No abstract

Biological encapsulants, such as viral capsids and ferritin protein cages, use many identical subunits to tile the surface of a polyhedron. Inspired by these natural systems, synthetic chemists have prepared an extensive series of artificial nanocages, with well-defined shapes and cavities. Rational control over the self-assembly of discrete, nanometre-scale, hollow coordination cages composed of simple components still poses considerable challenges as a result of the entropic costs associated with binding many subunits together, difficulties in the error-correction processes associated with assembly, and increasing surface energy as their size grows. Here we demonstrate the construction of a family of nanocages of increasing size derived from a single simple pentatopic pyrrole-based subcomponent. Reasoned shifts in the preferred coordination number of the metal ions employed, along with the denticity and steric hindrance of the ligands, enabled the generation of progressively larger cages, incorporating more subunits. These structural changes of the cages through these ‘mutations’ are reminiscent of differences in the folding of proteins caused by minor variations in their amino acid sequences; understanding how they impact capsule structure and thus cavity size may help to elucidate construction principles for still larger, more complex and functional capsules, capable of binding and carrying large biomolecules as cargoes.

The allosteric regulation of biomolecules, such as enzymes, enables them to adapt and alter their conformation to fit specific substrates, expressing different functionalities in response to stimuli. Different stimuli can also trigger synthetic coordination cages to change their shape, size, and nuclearity by reconfiguring the dynamic metal−ligand bonds that hold them together. Here we demonstrate an abiological system consisting of different organic subcomponents and Zn II metal ions, which can respond to simple stimuli in complex ways. A Zn II 20 L 12 dodecahedron transforms to give a larger Zn II 30 L 12 icosidodecahedron through subcomponent exchange, as an aldehyde that forms bidentate ligands is displaced in favor of one that forms tridentate ligands together with a pentaamine subcomponent. In the presence of a chiral template guest, the same system that produced the icosidodecahedron instead gives a Zn II 15 L 6 truncated rhombohedral architecture through enantioselective self-assembly. Under specific crystallization conditions, a guest induces a further reconfiguration of either the Zn II 30 L 12 or Zn II 15 L 6 cages to yield an unprecedented Zn II 20 L 8 pseudo-truncated octahedral structure. The transformation network of these cages shows how large synthetic hosts can undergo structural adaptation through the application of chemical stimuli, opening pathways to broader applications.