Another example of using integrated catalysis to synthesize products with high complexity is the application of a molecular machine in peptide synthesis. Leigh and coworkers reported using an artificial molecular machine to mimic nature's ribosome and synthesize oligopeptide with a predetermined sequence (FIG. ). Their system consists of a rotaxane, an axle with protected amino acids immobilized to it, and a bulky end-stopper. The rotaxane has a polypeptide arm that contains a cysteine moiety and a terminal glycylglycine amine group. The oligopeptide synthesis is accomplished by a series of O-S and S-N acyl transfers as the rotaxane moves along the axle. Though the system is only capable of incorporating up to 4 amino acids, it is still a valuable proof of concept that demonstrates how artificial synthesis can mimic nature.