Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 154

5.4.3 Mechanically Stretching to Induce Unfolding of

p -Stacked Structures

For folded -stacks, we have shown that heat is not very effective

at unfolding such nanostructures. Good organic solvents can unfold
π

π

-stacked nanostructures partially, yielding minor amount of
unfolded structures. However, molecular recognition proves to be
more effective at unfolding

-stacked nanostructures. Because of
this, molecular beacons become a very versatile tool to detect and
analyze DNA sequences. As a departure from the aforementioned
equilibrium approaches to unfolding, we will present here a
nonequilibrium approach to unfold

π

π

-stacked nanostructures—

mechanically stretching the foldamers.
Atomic force microscopy (AFM) has become a powerful tool
to study the unfolding phenomena of biological proteins. Single-
molecule force spectroscopy, which uses AFM to stretch one molecule
at a time, offered novel perspectives about structural information
and mechanical properties. How to connect the foldamer to the AFM
tip? Again, specific DNA hybridization provides the answer. However,
one AFM tip typically has enough space to host several molecules,
using available AFM tips to directly stretch a single molecule remains
challenging. The problem was solved by using self-assembled cone-
shaped dendrons to effectively space the reactive DNA molecules
attached to the dendron apexes. This controlled spacing removed
lateral steric hindrance, enhanced hybridization efficiency and
reproducibility, and greatly simplified the force distance curve.

Moreover, DNA hybridization offers tunable strength by varying
the number of base pairs. Because DNA hybridizes reversibly,
overwhelming AFM mechanical forces will not break the stretched
macromolecule, rather unzip DNA duplex; this enables repeated
pulling of the same macromolecule (Fig. 5.14).

−

S), foldamers
provide deeper insights into folding intricacies due to their beautiful
simplicity. A triblock polymer (

As a model of folded nanostructured

π

-stacks (N

π

DNA containing
three perylene moieties that fold into a thermophilic N

3D ), DNA

−

N

π

S

−

S, is being
investigated (Fig. 5.14). Because the foldamer specifically hybridizes
to the DNA molecules on the dendron-modified substrate and the AFM
tip, it effectively bridges the AFM tip and the substrate. Hydrophobic
effects strongly induce perylene moieties to fold into thermophilic
nanostructures. Mechanical stretching, however, forces the doubly
folded foldamer to unfold and adopt additional configurations in
repeated pulling: a singly folded state and a completely unfolded

π