Figure 1. Making macrocycles RaPID-ly. A team from The University of Tokyo has developed a method called RaPID for generating libraries of trillions of N-methylated peptide macrocycles displayed on mRNA. First, a cDNA library is generated (red bar) that encodes 8-15 residue-long peptides containing a mixture of natural and non-natural amino acids [a]. The cDNA library is transcribed to an mRNA library and subsequently linked to a second mRNA oligonucleotide that terminates in a puromycin residue (black circle) [b].

When the mRNA library is translated, the puromycin causes the growing peptide chain to remain linked to its mRNA template. The displayed macrocyclic peptides contain a linker region (gray), natural amino acids (yellow) and N-methylated amino acids (red) [c]. After selecting for macrocycles that bind to a desired target, binding molecules can be identified and/or amplified for a second round of selection by PCR of the linked mRNA template [d].