Chemistry Reference
In-Depth Information
Figure 14.1 (1) SAF-p1, (2) SAF-p2, and (3) SAF-p3. (Top) Sticky-ended coiled coil dimer
specified by complementary charge interaction and hydrogen bonding between Asn and Asn,
which led to the formation of straight cross-linked nanofibers. (Bottom) Control experiment of
the design of a blunt-ended dimer that failed to generate nanofibers. Scale bar ΒΌ 100 nm for all
images. Reprinted from Pandya et al. (2000). Copyright 2000 American Chemical Society.
sticky end at both termini. The structural specificity was gained and controlled by
molecularly engineering the dimer interface established by the amino acids at the
a, d, e, and g positions on both peptides. The selection of isoleucine at the a position
and leucine at the d position favors dimer formation as opposed to other oligomeriza-
tion states whereas asparagine was included at a different a site within each coil to
impart the preference of an out-registered structure and parallel orientation. Charge
interaction was designed to take place, as indicated in Figure 14.1, between the oppo-
site ends of each building block when they are fixed in a parallel arrangement. The
overall structure with the exposure of charged blocks recruited more partner peptides
to propagate into elongated nanofibers. Although it is almost impossible to visualize
the presence of an out-registered coiled coil dimer that leads to the higher ordered
assembly, the significance of the sticky-ended design was illustrated by synthesizing
a third peptide (SAF-p3) that was designed to form a stable in-registered structure
with SAS-p2 as a control experiment. The result showed that a mixture of SAF-p2
and SAF-p3 forming a blunt-ended dimer failed to generate fibers as opposed to
what was observed with the mixed SAF-p1 and SAF-p2 that were programmed to
form a sticky-ended dimer. The morphology of these fibers as characterized by
TEM was 10-mm length and 50-nm thickness, and X-ray fiber diffraction indicated
that the nanofibers are composed of coiled coils.
Concern arises regarding the disagreement between the fiber width determined by
TEM measurement and the theoretically calculated size for fibrils made of a single
