Toward Intelligent Materials - Molecular Self-Assembly: Advances and Applications - page 19

The symmetry of the nanotube shown in Fig. 1.10D,E reduces
the solvent-exposed surfaces in several respects. Most dramatically,
helical coiling (Fig. 1.8D) into a tube seals the pleat surfaces and
sequesters them from solvent so that only the strand termini are
exposed. The inner and outer tube surfaces have similar composition,
but differences in the radius of curvature of the inner and outer
surfaces will change the packing arrangements. Assuming that
for the ~40 nm-diameter nanotubes of KLVFFAL this difference is

negligible, this surface presented an opportunity to explore binding
of the histochemical Congo red (CR) to a simplified amyloid surface
[92]. The structure of the CR-binding site on amyloid has been
debated for almost 80 years [93-95], and the hallmark apple-green
interference and the broad red-shifted UV transition were both
observed on nanotube binding [92].
Electron diffraction and linear dichroism analyses revealed the
orientation of bound CR as parallel to the amyloid long axis, and was
most consistent with the alignment along the laminate grooves (Fig.
1.11). The structural model places the CR spanning six peptides (5 ×
4.7 Å), compatible with each sulfate charge specifically associating
with the positively charged lysines that line the laminate grooves.
The floor of the groove is composed of Leucine-17 (gray) and Leucine
22 (gray) side chains, creating a hydrophobic crevice. The repeating
array of binding sites are then defined by the out-of-register lysines
(blue), emerging from the hydrophobic surface like the knobs of a
herringbone stitch (Fig. 1.11, inset), to create an extended binding
crevice.
At saturation, these crevices or grooves order the CR molecules
linearly along three lysines in the groove, each closely packed (<20
Å) in well-ordered arrays (Fig. 1.11, inset). Such proximity is well
within the exciton coupling distance for the electronic transitions,
and exciton coupling theory [96,97] predicts a red-shift in

when
the transition dipoles are oriented end to end as Jelly-Scheibe (J)-
aggregates [98,99] and a blue-shift when organized side by side
as H-aggregates. As these interactions both appear to capture the
characteristic features of the CR binding to amyloid and offer an
opportunity to better characterize the laminate groove, an additional
effort focused on assigning the specific transitions [92].

λ

max