Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 26

scratched the surface and the opportunities are as rich as life itself.
Synthetic template-directed polymerization has already matured
into a robust process for chemical information transfer that allows
copying errors to create the necessary diversity for selection, but
the breadth of backbone scaffolds compatible with the process is not
yet known. Dynamic networks have been created with the -helix,

but multiple amino acids are needed to define the intermolecular
contacts, and unless these helices can be packed into higher ordered
arrays, the structural efficiency for information transfer may be too
low. In contrast,

α

β

-sheet associations are defined by both backbone
and side-chain interactions [115-124], and unlike either helical
coiled coils or the nucleic acid duplexes, each amino acid contributes
directly to intermolecular recognition. As a result, the β

-sheet

β

templates within the cross- fold are not masked by the association

event, so as a template the product inhibition that so plagues the
duplex coiled coils and nucleic acid systems is removed [125,126].
However, we have argued in this chapter that the conditionally
dependent phase transitions, in particular the great diversity of
crystalline peptide phases, provide a level of structural diversity
that will be necessary for non-equilibrium self-organizing
transitions. Changes in strand orientation and registry, altered sheet
complementarity, metallated assemblies, co-factor decorated arrays,
and multi-component supramolecular arrays are all responsive to
subtle changes in the chemical and physical environment. As the
energetic parameters that mediate complementarity continue to
emerge, it should become possible with these soluble well-behaved
assemblies to link different phase transitions to chemical changes
that function as kinetic feedback loops.
Such kinetic feedback loops, ones able to drive thermodynamic
ordering, are well known in biological networks, and are certainly
evident in the enzymatic reactions that control amplification of
genetic information. Related reactions have been developed for
both -helical [42,43] and nucleic acid [16,17] template-directed

reactions, but such reactions are just now being developed for the
cross-

α

β

assemblies [126,127]. The challenge will be to incorporate
the growing information about the structure and dynamics of these
assemblies to develop the appropriate template-directed chemistry.
The range of templates certainly includes not only the individual
strands of the

β

-sheets but also the laminate and pleat grooves of
the fibers and tubes as well as the internal cavities of the peptide
bilayer assemblies. The challenge will be to couple those reactions