Enhancing the mechanical properties of guanosine-based supramolecular hydrogels with guanosine-containing polymers

Article abstract of DOI:10.1021/ma402618z

We demonstrate that multivalent, polymeric 8-methoxyguanosine derivatives based on poly(dimethylacrylamide) can enhance the mechanical properties of the low molecular weight hydrogelator 8-methoxy-2′,3′,5′-tri-O- acetylguanosine at biologically relevant salt concentrations. It is proposed that these nongelling polymeric derivatives, under the conditions studied, can result in a significant enhancement of these supramolecular gels (e.g., for gels containing 1 wt % gelator G′ can be increased from ca. 2000 Pa with no additive to 80 000 Pa) by acting as supramolecular cross-linking units. Two competing mechanisms appear to play a role in these cogels. At low polymer concentrations the guanosine-containing polymers tend to act more as solubilizing agents for the gelator, thus weakening the gels, while at high guanosine-containing polymer concentrations the gels show a marked enhancement in mechanical properties consistent with them acting as supramolecular cross-linking agents. As such, the thermomechanical properties of these cogels depend on both the polymer:low molecular weight gelator ratio and the number of 8-methoxyguanosine repeat units present in the polymer additive. Thus, these polymeric guanosine-based additives impart the ability to tailor both the modulus and shear sensitivity of the gels. For example, cogels with a modulus ranging between ca. 95 and 80 000 Pa can be obtained through judicious selection of the type and amount of polymer additive.

Full text of DOI:10.1021/ma402618z

Article
pubs.acs.org/Macromolecules
Enhancing the Mechanical Properties of Guanosine-Based
Supramolecular Hydrogels with Guanosine-Containing Polymers
†
†,‡
‡
†
†,‡
Amanda E. Way, Angie B. Korpusik, Taylor B. Dorsey, Lauren E. Buerkle, Horst A. von Recum,
,
†,‡
and Stuart J. Rowan*
†Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio
4
4106, United States
‡
Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
S Supporting Information
*
ABSTRACT: We demonstrate that multivalent, polymeric 8-
methoxyguanosine derivatives based on poly-
(
dimethylacrylamide) can enhance the mechanical properties
of the low molecular weight hydrogelator 8-methoxy-2′,3′,5′-
tri-O-acetylguanosine at biologically relevant salt concentra-
tions. It is proposed that these nongelling polymeric
derivatives, under the conditions studied, can result in a
significant enhancement of these supramolecular gels (e.g., for
gels containing 1 wt % gelator G′ can be increased from ca.
2
000 Pa with no additive to 80 000 Pa) by acting as
supramolecular cross-linking units. Two competing mecha-
nisms appear to play a role in these cogels. At low polymer concentrations the guanosine-containing polymers tend to act more
as solubilizing agents for the gelator, thus weakening the gels, while at high guanosine-containing polymer concentrations the gels
show a marked enhancement in mechanical properties consistent with them acting as supramolecular cross-linking agents. As
such, the thermomechanical properties of these cogels depend on both the polymer:low molecular weight gelator ratio and the
number of 8-methoxyguanosine repeat units present in the polymer additive. Thus, these polymeric guanosine-based additives
impart the ability to tailor both the modulus and shear sensitivity of the gels. For example, cogels with a modulus ranging
between ca. 95 and 80 000 Pa can be obtained through judicious selection of the type and amount of polymer additive.
