Recent years have witnessed an increasing interest in hybrid molecular systems in which the programmability of DNA hybridization is used to introduce enhanced molecular control in synthetic systems. The first examples of DNA-functionalized supramolecular polymers have been reported only recently, but have already revealed structural and functional properties that are not easily obtained in either synthetic supramolecular polymers or DNA-only based systems. In this Topical Review, we provide an overview of the various forms of additional control offered by DNA hybridization for different types of supramolecular polymers and discuss how orthogonal supramolecular interactions in these hybrid systems can give rise to emergent structural and functional properties.
Recent years have witnessed an increasing interest in hybrid molecular systems in which the programmability of DNA hybridization is used to introduce enhanced molecular control in synthetic systems. The first examples of DNA-functionalized supramolecular polymers have been reported only recently, but have already revealed structural and functional properties that are not easily obtained in either synthetic supramolecular polymers or DNA-only based systems. In this Topical Review, we provide an overview of the various forms of additional control offered by DNA hybridization for different types of supramolecular polymers and discuss how orthogonal supramolecular interactions in these hybrid systems can give rise to emergent structural and functional properties.
At the cellular level, life is predominantly
built from aqueous,
dynamic molecular assemblies.[1,2] The transient nature
of these complex multicomponent systems introduces adaptability and
allows for rapid response to biological triggers with great efficiency.[3−5] In the quest to understand and emulate these natural systems, supramolecular
chemistry has become a topical research field in which supramolecular
polymers play a prominent role.[6] Although
the first synthetic supramolecular polymers were designed to assemble
in organic solvents, many water-soluble variants exist today, providing
an interesting platform for the development of molecular systems and
materials with life-like properties.[7] Extensive
studies using a wide variety of biophysical approaches have provided
detailed insight into the assembly mechanisms and exchange dynamics
of several of these water-soluble supramolecular polymers.[8−12] These studies have revealed a subtle interplay between various noncovalent
interactions that together govern their structural and dynamic properties,
but also showed that tuning these properties and introducing functionality
in these dynamic systems can be challenging. The latter is important
as future applications would require these systems to specifically
interact with other components, materials, cells, or tissues. However,
synthesis of these building blocks is not straightforward.DNA
has rapidly emerged as a highly versatile molecular building
block for the construction of precise nanometer structures and sophisticated
molecular machines and networks. In contrast to synthetic supramolecular
interactions, the programmability of DNA hybridization enables the
modular assembly of structures and reaction cascades with great precision
and structural control.[13] Recent years
have witnessed an increasing interest in hybrid molecular systems.[14,15]For example, DNA functionalization of covalent polymers provides
an additional level of control on the structure and macroscopic properties
of materials, allowing the construction of stimuli-responsive materials
such as hydrogels and other nanomaterials, DNA-surfactants that can
be applied as responsive drug delivery systems, and materials for
optoelectronic devices.[16−22] Only recently have the first examples of DNA-functionalized supramolecular
polymers been reported. In this Topical Review, we provide an overview
of the various forms of additional control offered by DNA hybridization
for different supramolecular polymers and discuss how orthogonal supramolecular
interactions in these hybrid systems can give rise to emergent structural
and functional properties.
Aromatic Oligomers
Some of the first
examples of DNA-functionalized supramolecular
polymers were reported by the group of Häner. In their pioneering
work, DNA-grafted supramolecular polymers consisting of oligomers
of aromatic compounds and oligonucleotides were constructed and used
to study how DNA can be used to gain control over the structural characteristics
of supramolecular polymers.[23] One class
of hybrid supramolecular building blocks consists of phosphodiester-linked
pyrene oligomers modified with oligonucleotides via solid-phase phosphoramidite
chemistry (Figure a). The supramolecular assembly of these monomers is initiated by
the formation of stair-like arrangements of the pyrenes within the
monomers and subsequent assembly of multiple monomers. The balance
between the lengths of the oligopyrenes and oligonucleotides directed
the morphology of the supramolecular assemblies. Oligopyrenes containing
7 pyrenes and 10 base oligonucleotides reversibly formed fibers with
lengths up to several hundreds of nanometers, while no fibrous structures
could be observed for oligopyrenes containing 4 or 1 pyrene unit(s).[23] Additionally, hepta-oligopyrene functionalized
with a single nucleotide assembled into 2D nanosheets, whereas micrometers
to tens-of-nanometers-long ribbons were formed when using DNA handles
containing 2 or more nucleotides, respectively.[24] Addition of an oligonucleotide complementary to that on
the hepta-oligopyrene units resulted in the formation of micrometer-size
fibrous networks due to cross-links formed by blunt-end stacking of
the grafted double-stranded DNA (Figure b).[9] The formation
of these networks was reversible by thermal denaturation of the double-stranded
DNA or by the addition of a scavenger oligonucleotide which separates
the strand complementary to the grafted handle via a strand displacement
reaction. The formation of cross-linked networks could also be achieved
by mixing supramolecular DNA-oligoperylene polymers grafted with complementary
strands (Figure c).[26] Increasing the temperature first disassembled
the resulting networks followed by full disruption of the supramolecular
polymeric assemblies at higher temperatures. Interestingly, subsequent
cooling of the mixture did not result in the reformation of networks,
but instead yielded one-dimensional polymer stacks containing mixtures
of the grafted strands. It was hypothesized that in these mixed polymers,
electrostatic repulsion between noncomplementary oligonucleotides
prevented hybridization of complementary strands between fibers, and
that the initial network formed by mixing preformed supramolecular
polymers with complementary strands represented a metastable state.
