Ribosomal Synthesis of Backbone-Cyclic Peptides Compatible with In Vitro Display

Backbone-cyclic peptides are an attractive class for therapeutic development. However, in vitro display technologies coupled with ribosomal synthesis are intrinsically inapplicable to such phenotypes because of loss of the C-terminal peptide region linking to genotype. Here, we report a methodology enabling the display of backbone-cyclic peptides. To achieve this, genetic code reprogramming was utilized to implement a rearrangement strategy involving the ribosomal incorporation of a designer initiator containing a thiazolidine-protected cysteine and 2-chloroacetoamide (ClAc) side chain, followed by an alpha-thio acid and cysteine at downstream positions. Upon expression of the linear peptide, spontaneous thioester rearrangement occurs between the alpha-thioester and the thiol group of the cysteine, liberating the alpha-thio group and resulting in cross-linking to the upstream ClAc side-chain group. Then selective deprotection of the thiazolidine-protected cysteine immediately promotes intramolecular native chemical ligation, as demonstrated for various sequences and ring sizes. In this approach, the backbone-cyclic peptides retain their C-terminal peptide regions via the side-chain thioether covalent linkage, making them compatible with in vitro display.