A Serine Ligation Method Based on Mitsunobu Reaction and O-to-N Acyl Transfer

A ligation protocol has been developed in which a peptide containing an N-terminal serine is linked via its side-chain alcohol to the C-terminal carboxylate of a second peptide by Mitsunobu esterification. N-Deprotection of the serine and subsequent exposure to a weak amine base triggers O-to-N acyl transfer, delivering the desired ligation product. A unique aspect of this strategy is the use of the C-terminal carboxylate as a nucleophile rather than as an electrophile. As a result, the ligation occurs without evidence of epimerization. A broad scope of C-terminal nucleophiles is tolerated, including proline and hindered beta-branched residues.