These findings illustrate the importance of pathway complexity on
the structural and functional properties of supramolecular polymers
containing orthogonal assembly motifs.
Figure 1
Using DNA to reversibly
control the structural characteristics
of supramolecular polymer assemblies. (a) Structure and schematic
representation of a DNA-modified heptapyrene monomer with the sequences
of the grafted, separator, and connector oligonucleotides. (b) DNA-modified
heptapyrene monomers are assembled into DNA-grafted ribbons in water.
Hybridization of the oligonucleotide on the heptapyrene with a complementary
handle (1b) results in the formation of fibrous networks driven by
the blunt-end stacking of the grafted DNA helices. This process is
reversible via disruption of the hybridized oligonucleotides, either
by thermal denaturation or by addition of an excess of a separator
strand (1a). Adapted and reproduced with permission from ref (25). (c) Schematic representation
of the assembly pathways of pyrene monomers functionalized with complementary
oligonucleotides (Py-a and Py-b). The monomers separately assemble
into supramolecular polymers which form networks upon mixing due to
hybridization of the complementary grafted oligonucleotides. Thermal
disruption of these networks or mixing of the different monomers leads
to the formation of mixed polymers which do not form networks. Adapted
and reproduced with permission from ref (26).
Using DNA to reversibly
control the structural characteristics
of supramolecular polymer assemblies. (a) Structure and schematic
representation of a DNA-modified heptapyrene monomer with the sequences
of the grafted, separator, and connector oligonucleotides. (b) DNA-modified
heptapyrene monomers are assembled into DNA-grafted ribbons in water.
Hybridization of the oligonucleotide on the heptapyrene with a complementary
handle (1b) results in the formation of fibrous networks driven by
the blunt-end stacking of the grafted DNA helices. This process is
reversible via disruption of the hybridized oligonucleotides, either
by thermal denaturation or by addition of an excess of a separator
strand (1a). Adapted and reproduced with permission from ref (25). (c) Schematic representation
of the assembly pathways of pyrene monomers functionalized with complementary
oligonucleotides (Py-a and Py-b). The monomers separately assemble
into supramolecular polymers which form networks upon mixing due to
hybridization of the complementary grafted oligonucleotides. Thermal
disruption of these networks or mixing of the different monomers leads
to the formation of mixed polymers which do not form networks. Adapted
and reproduced with permission from ref (26).A second example of hybrid supramolecular-building blocks
introduced
by the Häner group consists of DNA-modified tri- and pentaphenanthrenes,
which were also synthesized using phosphoramidite chemistry. When
two triphenanthrenes modified with complementary oligonucleotides
of 20 bases are mixed, the supramolecular interaction is dominated
by DNA hybridization resulting in the formation of 20 nucleotide dsDNA
containing phenanthrene overhangs at both ends. Hydrophobic interactions
between the phenanthrene “sticky ends” promoted the
formation of sheets and 50–200 nm vesicles in the presence
of spermine, which counteracts the electrostatic repulsion between
the dsDNA units (Figure a).[27] The same building blocks could also
be used to construct micrometers-long polymers grafted with ss-DNA-handles
by mixing unmodified triphenanthrene monomers with pentaphenanthrene
monomers modified with only one of the oligonucleotides (Figure b).[28] In these systems supramolecular polymerization is driven
by hydrophobic interactions between the phenanthrene groups, while
the oligonucleotides serve as handles grafted on the supramolecular
polymer to introduce additional functionalities. The light-harvesting
properties of the oligophenanthrenes were used to construct photonic
wires in which the energy harvested by light absorption in the oligophenanthrenes
was channeled via a cascade of (Förster) energy transfer steps
through precisely spaced cyanine dyes on complementary oligonucleotides
to finally excite a NIR Cy5.5 acceptor dye.