INTRODUCTION
are a variety of different classes of such gels: (1) the two
10
■
compounds form a gel only in combination, (2) both
compounds are gelators and either (a) interact with each other
Supramolecular gels, formed by the self-assembly of low
molecular weight gelators, show promise in numerous
11
1
2
to form cofibers or (b) self-sort resulting in two different
applications such as stimuli-responsive materials, sensors,
12
3
4
nanofibers within the gel, and (3) gelator plus a nongelling
additive component that is designed to impact the gel’s
biomaterials, liquid crystal materials, and personal care
5
formulations. Generally, such materials self-assemble into
13
thermomechanical or functional properties. In prior studies
we have focused on this latter two-component system by
designing nongelling additives and investigating how they can
be used to impact the properties of the supramolecular gel. For
long fibrillar structures that result in gelation of the liquid
medium. The self-assembly of the gelator is controlled by weak
and reversible noncovalent interactions, and as a consequence,
these gels are susceptible to external stimuli, which can impart
interesting mechanical properties such as temperature-sensitive
14
example, we and others have shown that surfactants added to
certain gelators can be used to help inhibit or prevent
crystallization of a gelator. In particular, we have recently
shown that the nonionic surfactant, Laureth-4, can result in
long-lasting hydrogels of an amphiphilic glucose-based gelator
and that both shear modulus and yield stress can be altered by
6
reversible sol−gel formation, sheer sensitivity, and healing.
However, it also means that most of these responsive gels are
relatively weak, although recent reports have shown that
nanocomposite supramolecular gels can exhibit enhanced
mechanical properties while still retaining some of their
15
7
controlling the ratio of the two components.
responsive behavior.
Another class of gelator that we have previously investigated
One key aspect of these systems that is receiving attention is
the design of a supramolecular gel that has specific targeted
properties. Some success toward this goal has been achieved
in two-component systems is guanosine (G) and its derivatives,
16
which are well-known hydrogelators. In the presence of metal
8
through chemically modifying gelators. An alternative
approach involves utilizing multiple-component gels where
their properties can be tailored by simply changing the ratio of
the different components. For a two-component system there
Received: December 22, 2013
Revised: February 7, 2014
Published: February 17, 2014
9
©
2014 American Chemical Society
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gelator self-assembles with Na+ ions into a continuous
guanosine helix (Figure 1aii). Nonetheless, the mechanical
properties of these gels can still be tailored with the addition of
TAcG. Specifically, the modulus and yield stress can be reduced
by the addition of TAcG by around 1−2 orders of magnitude at
room temperature.
In all the cases detailed above adding in the nongelling
additive results in a decrease in the mechanical properties of the
supramolecular gel at room temperature. Thus, it became of
interest to see if additives could be found that would enhance
the room temperature mechanical properties of the gels. Figure
1
b shows an idealistic schematic of one possible way to achieve
this. The concept here is to use polymeric multivalent
guanosine derivatives as potential supramolecular cross-linkers
for the 8-methoxyguanosine assemblies. In particular, reported
herein is the synthesis of polymerizable guanosine derivatives
(
1 and 2), their copolymerization with dimethyl acrylamide to
yield a series of water-soluble, low molecular weight polymers
3 or 4), and an investigation of the effect that these “cross-
(
linking” polymers have on their cogels with 8OMeTAcG at
biologically relevant Na salt concentrations (100 mM).
+
The addition of a polymeric additive to low molecular weight
gelators has not received much attention in the literature.
However, there are reports by a few of groups on how polymers
22
may impact the properties of a supramolecular gel. Some
early work by Hanabusa showed that selected commercial
polymers could be used to enhance the mechanical properties
23
of an organogelator, although no mechanism was proposed
for this enhancement. There are examples of systems in which
24
25
specific interactions, e.g. electrostatic, nucleobase, hydrogen
26
Figure 1. (a) (i) G-quartet formed around M+ ions and (ii) a
continuous G-helix formed around Na ions. (b) Schematic of
bonding, have been designed between the polymer and
gelator. In such cases a number of different mechanisms have
been proposed for how the polymer affects the gels
thermomechanical properties, including the polymer impacting
the supramolecular assembly and/or the crystallization of the
gelator. Other researchers have investigated the effect of mixing
polymeric gelators and low molecular weight gelators. The
goal of this specific study was to focus on polymers that have
side chains which are derived from the low molecular weight
gelator itself and as such can be directly incorporated into the
gel-forming supramolecular assembly.
+
guanosine-containing helical assemblies formed by 8OMeTAcG in the
+
presence of Na ions with (left) a polymerizable G-derivative (1) and
(
right) with a guanosine-containing polymer (2 or 3).
