In Vitro Selection of Thioether-Closed Macrocyclic Peptide Ligands by Means of the RaPID System

The Random nonstandard Peptides Integrated Discovery (RaPID) system enables efficient screening of macrocyclic peptides with high affinities against target molecules. Random peptide libraries are prepared by in vitro translation using the Flexible In vitro Translation (FIT) system, which allows for incorporation of diverse nonproteinogenic amino acids into peptides by genetic code reprogramming. By introducing an N-chloroacetyl amino acid at the N-terminus and a Cys at the downstream, macrocyclic peptide libraries can be readily generated via posttranslational thioether formation. Here, we describe how to prepare a thioether-closed macrocyclic peptide library, and its application to the RaPID screening.