Chemistry Reference
In-Depth Information
synthesis and generate materials with high gas absorptivity [
12
]. One image of CO
2
trapped inside such an array is shown in Fig.
6
[
5
], and has a remarkable similarity
to the Escher image in Fig.
4
[
6
].
The record holders for surface area per gram and for gas storage are materials
prepared by Yaghi and coworkers that they call metal-organic frameworks
(MOFs). The first of these MOFs [
14
] looked much like the Robsen design.
Increasingly, these beautiful structures, with dramatically increased porosity, look
like a vision of Escher's (Fig.
7
).
2 Discrete, Symmetric Assemblies
The spontaneous assembly of small molecular fragments into larger, high-symmetry
clusters has been accomplished in Nature for more than a billion years. Examples in
the natural world include the protein ferritin. This very ancient protein is found in
bacteria, plants, and animals. Mammalian ferritin is a 24-mer with octahedral
symmetry (such that each of the asymmetric subunits is related to the other 23 by
one of the symmetry operations of the pure rotation group O and its 24 symmetry
elements), but there is a microbial ferritin with 12 subunits and T symmetry. An
illustration of this structure is shown in Fig.
8
.
Assemblies with a segregated inner space are generally found in the natural
world as protective containers. In the case of the ferritins, a valuable piece of iron
hydroxyoxide is maintained in soluble form by preventing the aggregation of these
particles beyond the nanoscale. The discrete inner environment of such assemblies
can also be used to protect reactive species that cannot be isolated without a suitably
Fig. 8 View of microbial ferritin down the threefold axis of symmetry, with each of the three
symmetry-related portions colored differently