27
+
ions (such as K ) they generally self-assemble in to G-quartets
(
Figure 1ai), in which the guanine units bind to the metal ion
on the inside of the stack and the ribose units are on the
surface. Subsequently, these quartets assemble into octamers
and then into one-dimensional quadraplexes that eventually
yield percolating assemblies resulting in gel formation. Using
the cogel approach, we have shown that mixing G with a
nongelating, more hydrophobic guanosine derivative (2′,3′,5′-
tri-O-acetylguanosine, TAcG) results in the formation of stable
17
RESULTS AND DISCUSSION
There are a few reports of guanosine derivatives being
■
2
8
incorporated into synthetic linear polymers. Our approach,
which targeted the synthesis of the guanosine-containing
monomers (1 and 2) and their corresponding copolymers (3
and 4), is shown in Figure 2. Starting with the known 2′-3′-
18
gels in aqueous potassium chloride (0.354 M). Here the
guanine moiety in TAcG allows it to be incorporated into the
stack, and as such the surface of the quadraplex consists of both
ribose and triacetylribose units. By varying the ratio of the two
components, the thermomechanical properties of the gels can
be systematically altered. The data suggested that increasing the
ratio of the larger, more hydrophobic triacetylribose units
resulted in a shortening of the fibers (presumably on account of
steric congestion) as well as stronger fiber−fiber interactions at
slightly elevated temperatures (ca. 40 °C) on account of the
increased fiber surface hydrophobicity. A similar approach has
2
9
isopropylidene guanosine-5′-carboxylic acid (5) (prepared in
two steps from guanosine (G) in 40% overall yield) the 2′- and
3
0
3′-positions were deprotected with 1 M HCl to yield (98%)
guanosine-5′-carboxylic acid (6). The 8-position of the guanine
3
1
moiety was then functionalized with bromine (7, 80% yield)
before being converted to a methoxy group with sodium
30
methoxide to yield 8 (65%). The 2′- and 3′-positions were
then functionalized with acetyl groups using acetic anhydride
1
9
32
also been shown to work with other guanosine derivatives.
DMAP and triethylamine (45% yield). Finally, the 5′-position
2
0
For example, we recently reported a new guanosine
hydrogelator, 8-methoxy-2′,3′,5′-tri-O-acetylguanosine (8OMe-
TAcG), that forms gels at biologically relevant salt concen-
trations (ca. 100 mM NaCl, 5 mM KCl) and cell media,
something that is not possible with guanosine. In this case the
data suggests that rather than forming G-quartet stacks this
was reacted with 2,2′-(ethylenedioxy)bis(ethylamine)-
monoacrylamide, benzotriazol-1-yloxytripyrrolidinophospho-
nium hexafluorophosphate (PyBop), and diisopropylethyl-
amine (DIPEA) to yield the target polymerizable 8-
methoxyguanosine derivative, 1 (80%). The polymerizable
guanosine derivative 2, which does not have the 8-methoxy
21
1
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Figure 2. Scheme of polymerizable guanosine derivatives. Conditions:
3
3
(
a) 70% HClO , acetone, 3 h; (b) NaHCO , TEMPO, iodobenzene
4
3
diacetate, MeCN/H O, 3 h; (c) 1 N HCl, 65 °C, 1 h; (d) Br , H O, 1
2
2
2
h; (e) NaOCH , DMSO, MeOH, 65 °C, 18 h; (f) (CH CO) O,
3
3
2
MeCN, TEA, DMAP, 4 h; (g) 2,2′-(ethylenedioxy)bis(ethylamine)-
monoacrylamide, PyBop, DIPEA, DMF, 18 h; (h) 4-cyano-4-
[
(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid, DMA, AIBN,
DMF, 70 °C, 10 h.
