Methods of Chemical Ligation
There are various techniques described in literature. The most practical and robust method for the chemoselective reaction of unprotected peptides is native chemical ligation. Native chemical ligation has overcome the limitations of the classical synthetic organic chemistry approach to the total synthesis of proteins, and enables the routine total or semi- synthesis of protein molecules. The original chemical ligation methods involved the formation of a non-native bond at the ligation site. Subsequently, native chemical ligation was developed. In native chemical ligation, an unprotected peptide-thioester reacts with a Cys-peptide to give a ligation product with a native amide ('peptide') bond at the ligation site. In this method, the initial thioester-linked ligation product intermediate rearranges to form an amide bond.
Native chemical ligation relies on the presence of a cysteine residue at the ligation site. Methods using removable auxiliary groups can in some instances extend the use of native chemical ligation to non-cysteine residues, as can the use of desulfurization subsequent to the ligation (e.g. converting a Cys to an Ala).
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