Total Chemical Synthesis And Modification Of Proteins-Development Of Remote Amide Thiolation

Proteins that contain non-natural amino acid residues (such as post-translationally modified proteins and proteins functionalized with synthetic probes) are highly important molecules in chemical biology. These molecules can't be readily obtained through biological expression, so that the only approach to produce them is to use chemical synthesis. Currently the most important method in the field of chemical synthesis of protein is cysteine-based native chemical ligation, which can efficiently condense peptide segments in aqueous solutions under mild conditions to generate natural as well as non-natural proteins. However, there are some limitations in this method. Firstly, a cysteine residue is needed at the ligation site while the abundance of cysteine in proteins is only 1.7%. Secondly, the other part involved in the native chemical ligation must be a thioester, which is usually unstable in solution and can not be prepared by commom Fmoc-SPPS. Therefore, developing new methods to break through the above limitations could be valuable for the total synthesis of proteins.In this paper, we developed a new remote peptide ligation method to overcome cysteine limitations of ligation site. We synthesized a series of model compounds with different sulfur-displ tying auxiliaries on the nitrogen of the last amide and conducted their ligation with thioesters under mild conditions successfully. Therefore, the ligation residues which are applicable in native chemical ligation are no longer limited to cysteine and glycine.