Figure 3. (a) Images of 2 wt % 5:5 8OMeTAcG:1 and 2 cogels in 100
−1
mM NaCl; stress sweep experiments (ω = 6.28 rad/s , T = 25 °C) of
wt % 100 mM NaCl gels of (b) 8OMeTAcG:1, (c) 8OMeTAcG:2;
and (d) 8OMeTAcG:1 gels with varying wt % of 1 while holding
OMeTAcG concentration at 1 wt %.
2
substituent, was prepared using similar chemistry (via
acetylation and peptide coupling of 6) in 17% overall yield
starting from G (Figure 2).
8
Cogels of 8OMeTAG with the Monomer 1 or 2. Initial
studies were carried out to determine if the acrylamide-
functionalized guanosine monomers (1 and 2) were able to
form gels with 8OMeTAcG. Mixtures of 8OMeTAcG and 1 or
ature NMR (VT-NMR) studies were undertaken; NMR will
show the guanosine derivatives in the solution phase of the
2
0,34−36
sample but not in the gel phase.
In these experiments
2
were made in varying ratios (10:0, 7:3, 5:5, 3:7, 0:10) in 100
the NMR of a 1:1 mixture of the two components is taken as
the sample is cooled from 75 to 25 °C and the percentage of
the components in the sol determined by integration of their
acetyl protons. 75 °C is well above the gel−sol transition
mM NaCl where the total content of the two compounds was
set at 2 wt % (Figure 3a). Gratifyingly, within 15 min stable gels
could be formed under these conditions at 8OMeTAcG:1 (or
2
) ratios ranging from 10:0 to 3:7. Neither 1 nor 2 formed a gel
by itself, most likely on account of their enhanced, relative to
OMeTAcG, water solubility. To confirm that 1 and 2 can be
incorporated into the 8OMeTAcG assemblies, variable temper-
temperature (T
) of 8OMeTAcG (ca. 63 °C) and is a
gel−sol
temperature where the ratio of the two components in the
solution is measured to be 1:1 by NMR. These variable
temperature experiments suggest that 69% of 1 and 78% of
8
1
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OMeTAcG are incorporated into the 8OMeTAcG:1 gel
Article
8
formed gels by themselves in 100 mM aqueous NaCl, even at
high concentrations (up to 50 wt %), which is presumably
related to their good water solubility.
phase at room temperature. 2 is incorporated to a lesser degree
62%) in the 8OMeTAcG:2 cogels with 80% 8OMeTAcG
(
being present in the gel phase (Figures S1 and S2). It is
important to note that guanosine derivatives tend to form
stacks of G-quartets while, from our previous work, the
VT-NMR experiments were performed on 1:1 samples of
8OMeTAcG and 44 and 34 to determine the extent of
incorporation of each component into the gel phase (Figures
20
8OMeTAcG prefers to form continuous helical assemblies on
S5 and S6). It was found that in 8OMeTAcG:3 ca. 82% of
4
account of the presence of both the methoxy moiety and the
bulky acetyl groups. Thus, it is reasonable that 1 (with its 8-
methoxy substituent) is more easily incorporated into the
helical assembles of 8OMeTAcG than 2.
8OMeTAcG and 74% of the 8-methoxyguanosine repeat units
from 34 are incorporated into the gel phase, while in
8OMeTAcG:4 about 79% of 8OMeTAcG and 68% of the
4
guanosine repeat units from 4 are incorporated. Thus, the
4
Stress sweeps (from 0.1 to 1000 Pa and a frequency of 6.28
incorporation of the polymeric guanosine derivatives (3 and
4
rad/s) were performed on all cogels at 25 °C. As was the case
44) and the amount of the gelator in the gel phase, at the 1:1
ratio, are similar to those of their respective monomers (1 or
2). The thermal properties of the 8OMeTAcG and the 1:1
8OMeTAcG:1, 2, 3 , and 4 gels were analyzed by differential
1
8,20
with our previous studies,
increasing the amount of the
nongelator (1 or 2 in this case) results in a systematic decrease
in the mechanical properties of the cogel (Figure 3a−c, Figures
S3 and S4). It is worthy of note that the G′ of the 1 cogels (i.e.,
x
4
scanning calorimetry (DSC) (Figure 4). The DSC for each gel
shows a peak that is consistent with the Tgel−sol which for the
gelator 8OMeTAcG itself is ca. 63 °C. Incorporation of
monomer 1 into the gel does not change the Tgel−sol
temperature (63 °C). However, addition of homo-polyDMA
6
6
000 Pa for 7:3) is significantly greater than the 2 cogels (i.e.,
70 Pa for 7:3), consistent with 1 being incorporated better/
more strongly (relative 2) within the 8OMeTAcG gelator
assemblies.
