Recognition Elements PPI Library
Protein-protein interactions (PPIs) are essential components in cellular signaling pathways as well as in important processes such as viral infection, replication, and immune suppression. The unknown or uncharacterized PPIs involved in such interaction networks often represent compelling therapeutic targets for drug discovery. To date, however, the main strategies for the discovery of small molecule modulators of PPIs are typically limited to structurally characterized targets. Recent developments in molecular scaffolds that mimic the side chain display of peptide secondary structures have yielded effective designs. 
Set of Recognition Elements includes gamma-, beta-turns, dipeptide mimetics. Turns are defined as regions where a peptide chain reverses its overall direction. Gamma-turns involve three residues and a hydrogen bond is often formed between residues i and i + 2 so that a pseudo-7-membered ring is formed. Chain reversal in beta-turns involves four residues and a hydrogen bond may then be formed between residues i and i + 3 so that forms a pseudo-10-membered ring. A common method to mimic the bioactive conformation of peptides is to synthesize conformationally constrained analogs via backbone-backbone or backbone-side chain cyclization.
 L. R. Whitby and D. L. Boger, “Comprehensive peptidomimetic libraries targeting protein-protein interactions,” Acc. Chem. Res., vol. 45, no. 10, pp. 1698–1709, 2012, doi: 10.1021/ar300025n.