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Expanding the structural diversity of ribosomally-translated peptides

The ribosome is able to create peptides with a diverse array of amino acids beyond the canonical 20, but there remain limitations to what it can accept. To overcome this, ribosomally translated peptides can be modified chemically using reactions that are either selective for a limited number of the canonical amino acids, or are selectively reactive with a functional group that is amenable to ribosomal translation. In our research, we are attempting to push the boundaries of what can be achieved using the ribosome to create modified peptides, especially glycopeptides. Carbohydrate-modified amino acids seem to be particularly unsuited to direct ribosomal translation, and glycosylation of peptides and proteins can have very important influences on in vivo activity, necessitating an indirect approach. Despite the complexity of an in vitro translation system, we have been able to show selective modification of an mRNA displayed macrocyclic peptide library with carbohydrates in this highly heterogenous setting, through cysteine thiol activation and elimination followed by conjugate addition. This gives a thioglycoside that is a very good mimic of natural O-glycosylation of serine, but lacks stereochemical control and is only suited to one or two modifications in a peptide. Current research in this area is focussed on developing a new chemical handle that can give stereochemically controlled glycosylation at both serine and threonine, with no limitations on the number of modifications and minimal artificial linker. In addition, a further ribosomally-translated chemical handle is envisaged for generating a natural N-glycosidic linkage with complete control of the modification site.

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Selective glycosylation of macrocyclic peptide libraries by cysteine activation and elimination using DBAA, followed by conjugate addition of a thiol (Chem Sci, 2016 doi: 10.1039/C6SC04381J)

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