Diversity Scaleof Library Matters: Impact of mRNALibrary Diversity Scales on the Discovery of Macrocyclic PeptidesTargeting a Protein by the RaPID System

Macrocyclic peptides make up a unique class of modalities known for their high affinity, specificity, and ability to modulate protein–protein interactions, including receptor activation. Messenger RNA display, including the Random Nonstandard Peptides Integrated Discovery (RaPID) system, stands out in identifying target-specific macrocyclic peptides, producing potent binders with low to subnanomolar dissociation constants against diverse targets. It has often been discussed that this success is partly attributed to the vast library of over a trillion different peptide sequences expressed from the corresponding mRNA sequences. However, the impact of library scales on the identification of various binders has not been experimentally validated. Here, we report the RaPID selections against an ectodomain of a receptor tyrosine kinase MET using peptide libraries ranging from 10⁶ to 10¹⁴ unique members of mRNAs. We thoroughly analyzed the outcomes, including the binding kinetic properties, of the enriched peptide families. This study provides valuable guidelines for designing libraries with various numbers of sequences and selection conditions to enrich macrocyclic peptides with the desired characteristics.