C O M M U N I C A T I O N S
π-stacking interactions have been observed in other aromatic
oligomers and polymers.15-17 The association energy between the
four phenyl rings on two DCL modules is comparable to that for
the four hydrogen bonds within each DCL module. As the force
builds to the threshold, both the π-stacking and hydrogen bonding
within each cluster unfold simultaneously. Clusters with varying
number of DCL modules will gain different length upon unfolding,
resulting in a distribution of ∆L. Computer modeling also shows
that the large alkyl loops on both sides of a DCL module repel
each other due to sterics as DCL modules π-stack with each other,
making the smallest cluster formed between two DCL modules most
favorable (see Supporting Information). The unfolding of the
smallest cluster containing two DCL modules should gain 12 nm
in contour length, which agrees well with the most probable ∆L
value observed experimentally (Figure 2).
Figure 2. AFM single-molecule force-extension curve for DCL modular
polymer. The solid line is the fitting with WLC model at a 0.55 nm
persistence length (L is the contour length during stretching).
In conclusion, a new class of well-defined modular polymers
has been synthesized using a double-closed-loop peptidomimetic
â-sheet module. Single-molecule nanomechanical studies on these
polymers show more uniform sawtooth patterns, suggesting that
the DCL modular polymer undergoes sequential unfolding upon
stretching. Further studies toward the bulk mechanical properties
and the design of well-defined modules having varying H-binding
strength are currently underway in our laboratory.
2). The patterns in the force-extension curves were more uniform
for the DCL modular polymer than for the UPy system, which was
attributed to its more uniform structure. The number of peaks in
the stretching curves ranges from 2 to 7 with the most probable
range being 3-5. The chain detaches from the surface typically
after 60-120 nm stretching. The most probable peak force for
unfolding each module is ∼ 50 pN (see the histograms in the
Supporting Information). This force value is lower than that of our
UPy modular polymers which had an unfolding force of ∼100-
200 pN. This is consistent with the binding strengths of the two
modules: the dimerization constants (Kdim) measured in chloroform
for the UPy and the current peptidomimetic â-sheet units are ∼107
and 104, respectively.10,14
The consistent sawtooth pattern in the single-molecule force-
extension curves suggests the DCL monomer units unfold sequen-
tially as the polymer is stretched. As the AFM tip picks up a single
chain and is retracted from the surface, tension builds up on the
bridging polymer chain. Once the force builds up enough to unfold
one of the DCL units, a rupturing event occurs and the force drops.
This sequence of events can happen repeatedly as the polymer chain
is stretched, resulting in the sawtooth pattern seen in Figure 2.
Further evidence for single-chain stretching comes from fitting the
single-molecule data to the wormlike chain (WLC) model,5 which
is used to predict the relationship between the extension of a
polymer chain and its entropic restoring force. Our force-extension
data fit nicely to this model with a persistence length (b) of 0.55
nm (Figure 2). This value is reasonable as compared to the 0.4 nm
values for proteins.4 The persistence length for DCL polymer is
larger than for the UPy polymer or natural proteins because the
DCL polymer has a more rigid backbone than these systems.
Biological modular polymers such as titin show a constant
increase in contour length (∆L) between consecutive peaks in their
force-extension curves due to the sequential unfolding of identical
modules as the polymer is stretched.4 For DCL polymers, the ∆L
ranges from 5 to 21 nm with the most probable value of 12.5 nm
(see the histograms in the Supporting Information). We propose
that this nonconstant peak spacing is due to the formation of tertiary
structures by π-π stacking between adjacent DCL modules.
Computer modeling shows that the four phenyl rings on each DCL
module are coplanar. Strong π-π-stacking interactions between
phenyl rings on adjacent DCL modules drive them to form small
clusters along polymer chains (see Supporting Information). Similar
Acknowledgment. We are thankful for the financial support
from the Arnold and Mabel Beckman Foundation. We thank
Professor James Nowick for helpful discussion and Jane Bai for
the help on AFM experiments and WLC modeling.
Supporting Information Available: Synthesis and characterization.
This material is available free of charge via the Internet at http://
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