Spherical hosts

The discovery that members of the resorcin[4]arene family self-assemble to form 1, owing to its classification as an Archimedean solid, prompted us to examine the topologies of related spherical hosts with a view to understanding their structures on the basis of symmetry. In addition to providing grounds for classification, we anticipated that such an approach would allow us to identify similarities at the structural level, which, at the chemical level, may not seem obvious and may be used to design large, spherical host assemblies similar to 1. 

Thus, it is herein that we now describe the results of this analysis which we regard as the development of a general strategy for the construction of spherical molecular hosts. [11] We will begin by presenting the idea of self-assembly in the context of spherical hosts and then, after summarizing the Platonic and Archimedean solids, we will provide examples of cubic symmetry-based hosts, from both the laboratory and nature, with structures that conform to these polyhedra. 

Fig. 9.18 Ferritin, a spherical host based upon the rhombic dodecahedron. X-ray structure of the polypeptide subunit.

Indeed, we anticipate that the Platonic and Archimedean solids may be used for the construction of hosts which conform to those solids not yet realized and additional members of each family, where supramolecular synthesis, via self-assembly, will play a major role in their design, ushering in an era of spherical host-guest chemistry. 

To construct a spherical host from two subunits (n = 2), each unit must cover half the surface of the sphere. This can only be achieved if the subunits exhibit curvature and are placed such that their centroids lie at a maximum distance from each other. These criteria place two points along the surface of a sphere separated by a distance equal to the diameter of the shell. As a consequence of this arrangement, there exist two structure types one with two identical subunits attached at the equator and one belonging to the point group Ax/ which is topologically equivalent to a tennis ball. In both cases, the subunits must exhibit curvature. 

To construct a spherical host from three subunits (n = 3), each must cover one-third the surface of the sphere. Following the design conditions described previously, placing three identical subunits along the surface of a sphere results in an arrangement in which their centroids constitute the vertices of an equilateral triangle. As a result, there is only one structure type, that belonging to A/i- Each of the subunits must exhibit curvature. 

Based on the above study, larger pseudo-spherical host assemblies have been constructed from extended tetra-urea derivatives structures 29 and are... 

Figure 21. X-ray crystal structure of ferritin. A spherical host based upon the rhombic dodecahedron, (a) carbon trace of the polypeptide subunit, (b) the assembly displayed by the subunits. Dark ovals represent single subunits.

Fig. 4 Spherical hosts based on the Platonic solids (a) spherand (tetrahedron) (b) metal-organic cage (tetrahedron) (c) cyclophane-based cube (cube) (d) the [(V06)(RP03)g] ion (R=fBu, OSiMe ) (octahedron and cube) ( e DNA-based cube (printed with permission by Professor Nadrian Seeman) (cube) and (f) the rhinovirus (icosahedron). (View this art in color at www.dekker.com.)...

Diederich, F. and Dick, K. (1984) Spherical Host Molecules For the Complexation of Aromatic Hydrocarbons in Aqueous Solution , Angew. Chem. 96,789-790 Angew. Chem. Int. Ed. Engl. 23, 810-812. 

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