An N-terminal selective thiazoline peptide macrocyclisation compatible with mRNA display and efficient SPPS

Discovery of new to nature ‘de novo’ macrocyclic peptides has been greatly facilitated by the integration of genetic recoding approaches with peptide display technologies. Perhaps most important among the changes that can made to a peptide to allow its use in a biological setting is macrocyclisation, which has beneficial impacts on target affinity, selectivity, stability, and cell permeability. However, introducing macrocyclisation into a linear sequence is unlikely to be successful unless the sequence is already primed to adpot an appropriate conformation. As a result it is important to include cyclisation already at the discovery stage, meaning there is a need for more diverse cyclisation options that can be deployed in the context of peptide display techniques such as mRNA display. In this work we show that meta-cyanopyridylalanine can be ribosomally incorporated into peptides, forming a macrocycle in a spontaneous and selective reaction with an N-terminal cysteine generated from bypassing the initiation codon in translation. This reactive amino acid can also be easily incorporated into peptides during standard Fmoc solid phase peptide synthesis, which can otherwise be a bottleneck in transfering from peptide discovery to peptide testing and application. We demonstrate the potential of this new method by discovery of macrocyclic peptides targeting influenza haemagglutinin, and molecular dynamics simulation indicates the mCNP cross-link stabilises a beta sheet structure in a representative of the most abundant cluster of active hits. Our new approach generates macrocycles with a more rigid cross-link and with better control of regiochemistry when additional cysteines are present, also allowing easy access to spontaneously forming bicyclic peptides, and so is a valuable addition to the mRNA display toolbox.