Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 3

simple polypeptides display complex conditionally responsive phase
behaviors [21,22] that transitions into extended crystalline arrays
[23] with long-range order that rivals today's bilayer membranes
[24]. Here we have reviewed some of the wide diversity of forms
accessible through self-assembly of the polypeptide backbone and
demonstrate, even in this limited sampling, the accessibility and
diversity of this scaffold for the creation of intelligent materials
[8]. This effort has revealed the foundation on which a synthetic
approach to chemical evolution can be constructed for the next
generation of new materials.

1.2 Forming Ordered Networks

As early as the late 1950s Sidney Fox was investigating peptide
assemblies prepared from the amino acids that were generated in
the Miller-Urey experiments [25]. The aggregated microspheres
emerging from random condensation of mixtures of amino acids
were suspected of having many remarkable material properties
[26], but little detailed structural information, not even the peptide
sequences, were available at that time. New structural methods
have changed our capabilities dramatically. We now know that
proteins are constructed with a surprisingly small number of
simple secondary folds, and of these, the

α

-helix has been the most
extensively used in the development of new materials [27,28]. The
spiraling backbone of the helix projects each amino acid side chain
in a prescribed pattern along the surface of a cylinder where their
role in protein function can range from catalysis to co-factor binding
and membrane anchoring, to stabilizing struts in structural scaffolds
and even dynamic motions for complex signal transmission [29].
With native proteins as guides, higher order structures have now
been constructed by exploiting face-complementary helices to create
long-range molecular order for both structure and catalysis [30-34].
A representative example can be seen in the work of Conticello and
co-workers in their design of macroscale helical assemblies [35].
By constructing the first half of a helix bearing negative charges
(glutamic acid) and the second half positive charges (arginine and
lysine) (Fig. 1.1), the resulting amphiphilic helix progressively
assembles as staggered arrays. These centimeter-long fibers have
now been constructed to be responsive to pH, small molecules, and
metal ions [36-38].