Figure 3. A self-templating system built from peptide units.

a | A de novo designed peptide ligase. α-helical peptides A and B bind to the electrostatically complementary α-helical peptide C to form the C·A·B ternary complex, which is composed of two coiled coils. Peptide A has a modified amino terminus that reacts with the chemically modified carboxyl terminus of peptide B on formation of the ternary complex. The reaction of peptides A and B is a ligation that forms the product C·P (C·P^* represents the chemical reaction between A and B to produce P). b | Peptide replicator schematic based on the reaction illustrated in part a, showing the reaction of peptide A with peptide B on formation of a ternary complex with peptide C. Peptides A^L, B^L, and C^LL are composed of L-amino acids, whereas peptides A^D, B^D, and C^DD are composed of D-amino acids. Peptides C^LL and C^DD are produced autocatalytically in a template-directed fashion through the reaction of precursors A^L with B^L, and A^D with B^D, respectively. Therefore, this replicator is stereochemically selective, only producing products (C^LL and C^DD) that are isomerically pure. Part b modified, with permission, from Ref. 65 © American Chemical Society (2001) and from Nature Ref. 66 © (2001) Macmillan Magazines Ltd.