Figure 2
Combining the intrinsic
light harvesting properties of phenanthrene
based polymers to transfer energy toward a DNA photonic wire. (a)
Vesicles are formed by the assembly of two triphenanthrenes modified
with complementary oligonucleotides. Adapted and reproduced with permission
from ref (27). (b)
Supramolecular polymers are assembled by mixing triphenanthrene (A)
with DNA-modified pentaphenanthrene (B) monomers. By addition of labeled
oligonucleotides complementary to the phenanthrene grafted oligonucleotide
(C and D), a photonic wire is constructed. After excitation of the
phenanthrenes, the harvested energy is transferred along the supramolecular
polymers toward the photonic wires where it is transferred via FRET
between the cyan dyes to the final Cy5.5 acceptor. Adapted and reproduced
with permission from ref (28). Copyright 2018, Wiley-VCH.
Combining the intrinsic
light harvesting properties of phenanthrene
based polymers to transfer energy toward a DNA photonic wire. (a)
Vesicles are formed by the assembly of two triphenanthrenes modified
with complementary oligonucleotides. Adapted and reproduced with permission
from ref (27). (b)
Supramolecular polymers are assembled by mixing triphenanthrene (A)
with DNA-modified pentaphenanthrene (B) monomers. By addition of labeled
oligonucleotides complementary to the phenanthrene grafted oligonucleotide
(C and D), a photonic wire is constructed. After excitation of the
phenanthrenes, the harvested energy is transferred along the supramolecular
polymers toward the photonic wires where it is transferred via FRET
between the cyan dyes to the final Cy5.5 acceptor. Adapted and reproduced
with permission from ref (28). Copyright 2018, Wiley-VCH.
Squaramide Bola-Amphiphiles
Several other examples have
been reported where DNA hybridization
is used to recruit DNA-modified molecular cargo on supramolecular
polymers. Both the Häner group and the group of Kieltyka demonstrated
reversible recruitment of DNA-functionalized gold nanoparticles along
supramolecular polymers.[29,30] The latter group used
squaramide based bola-amphiphiles to form hundreds-of-nanometers-long
supramolecular assemblies, driven by hydrogen bonding and hydrophobic
interactions. Squaramide monomers were decorated with 16 nucleotide
DNA-handles via copper mediated cyclo-addition reactions and mixed
with inert monomers to obtain DNA-grafted squaramide polymers (Figure a). By using unique
handle sequences for particles with a diameter of 5 or 15 nm in combination
with two squaramide monomers decorated with complementary handles,
the system enabled the specific recruitment of one or both gold particles
(Figure b). The DNA-based
tethering of the particles also allowed subsequent release of the
particles by using toehold-mediated strand displacement. By sequential
addition of the DNA-modified particles and displacer strands, sequential
loading and release of gold nanoparticles was shown, demonstrating
a high level of control over the recruitment and release of components
on the polymer scaffold.
Figure 3
Selective and reversible recruitment of cargo
on supramolecular
polymers mediated by DNA hybridization. (a) Structures of an inert
and a DNA-modified squaramide bola-amphiphile. (b) Supramolecular
copolymers of squaramide derivatives were assembled containing two
monomers grafted with DNA handles with different sequences (a and
c). This enabled the selective recruitment of two gold particles of
5 and 15 nm functionalized with DNA handles containing a sequence
complementary to one of the DNA-grafted squaramides (a* and c*). By
addition of an oligonucleotide (b,a) fully complementary to the handle
on one of the gold nanoparticles, the DNA handle on the polymer is
displaced, which selectively releases the particle. Using these techniques,
the system allows the sequential recruitment and release of the 15
nm particles after which the 5 nm particles can be recruited. Scale
bars represent 50 nm. Adapted and reproduced with permission from
ref (30). Copyright
2017, Wiley-VCH.
