Cooperative systems constructed using crystalline metal complexes of short flexible peptides

Biological systems display a range of sophisticated functions that cannot be performed by artificial systems, through intricate cooperative structural changes involving multiple functional units. The designability and structural flexibility of peptides are demonstrated by biological systems that display cooperative structural changes; these properties also make them well-suited for the formation of artificial systems that display such changes. The problem with the use of peptide frameworks is that long peptide residues, which are not suitable for gram-scale use, are required for the formation of stable ordered structures. However, if ordered structures containing peptides could be constructed by coordinating them to metal ions, peptides could be widely used to develop sophisticated functional materials. Crystal packing can be used for the design of functional materials made from simple molecules because it provides a way to place the components relative to each other. Although crystalline systems have been reported in which the small size of the cavities has been attributed to the flexibility of the peptide, recently, large systems with giant cavities have been developed with flexible peptides. In this review, we summarize the formation of cooperative multicomponent systems in the crystalline state using metal complexes of simple peptides, along with recent advances in the construction of giant artificial systems using short peptides.