Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 24

Moreover, simple washing experiments identified two equal
populations of TFA ions, with the surface population being readily
exchanged so as to differentiate the outer surface of these assemblies
from the inter-leaflet cavity [24]. Such differential access to these
cavities in such highly ordered supramolecular architectures can
now be exploited in kinetic schemes to push the system away from
thermodynamic equilibrium.
Previous recognition of the potential properties of such ordered
and densely charged surfaces had motivated the deposition of Au and
Pd nanoparticle or polyanions (e.g., anionic poly(styrenesulfonate))
to extend the functionality of the nanotubes [106]. Further, the
addition of excess divalent anions (HPO 3 2- 4 2-

) was found to
sufficiently passivate the surfaces of these positively charged tubes
to achieve even higher order assemblies [107]. Fibers composed
of bundled hollow nanotubes achieved lengths in excess of several
millimeters and the ends could be well resolved by electron
microscopy (Fig. 1.14A). Monte Carlo simulations [108] indicate
that the maximum attractive force between charged surfaces occurs
when buffered by a monolayer of counter-ions, and that divalent
anions reduce the entropic loss of binding [109]. Both SO

, SO

4 2- and

4 2-

HPO bundle the tubes at ~3:1 anion:peptide molar ratio (Fig.

1.14B), whereas >100:1 concentration ratios were required for
NaCl-induced bundling.
These ordered fibers compare favorably to the length scale
achieved with more typical lipid and general amphiphile assemblies,
and yet add several distinct features. The most obvious is the
ordered packing and precise crystalline arrangements available to
the individual peptide strands. The tube and fiber surfaces reflect
this long-range array, most nicely demonstrated by the ordered
fluorophores [101] and precisely arrayed metal ions [81]. Second,
the laminate grooves, constrained by NMR and diffraction analyses
and optically probed with CR and Alexa binding events, present
ordered binding sites because of the peptide crystalline phases
that are not attainable with typical amphiphiles. Third, while the
crystalline domains are depicted in precise arrays, the accumulating
evidence suggests they are both plastic and dynamic. For example,
similar peptide amphiphiles solubilize and stabilize known integral
membrane proteins as complex as photosynthetic reaction centers
[110]. And finally, given the variability in amino acid sequence and the
strand and sheet registry available, the surface of the architectures
available may have only been scratched and the extent to which such