-mercaptolysine to form an isopeptide bond, excess aryl thiol was required for efficient ligation, necessitating purification prior to desulfurization. To overcome these obstacles, we found that functionalization of the aryl thiol with a hydrazide moiety enabled, after the ligation step, its capture by resin-aldehyde to permit direct desulfurization. Altogether, these approaches should facilitate protein synthesis with improved efficiency in yields and time.