Selective and reversible recruitment of cargo
on supramolecular
polymers mediated by DNA hybridization. (a) Structures of an inert
and a DNA-modified squaramide bola-amphiphile. (b) Supramolecular
copolymers of squaramide derivatives were assembled containing two
monomers grafted with DNA handles with different sequences (a and
c). This enabled the selective recruitment of two gold particles of
5 and 15 nm functionalized with DNA handles containing a sequence
complementary to one of the DNA-grafted squaramides (a* and c*). By
addition of an oligonucleotide (b,a) fully complementary to the handle
on one of the gold nanoparticles, the DNA handle on the polymer is
displaced, which selectively releases the particle. Using these techniques,
the system allows the sequential recruitment and release of the 15
nm particles after which the 5 nm particles can be recruited. Scale
bars represent 50 nm. Adapted and reproduced with permission from
ref (30). Copyright
2017, Wiley-VCH.
Peptide Amphiphiles
A particularly impressive demonstration of the ability to reversibly
and rationally control the formation of superstructured networks was
recently reported by Stupp and co-workers through introducing DNA
handles on peptide amphiphile (PA) based supramolecular polymers.
PA monomers consist of an aliphatic chain and an amino acid sequence
containing a β-sheet forming region and a hydrophilic region.
To obtain DNA-modified PA derivatives, amine-functionalized oligonucleotides
with lengths from 10 to 45 nucleotides were functionalized with dibenzocyclooctyne-sulfo-N-hydroxylsuccinimidyl (DIBAC-NHS) and subsequently conjugated
to an azide-modified PA using strain promoted alkyne azide click chemistry
(Figure a).[31] Mixing of DNA-modified and inert PAs resulted
in the formation of 10–15-nm-wide, one-dimensional nanofibers
containing a stoichiometric distribution of DNA handles driven by
hydrophobic interactions and β-sheet formation. Pure DNA-modified
PA’s did not form fibers but assembled into spherical micelles,
probably due to steric and electrostatic repulsion between the DNA
handles.
Figure 4
Reversible formation of superstructures in peptide amphiphile–DNA
networks. (a) Structure of a peptide amphiphile (PA) monomer modified
with oligonucleotides via DIBAC conjugation. (b) PA copolymers of
inert and DNA-modified monomers cross-link due to hybridization of
complementary grafted oligonucleotides which subsequently cluster
within the fibers. (c) Simulations show that homogeneous hydrogels
are obtained when molecular exchange of DNA monomers between PA fibers
is prohibited. The magnified view shows individual fibers (blue) with
a stochastic distribution of DNA monomers (pink) along the fibers.
(d) When molecular exchange is allowed, the simulations result in
the formation of bundled fibers. The magnified view shows a bundle
of fibers (blue) enriched with DNA (pink) in a matrix of individual
fibers depleted of DNA monomers. (e) Simulations showing that the
bundle growth rate is a function of the molecular attraction between
monomers and the interfiber hybridization of the grafted oligonucleotides
(respectively, the intra- and interfiber energies (Eintra, Einter)). Bundles form
within the energy range 5 kBT < Eintra. (f) SEM micrographs of
astrocytes cultured on cross-linked, bundled (left), and individual
(right) PA-DNA fibers. The cells display a reactive phenotype on the
bundled fibers while a naive phenotype is observed on the individual
fibers. Cells are falsely colored in blue and bundled PA-DNA fibers
in pink. The magnified view (lower images) shows the cell–substrate
interaction. Scale bars represent 5 μm (upper images) and 2
μm (lower images). Adapted and reproduced with permission from
ref (31). Copyright
2018, American Association for the Advancement of Science.
Reversible formation of superstructures in peptide amphiphile–DNA
networks. (a) Structure of a peptide amphiphile (PA) monomer modified
with oligonucleotides via DIBAC conjugation. (b) PA copolymers of
inert and DNA-modified monomers cross-link due to hybridization of
complementary grafted oligonucleotides which subsequently cluster
within the fibers. (c) Simulations show that homogeneous hydrogels
are obtained when molecular exchange of DNA monomers between PA fibers
is prohibited. The magnified view shows individual fibers (blue) with
a stochastic distribution of DNA monomers (pink) along the fibers.