It is important to mention that in the above studies
increasing the amount of 1 or 2 decreases the amount of
gelator, 8OMeTAcG (total wt % was held at 2%), which is
presumably also contributing to the decrease in G′. Therefore,
similar stress sweep experiments were performed on cogels of
(3 ) does result in a significant drop in the T
0
(to ca. 51
gel−sol
°C), suggesting that this polymer has a significant destabilizing
effect on the gel. For the 8-methoxyguanosine-containing
polymer additives 3 there is a systematic increase in the Tgel−sol,
x
i.e. 3 (∼62.5 °C), 3 (∼65 °C), and 3 (∼67 °C), consistent
4
7
10
8
8
3
OMeTAcG:1 that kept the concentration of the gelator
OMeTAcG at 1 wt % but varied in the amount of 1 (Figure
d). Again, a drop in G′ is seen with an increase in 1; however,
with an increase in the stability of the gel with an increase in the
number of these binding units in the backbone. The Tgel−sol of
the 8OMeTAcG cogels with the guanosine derivatives; 2 (∼56
almost no difference in the yield points is observed.
°C) and the copolymer 4 (∼58 °C) are significantly lower
4
Synthesis of Guanosine-Containing Copolymers 3
and 4 and Their Cogels with 8OMeTAG. Both 1 and 2
were copolymerized with the water-soluble monomer dime-
thylacrylamide (DMA) via RAFT polymerization to yield 3 and
than any of the 8-methoxyguanosine derivatives and only
slightly higher than 3 , which is consistent with this guanine
0
moiety not being as effectively bound within the 8OMeTAcG
assemblies as the 8-methoxyguanosine units.
4
, respectively (Figure 2). All copolymers were targeted with a
degree of polymerization (DP) of ca. 30. 1 was copolymerized
with DMA at three different DMA:1 ratios25:5, 20:10, and
1
5:15while 2 was polymerized with DMA in a ratio
(
DMA:2) of 25:5. The three copolymerizations of 1 and
DMA yielded polymers with different ratios of the two repeat
units (DMA:1) but similar DPs; 3 (21:4, M = 4500, DP =
4
n
2
5), 3 (18:7, M = 6000, DP = 25), and 3 (15:10, M = 7400,
7 n 10 n
DP = 25) (see Table S1). The copolymerization of DMA and 2
yielded a polymer (4 ) with a molecular weight of 4300 g/mol
4
(
DP = 25) and a ratio of 21:4 (DMA:2 repeat units). The
homopolymer of poly(dimethylacrylamide) 3 (M = 2800 g/
0
n
mol, DP = 28, Đ = 1.15) was also synthesized in the similar
manner to the guanosine-containing polymers as a control.
Cogels were made in 100 mM aqueous NaCl keeping the
amount of 8OMeTAcG (1 wt %) the same and varying the
ratio of 3 or 4 (1:0.25, 1:0.5, 1:1, and 1:1.5, based on molar
Figure 4. DSC of 2 wt % 8OMeTAcG and 1:1 8OMeTAcG: (1, 2, 34,
3 , 3 , and 4 ) cogels in 100 mM NaCl.
