• Selenium-Mediated Peptide Ligations at Proline-Proline Junctions

The last decade has seen a renaissance in the number of polypeptide therapeutics approved for the treatment of numerous diseases, meaning there is a significant need for technologies to efficiently access these biomolecules. Currently, native chemical ligation (NCL) is the most widely used methodology for the convergent assembly of peptides to afford polypeptides. Whilst this technology has revolutionized protein science, it is marred by its reliance on a suitably located low abundant amino acid, cysteine (ca. 1.3%) within the target polypeptide. To combat these limitations, the advent of thiolated and selenolated amino acids in conjunction with desulfurization and deselenization reactions have significantly widened the scope of NCL to include more naturally abundant amino acids.

As part of this work, a novel one-pot additive and additive-free selenoproline-selenoester ligation-deselenization strategy has been developed to achieve, for the first time, ligations at the most difficult junction; the proline-proline junction.

The first goal was to develop an efficient route towards a PMB-protected trans- and cis-γ-selenoproline building block. The power of the trans-γ-selenoproline building block is demonstrated in four successful model additive/additive-free ligation-deselenization reactions between peptide proline selenoesters and peptide dimers bearing an N-terminal trans-γ-selenoproline, affording the native peptide bond in 16 h. The use of the cis-γ-selenoproline building block in model additive/additive-free ligations proved unsuccessful and led to the accumulation of an unrearranged trans-selenoesterification intermediate. The observation of this intermediate provided strong evidence that the selenium-mediated ligations proceed through an NCL-like mechanism.

In light of the success, the utility of this technology was extended to the first total synthesis of an antimicrobial peptide, lumbricin-1, which prior to this technology could not be accessed synthetically. The simplicity and efficiency of the ligation-deselenization chemistry should serve as a powerful means to access a library of synthetic proteins, including those of therapeutic relevance, in the future.