(d) When molecular exchange is allowed, the simulations result in
the formation of bundled fibers. The magnified view shows a bundle
of fibers (blue) enriched with DNA (pink) in a matrix of individual
fibers depleted of DNA monomers. (e) Simulations showing that the
bundle growth rate is a function of the molecular attraction between
monomers and the interfiber hybridization of the grafted oligonucleotides
(respectively, the intra- and interfiber energies (Eintra, Einter)). Bundles form
within the energy range 5 kBT < Eintra. (f) SEM micrographs of
astrocytes cultured on cross-linked, bundled (left), and individual
(right) PA-DNA fibers. The cells display a reactive phenotype on the
bundled fibers while a naive phenotype is observed on the individual
fibers. Cells are falsely colored in blue and bundled PA-DNA fibers
in pink. The magnified view (lower images) shows the cell–substrate
interaction. Scale bars represent 5 μm (upper images) and 2
μm (lower images). Adapted and reproduced with permission from
ref (31). Copyright
2018, American Association for the Advancement of Science.Mixing fibers decorated with complementary oligonucleotides
resulted
in the formation of hydrogels, which could be reversed by breaking
the DNA-mediated interaction between fibers using toehold-mediated
strand displacement (Figure b). Structural analysis of the gels using scanning electron
microscopy (SEM) showed a superstructure consisting of large, micrometer-sized
bundles of fibers segregated within a network of individual nanoscale
fibers. Coarse-grained simulations of this system suggested that the
formation of these superstructures relies on the formation of clusters
of cross-linked DNA-modified monomers within the fibers. The cross-linked
fibers then intertwine to form bundles which subsequently intertwine
with each other to form the observed superstructures. Confocal microscopy
revealed that the formation of the cross-linked fibers was complete
in 10 min and was accompanied by clustering of the DNA-modified PA
within the fiber. It was shown experimentally and by simulations that
it is crucial that the interactions between the PA monomers are sufficiently
strong to form stable fibers but not too strong to prevent dynamic
redistribution of the DNA-grafted monomers to form the bundled structures
(Figure c,d). Additionally,
the hybridization interactions need to be strong enough to maintain
the cross-links between the fibers. Taken together, these simulations
showed that the bundles can form when the balance between the intra-
and interfiber interactions lies in a remarkably narrow distribution
(Figure e). The density
of DNA-PA also proved important for the formation of the cross-linked
superstructures. Very low densities did not support the formation
of sufficient cross-links, but at too high DNA-PA densities the system
formed a three-dimensional gel before the formation of the cross-linked
fibers could occur. The formation of cross-linked fibers in the gel
increased the stiffness of the gels and could be tuned in a predictable
manner by changes in the length and GC content of a cross-linker DNA
strand, salt concentration, and the length of the aliphatic chain
and type of amino acids in the PA monomers. Additionally, the DNA
cross-links and the subsequent formation of the highly bundled fibrous
structures could be disrupted in a reversible fashion via thermal
disruption or toehold-mediated strand displacement. This ability to
switch the structural properties of the fibrous networks was subsequently
used to study the response of neural cells (astrocytes) to changes
in their environment. When the cells were cultured in the PA-DNA hydrogels
containing the higher order bundles, a reactive phenotype was observed
while naive cells were observed in hydrogels existing from individual
fibrous networks (Figure f). Interestingly, the phenotype of the cells could be switched
from reactive to naive or visa versa by addition of an invader or
anti-invader strand to, respectively, disrupt or reform the DNA cross-links
in the network.
Benzene-1,3,5-Tricarboxamide (BTA) Polymers
In the work described so far, DNA hybridization either served to
recruit DNA-cargo on a supramolecular polymer, or was used to control
the formation of networks and other superstructures. However, DNA
hybridization and supramolecular polymerization can also enhance each
other when two complementary DNA strands are attached to the same
supramolecular polymer in an antiparallel orientation. This phenomenon
was studied by Brunsveld and co-workers using C3 symmetric amphiphiles
consisting of a bis-pyridine benzene-1,3,5-tricarboxamide (BiPy-BTA)
core decorated with water-soluble ethylene glycol (EG) tails (Figure a). Driven by hydrophobic
interactions and intermolecular hydrogen bonds between the monomer
cores, these BiPy-BTA-EG monomers assemble into stable fluorescent
polymers.[3200] DNA-functionalized BiPy-DNA
monomers were obtained by conjugation of azide-modified oligonucleotides
to bis-pyridine decorated BTA derivatives (BiPy-BTA) modified with
a single dibenzocyclooctyne (DBCO) via strain-promoted cycloaddition.[32] To systematically study the effect of supramolecular
templating on DNA hybridization, a 13 nucleotide template strand was
conjugated via its 3′ end, whereas a series of 13 base DNA
sequences containing between 4 and 7 complementary bases was conjugated
to the BiPy-BTA via their 5′-end (Figure c). Supramolecular-templated hybridization
was monitored using FRET between Cy3- and C5-dyes conjugated to each
strand and compared to the nontemplated interaction. Supramolecular
templating enhanced DNA hybridization by at least 6 kcal/mol, equivalent
to the effect of 4 additional base pair interactions. The increased
interaction strength can be understood by the high local concentration
of the DNA-strands, which was estimated to be between 2 and 20 mM.
