Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 13

incorporated into the scaffold by extending the native N-terminus to
HAQKLVFFA [81]. Indeed, this peptide was shown to bind Cu(II) and
Zn(II) ions through chelation to two side-chain imidazoles, one from
each adjacent H-bonded

-strand (Fig. 1.5D), forcing parallel strand
registry [82]. The Cu(II)-HAQKLVFFA assembly, like the KLVFFAQ
peptide, form parallel sheet assemblies that are fibers.
Taken together, knowledge of several critical interactions and
folding constraints are indeed allowing for bottom-up approaches
in the design of cross-

β

architectures (summarized in Fig. 1.7), and
these results suggest that the strategies should be transferable, and
predictive, in other

β

-sheet rich assembly designs [83-87]. However,
higher order morphologies, e.g., tubes, will depend on developing
constraints for facial complementarity, or sheet-sheet interactions,
and overall ribbon curvature, or pitch of the resulting ribbons. These
cross-strand pairing examples do suggest initial approaches for
probing the laminate interface and possibly other critical interactions
that play critical roles in stabilizing the nanotube morphology.

β

Sheet-Sheet Association: In the context of the growing examples

of cross-

β

assemblies, it remains remarkable that the assembly of

A -sheets

or the hollow nanotubes that contain >100 sheet laminates can be
regulated by a single protonation event (Fig. 1.3). This dramatic
lamination difference (Fig. 1.8D) has been argued to result from a
change in

β

(16-22) as either 5 nm-diameter fibers containing a few

β

-sheet complementarity that increases the stability of the
laminate [23]. Topologically, as the laminate number increases, the
growing ribbon architecture can no longer access the twisted fibers,
but the helical fold of the nanotubes does become energetically
available. Nanotube structural models developed through molecular
dynamics (MD) analyses that are initially constrained by solid-state
NMR and match diffraction data, suggests that the distribution of
the F-F dyad on each

β

-sheet face may contribute significantly to
extended lamination (Fig. 1.3C,F) [23]. Indeed, congeners resulting
from replacement of the F-F dyad with either I-I or L-L assemble
only as fibers. Structural models have not yet resolved the precise
arrangement of the F-F dyad for optimal complementarity; also
it is not really clear whether the morphology is due to proper
alignment of the F-F dyad or other contributing constraints, but one
reasonable hypothesis is that cross-sheet constraints, like cross-
strand constraints, are important for morphological control.

β