Systematic construction of progressively larger capsules from a 5-fold linking subcomponent

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.

A Diverse Array of Large Capsules Transform in Response to Stimuli

“…21 The results are in line with the expected size of Zn-1, consistent with the crystal structure of a previously described Co II analog. 18 Electrospray ionization mass spectrometry (ESI-MS) further confirmed its Zn II 20 L 12 composition, while traveling wave ion mobility-mass spec-trometry (TWIM-MS) data excluded the presence of other isomers (Figure 2d,e). Small-angle X-ray scattering (SAXS) analysis offers a statistically robust method for the structural characterization of large cages.…”

“…Different stimuli, such as the addition of subcomponents, metal ions, or a change in temperature, triggered the transformation of one cage into another. The use of graph theory to extend our original series of truncated polyhedra 18 allowed us to characterize a new pseudo-truncated octahedral structure, which crystallized with either D 4 , D 2 , or C 2 symmetry. In contrast to the structural diversity we obtained previously, 18 here a complex transformation network between four large cages and one sandwich structure was realized, with different stimuli triggering structural interconversions.…”

“…A distinct strategy was previously developed to construct a series of progressively larger capsules with formulas of M 5n L 2n , where n = 1, 2, 3, or 6, by increasing the dihedral angles between pentagonal faces at vertices. 18 We reasoned that M 5n L 2n structures with values of n missing from this series might be obtained by modifying the angles between the pentagonal faces of these structures. Based on graph theory 23 we proposed a structure for the Zn II 20 L 8 cage (n = 4, Figure S1), having a measured angle of ∼90°.…”

“…We infer the structure of Zn-3 to have the same truncated rhombohedral framework as a reported truncated rhombohedron with Co II centers. 18 Tetrabutylammonium G could not be removed even by washing with diethyl ether three times, highlighting the strong interactions between host and guest, which might lead to the cage transformation as discussed below. This truncated rhombohedral structure possesses eight triangular windows and three edge window pockets, where two pentagonal faces share the same edge.…”

“…We infer that the minor species previously observed in the ESI-MS of icosidodecahedron Co-2 may correspond to such a structure. 18 To characterize Zn-4 in solution, a mass spectrum was measured immediately after dissolving crystalline samples in CH 3 CN (Figure S38). Species corresponding to G 8 ⊂Zn-4 in a succession of charge states were found, consistent with the crystal structure, where eight G anions bound in the window pockets.…”