Using the dynamic nature of the assemblies, the hybridization of the
grafted oligonucleotides with 4 complementary bases could subsequently
be reversed via addition of inert monomers which insert between the
DNA-modified monomers in the assemblies, thus decreasing the local
concentration of DNA strands on the supramolecular polymer (Figure e). Time-dependent
studies showed that the rearrangement process took about 2 h and was
determined by the kinetics of monomer exchange.
Figure 5
(a) Inert and DBCO-modified
bis-pyridine decorated BTA derivatives
are mixed and (b) subsequently form coassemblies. (c) Oligonucleotides
with various sequences are then added to react with the DBCO-modified
monomers. (d) After mixing BTA assemblies grafted with complementary
oligonucleotides, the addition of magnesium results in hybridization
of the complementary DNA-handles enabled by the dynamic rearrangement
of the BiPy-BTA monomers. (e) Subsequent addition of inert monomers
leads to dilution of the DNA-grafted monomers over the assemblies
and disruption of the hybridized handles and redistribution of the
monomers. Reproduced with permission from ref (32). Copyright 2018, Wiley-VCH.
(a) Inert and DBCO-modified
bis-pyridine decorated BTA derivatives
are mixed and (b) subsequently form coassemblies. (c) Oligonucleotides
with various sequences are then added to react with the DBCO-modified
monomers. (d) After mixing BTA assemblies grafted with complementary
oligonucleotides, the addition of magnesium results in hybridization
of the complementary DNA-handles enabled by the dynamic rearrangement
of the BiPy-BTA monomers. (e) Subsequent addition of inert monomers
leads to dilution of the DNA-grafted monomers over the assemblies
and disruption of the hybridized handles and redistribution of the
monomers. Reproduced with permission from ref (32). Copyright 2018, Wiley-VCH.The work by Brunsveld and co-workers
shows the potential of using
supramolecular polymers as a dynamic platform to enhance molecular
interactions by increasing their effective local concentration, a
strategy that is also frequently used in biology to control signal
transduction and increase the efficiency of metabolic reaction cascades.
Our group recently showed that DNA-mediated recruitment of proteins
on another type of supramolecular BTA polymer can efficiently
promote protein–protein interactions. The BTA system used in
these studies consisted of a BTA core containing three amphiphilic
dodecyl-EG4 side-arms that shield the hydrogen bonds in
the core from water, while facilitating water solubility of the assembled
polymer. BTA-DNA monomers were obtained by conjugation of a 10 base
alkyne-functionalized oligonucleotide to an azide-functionalized BTA
via a copper-mediated alkyne–azide cycloaddition reaction (Figure a).[33] Copolymer assembly of BTA and BTA-DNA resulted in the formation
of μm-long 1D supramolecular BTA polymers. These polymers were
found to be remarkably robust, even with a high percentage of
DNA-functionalized BTA. To study specific recruitment of proteins
to the BTA polymers, TEM1-β-lactamase was used as a reporter
enzyme along with its inhibitor protein BLIP. Each protein was functionalized
with a specific 21-base oligonucleotide, which allowed recruitment
on the BTA-polymer in the presence of specific recruiter strands that
are complementary to the DNA-strands on the BTA and on one of the
proteins (Figure b).
Figure 6
Controlling
protein activity by dynamic recruitment on a supramolecular
polymer platform. (a) Chemical structures of an inert BTA monomer
and a DNA-modified monomer obtained via copper mediated cyclo-addition.
(b) Supramolecular polymers are obtained by assembly of inert and
DNA-modified BTAs. A DNA-modified enzyme and inhibitor protein can
be selectively recruited driven by the hybridization of specific recruiter
strands with the oligonucleotides on the BTA and the proteins. The
high local protein concentration results in protein complex formation
and subsequently a decrease in enzyme activity. (c) Normalized enzyme
activity as a function of inhibitor concentration (black dots). The
fitting of the enzymatic activities was derived from a Michaelis–Menten
model for competitive inhibition, yielding an apparent inhibition
constant (Ki,app) of 2.3 ± 0.2 nM
(red line). (d) Upon the addition of inert BTA monomers, dynamic exchange
of BTAs results in dilution of the recruited proteins and subsequently
an increase of the total protein activity. (e) Upon addition of the
DNA-grafted BTA polymers to the proteins and recruiter strands, the
equilibrium of protein inhibition is reached within minutes. Adapted
and reproduced with permission from ref (33). Copyright 2018, Springer Nature.
