Native Chemical Ligation - Definition

Definition

In native chemical ligation, the thiolate group of an N-terminal cysteine residue of an unprotected peptide 2 attacks the C-terminal thioester of a second unprotected peptide 1 in an aqueous buffer at pH 7.0, 20°C3. This intermediate rearranges by an intramolecular S,N-acyl shift that results in the formation of a native amide ('peptide') bond 4 at the ligation site (scheme 1).

Remarks :

• Thiol additives :

NCL reaction is catalyzed by in situ transthioesterification with thiol additives. The most common thiol catalysts to date have been either a mixture of thiophenyl, 4-mercaptophenylacetic acid (MPAA), or 2-mercaptoethanesulfonate (MESNa). (ref)

• Regioselectivity of NCL :

The main property of the NCL method is the reversibility of the first step, the thiol(ate)–thioester exchange reaction. Native chemical ligation is exquisitely regioselective because that thiol(ate)–thioester exchange step is freely reversible in the presence of an exogenous thiol added as catalyst. The high yields of final ligation product obtained, even in the presence of internal Cys residues in either/both segments, is the result of the irreversibility, under the reaction conditions used, of the second (S-to-N acyl shift) amide-forming step.

• Chemoselectivity of NCL :

No side-products are formed from reaction with the other functional groups present in either peptide segment (acids or basics amino groups, phenolic hydroxyls, etc.).