Chemical ligation of unprotected peptides directly from a solid support

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Abstract
In this article we describe a new, convenient procedure to carry out intramolecular (cyclization) and intermolecular native chemical ligations of unprotected peptides directly from a solid support. Our solid‐phase ligation approach eliminates the need to manipulate peptide „thioacid and peptide αthioester intermediates in aqueous solution before the ligation step, thereby leading to a reduction in handling losses and significantly increasing the overall efficiency of the chemical ligation strategy. A key step in our ligation scheme is the ability to generate fully unprotected peptides tethered to a solid support through an ”thioester linkage. This can be achieved efficiently using optimized Boc‐solid‐phase peptide synthesis on a 3‐mercaptopropionamide‐polyethylene glycol‐poly‐(N,N‐dimethylacrylamide) copolymer support (HS‐PEGA). Once the synthesis is complete, the fully protected peptide „thioester resin is treated with HF to give the corresponding fully unprotected peptide „thioester resin. Using this procedure several polypeptides ranging from 15 to 47 residues were synthesized successfully. These peptide‐resins were then used to perform both intramolecular (head‐to‐tail cyclizations) and intermolecular solid‐phase ligations. The intramolecular solid‐phase ligations proceeded much faster than their intermolecular counterparts, but in both cases the reactions were observed to be remarkably clean. The presence of aromatic thiol cofactors significantly accelerated the relatively slow intermolecular ligations. This novel methodology was then extended to provide a general method for performing sequential intermolecular ligations, allowing easy access to much larger polypeptide and protein systems.