Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 22

grooves appear to have at least some of
the features of small molecule binding sites in proteins. For example,
Thioflavin-T, another histochemical dye used extensively for amyloid,
does not bind to the nanotubes while the fluorescent Alexa dyes,
which bear more structural resemblance to CR, bind strongly and
extensively along the grooves [101]. So, these assemblies may reflect
some of the properties of proteins and certainly capture some of
the ability to access the long-range order inherent in lipids and
membranes. Already these few metal and organic binding associations
have enriched our understanding of the kinetics of cross-

At this point, the cross-

β

β

assembly
[50,88,102], revealed new approaches for morphological control
[21,23], and created probes of diagnostic features of the final
structural array [92], but there is additional evidence now for even
more higher order supramolecular assemblies that further exploit
the unique surfaces created by the cross-

β

arrays.

As the forces controlling the
transition to crystallinity are resolved, it becomes increasingly
possible to create even greater levels of molecular diversity. In the
DNA-directed synthetic polymerization process, the thermodynamic
assemblies are trapped through reductive amination and recycled
on the template to give increasingly higher molecular weight
products. This simple thermodynamic assembly/kinetic trap
process provides the foundation for transmitting the molecular-
level encoded information but lacks the feedback loops that keep
the system far from equilibrium and positioned for self-organizing
behavior. Most notably, these templates are typically limited by
product inhibition, even if some chemical solutions have been
found [15]. In contrast, the

Supramolecular Architectures:

-sheet peptide templates are not
limited by assembly and their continued growth provides access to
divergent ordered phases. Similar conditionally dependent phase
transitions are characteristic of simple amphiphilic molecules, and
the more complex phospholipids of biological membranes access
diverse phases with remarkable paracrystalline long-range order.
The polypeptides appear to access even greater diversity in their
paracrystalline phases [103-105], arising via a consecutive series of
phase changes [22,67], and it may now be possible to harness such
complex behavior for self-organization.
A striking example of the complexity that can be achieved in the
peptide assemblies is found in the peptide bilayer membranes that
make up the nanotube of A

β

β

(16-22) [24]. Under acidic conditions
where the KLVFFAE peptide organizes in anti-parallel

β

-sheets