Controlling
protein activity by dynamic recruitment on a supramolecular
polymer platform. (a) Chemical structures of an inert BTA monomer
and a DNA-modified monomer obtained via copper mediated cyclo-addition.
(b) Supramolecular polymers are obtained by assembly of inert and
DNA-modified BTAs. A DNA-modified enzyme and inhibitor protein can
be selectively recruited driven by the hybridization of specific recruiter
strands with the oligonucleotides on the BTA and the proteins. The
high local protein concentration results in protein complex formation
and subsequently a decrease in enzyme activity. (c) Normalized enzyme
activity as a function of inhibitor concentration (black dots). The
fitting of the enzymatic activities was derived from a Michaelis–Menten
model for competitive inhibition, yielding an apparent inhibition
constant (Ki,app) of 2.3 ± 0.2 nM
(red line). (d) Upon the addition of inert BTA monomers, dynamic exchange
of BTAs results in dilution of the recruited proteins and subsequently
an increase of the total protein activity. (e) Upon addition of the
DNA-grafted BTA polymers to the proteins and recruiter strands, the
equilibrium of protein inhibition is reached within minutes. Adapted
and reproduced with permission from ref (33). Copyright 2018, Springer Nature.Recruitment of both proteins on the BTA-scaffold
resulted in inhibition
of enzyme activity at low nanomolar concentrations of the inhibitor
protein, representing a 1000-fold increase in apparent inhibition
constant compared to the Kd of 1.5 μM
obtained in solution (Figure c). The increased apparent affinity could be quantitatively
understood by the increased local concentration of the proteins on
the BTA polymer scaffold, which could be tuned by changing the density
of DNA handles on the polymer. Indeed, enzyme activity could be restored
by the addition of excess inert BTA monomers to dilute the BTA-DNA
density from 5% to 0.25%. The restoration of enzyme activity by BTA-dilution
took several hours, which is in accordance with the kinetics known
for dynamic exchange of the BTA monomers (Figure d). Remarkably, recruitment of the DNA-modified
proteins on the BTA template and subsequent formation of the enzyme–inhibitor
complex was found to be fast, reaching equilibrium within minutes
(Figure e). Since
this process is much faster than the kinetics of BTA-monomer exchange,
protein complex formation on the BTA-scaffold was hypothesized to
involve rapid association and dissociation between DNA-duplexes along
the polymer. The 10-nucleotide interaction between the DNA strand
on the BTA polymer and the recruiter strand thus allows rapid exchange
of DNA-conjugated proteins (kdiss ∼
1 s–1),[34] while also
providing sufficient thermodynamic driving force to recruit proteins
and increase their effective local concentration.The remarkable
efficiency of DNA-functionalized BTA polymer to
template the formation of protein–protein interactions, suggested
that they might also provide attractive scaffolds to enhance the speed
of DNA-based molecular circuits.[35] These
circuits typically consist of a series of sequential toehold-mediated
strand exchange reactions, using low nanomolar concentrations of individual
reactants to avoid background and nonintended side reactions. As the
reaction rates of the DNA networks depend on the concentration of
the reactants, the operations are slow, taking several hours or even
days, hampering their translation into practical applications. The
speed of DNA-based molecular computing has been increased by confining
the reactants on the surfaces of DNA-functionalized nanoparticles
or DNA-origami scaffolds. However, immobilizing oligonucleotide reactants
on a DNA origami scaffold requires careful positioning of the circuit
components to facilitate efficient interaction between reactants.
To explore whether DNA-functionalized supramolecular polymers could
provide a more dynamic and operationally more simple alternative platform,
we first tested whether confinement of DNA reactants on the BTA-polymer
could enhance the kinetics of toehold-mediated strand displacement
and strand exchange reactions, two fundamental reactions in DNA computing.
DNA reactants were extended with a sequence complementary to the 10
bases of the BTA-DNA conjugate (Figure a,b). Recruitment of these components on the BTA-DNA
polymers increased the speed of single displacement and exchange reactions
up to a 100-fold. The templated strand exchange reaction was also
favored thermodynamically by multivalent interactions with the polymer
template. The general applicability of the BTA-DNA scaffold to increase
the speed of DNA-based computing was subsequently demonstrated for
three well-known and practically important DNA-computing operations:
multi-input AND gates, catalytic hairpin assembly (CHA), and hybridization
chain reactions (HCR). The product of the latter amplification reaction
is of a polymer of alternating, partially overlapping DNA strands
(Figure c). Dual color
imaging of Cy3-labeled BTAs and Cy5-labeled reaction product using
a combination of total internal reflection (TIRF) and stochastic optical
reconstruction microscopy (STORM) indeed showed a clear overlap between
the DNA polymer product and the BTA-polymer backbone, consistent with
the BTA-polymer acting as template of the HCR reaction (Figure d).
Figure 7
Acceleration of DNA-based
computing on supramolecular polymers.
(a) Schematic representation of a strand exchange reaction using freely
diffusing oligonucleotide reactants with the kinetic characterization
of a reaction with toeholds of various lengths. (b) Schematic representation
of a strand exchange reaction templated by a BTA polymer. The reactants
are recruited to the polymer via the hybridization of a sequence complementary
to the oligonucleotide on the BTA. This results in a high local concentrations
which consequently increases the association kinetics of toehold binding.
Additionally, due to the multivalent anchoring of the product to the
supramolecular polymer, the product is stabilized resulting in increased
operation yields. (c) Schematic representation of a hybridization
chain reaction (HCR) templated by the BTA polymers. Starting upon
the addition of an input, the sequential opening of two labeled metastable
hairpin strands results in the formation of a polymeric DNA assembly.
(d) TIRF images of immobilized BTA polymers containing Cy3 labeled
monomers with corresponding STORM images of the HCR product containing
Cy5 labeled oligonucleotides after the HCR was initiated by addition
of, respectively, 1 and 5 nM input strand. The scale bars represent
2 μm. Adapted and reproduced with permission from ref (35).
Acceleration of DNA-based
computing on supramolecular polymers.
(a) Schematic representation of a strand exchange reaction using freely
diffusing oligonucleotide reactants with the kinetic characterization
of a reaction with toeholds of various lengths. (b) Schematic representation
of a strand exchange reaction templated by a BTA polymer. The reactants
are recruited to the polymer via the hybridization of a sequence complementary
to the oligonucleotide on the BTA. This results in a high local concentrations
which consequently increases the association kinetics of toehold binding.
Additionally, due to the multivalent anchoring of the product to the
supramolecular polymer, the product is stabilized resulting in increased
operation yields. (c) Schematic representation of a hybridization
chain reaction (HCR) templated by the BTA polymers. Starting upon
the addition of an input, the sequential opening of two labeled metastable
hairpin strands results in the formation of a polymeric DNA assembly.
(d) TIRF images of immobilized BTA polymers containing Cy3 labeled
monomers with corresponding STORM images of the HCR product containing
Cy5 labeled oligonucleotides after the HCR was initiated by addition
of, respectively, 1 and 5 nM input strand. The scale bars represent
2 μm. Adapted and reproduced with permission from ref (35).
Summary and Conclusions
DNA hybridization is arguably the
most studied and best understood
natural supramolecular interaction, but hybrid systems that combine
synthetic supramolecular polymers with DNA-based assembly have only
recently been reported. Nonetheless, these studies have already revealed
interesting emerging properties that are not easily obtained in either
synthetic supramolecular polymers or DNA-only based systems. A relatively
straightforward application of grafting DNA-strands on supramolecular
polymers is to allow the functionalization of supramolecular polymers
with DNA-modified molecular cargo such as small molecules, proteins,
and nanoparticles. The reversible nature of DNA hybridization
and the ability to precisely tune both its affinity and dynamics provides
much higher control than using other forms of noncovalent or covalent
interactions. DNA-based interactions can also be easily controlled
externally, both generally using salt concentration or temperature,
and specifically using DNA-displacement reactions. Interactions between
complementary strands on different polymers can be used to create
networks and other higher order superstructures. The formation of
these structures is not only determined by the relative thermodynamic
strengths of orthogonal supramolecular interactions, but also by their
dynamics, giving rise to kinetically trapped states and pathway complexity.
DNA hybridization and supramolecular polymerization can also reinforce
each other, with supramolecular polymerization stabilizing the interactions
between complementary strands within the same polymer, and DNA-interactions
steering internal ordering in supramolecular polymers. Vice versa,
by dynamically confining DNA reactants on their surface, supramolecular
polymers can act as adaptive templates to increase the kinetics and
efficiency of DNA-based molecular computing. The reversibility and
ability to tune the dynamics of both supramolecular polymerization
and DNA hybridization provides access to modular and uniquely tunable
systems that can already emulate some of the adaptivity and specificity
of their natural counterparts. While these studies have mainly focused
on understanding fundamental properties, they also provide a first
glimpse of possible applications, such as the development of responsive
materials in tissue engineering, controlled drug delivery, molecular
sensors and the construction of synthetic signaling cascades.
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