x
4
7
10
4
equivalents of 8OMeTAcG to the repeat units of 1 or 2 in the
polymer; see Table S2 for wt % details). For example, for the
Circular dichroism measurements were performed on 0.3 wt
1
:0.25 8OMeTAcG:3 system the amount of 3 was added
% 8OMeTAcG, 1:1 8OMeTAcG:1, and 1:1 8OMeTAcG:34
4
4
such that the polymer 8-methoxyguaninosine repeat units were
.25 mol equiv of 8OMeTAcG. For the 3 cogel the same gram
gels in 100 mM NaCl (Figure 5a) to better understand the
nature of their assembly. The CD spectrum for 8OMe-
37
0
0
amount of polymer was added to the system as with the 34
cogel. It is important to note that these cogels do not form
immediately upon initial mixing but do form upon standing for
TAcG:1 and 8OMeTAcG:3 are similar to that of 8OMeTAcG
4
alone with diagnostic peaks at ca. 280, 250, and 210 nm. The
peak at 280 nm is evidence for a tilting or deformation of the
tetrameric G planes, consistent with the formation of
continuous G-helix assemblies rather than G-quartet forma-
a period of time. For example, the 3 comixtures gelled (as
4
determined by the upside down test) within 2−3 h and the 4
4
3
8,39
cogels formed in 5−6 h, while the 3 and 3 comixtures took
tion.
To obtain further insight into the assembly of the gels,
7
10
12−14 h to form the gels. Furthermore, none of the polymers
transmission electron microscopy (TEM) was performed.
1
813
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Figure 6. Comparisons of stress sweeps of 1:1 8OMeTAcG cogels
with 1, 3 , 3 , 3 , and 3 in 100 mM NaCl at 25 °C.
0
4
7
10
methoxyguanosine units binding into the helical 8OMeTAcG
assemblies and the polymers acting in some way as cross-
linking units. A key aspect that appears to play a role in these
systems is the 8OMeTAcG gel/sol content of the gels. NMR
data (see Figure S9) show that the amount of 8OMeTAcG
incorporated into the gel content increases, from ca. 68% for
the 3 cogel to 87% for the 3 cogel, with the number of 8-
0
10
methoxyguanosine units in the polymer backbone.
Rheology studies at different gelator:polymer ratios (Figures
S10−12) indicate a more complicated behavior than suggested
by Figure 6. While the 1:1.5 8OMeTAcG:3 gels show a similar
x
trend to the 1:1 cogels, the 1:0.25 show a completely different
behavior. In these samples the strongest cogel is formed with
the polymer with the fewest number of 8-methoxyguanosine
repeat units (3 ). NMR data (Figure S9) suggest that at these
4
lower concentrations this may be a result of the polymers 37
and 310 aiding the solubility the 8OMeTAcG gelator; the
amount of 8OMeTAcG present in the sol phase of the 3 , 3 ,
4
7
and 310 1:0.25 cogels is 12, 22, and 21%, respectively.
Figure 7a−d shows the data for the cogels of 3 , 4 , 3 , and
4
4
7
310
, respectively, at the different gelator to polymer ratios and
highlights a number of other interesting points. One of the first
things to note is that the mechanical enhancement effect of 34
Figure 5. (a) CD spectra of 1 wt % 8OMeTAcG, 1:1 8OMeTAcG:1,
and 1:1 8OMeTAcG:34 in 100 mM NaCl. TEM images of (b)
(
Figure 7a) is greater than 4 (Figure 7b) at all ratios, e.g. 11
4
8
OMeTAcG and (c) 8OMeTAcG:3 in 100 mM NaCl.
000 Pa versus 4000 Pa at a ratio of 1:0.25. This is again
consistent with the previous data suggesting 8-methoxyguano-
sine units are incorporated more efficiently in to the helical
8OMeTAcG assemblies. However, both follow the trend of a
reduction in the gels’ storage modulus and yield stress as the
concentration of polymer increases. Interestingly, a very
different behavior is observed for the two polymers that have
more 8-methoxyguanosine repeat units within their backbone
(3 and 3 ). In the 8OMeTAcG:3 cogel, both G′ and yield
4
Images of 8OMeTAcG and 8OMeTAcG:310 (Figure 5b,c)
show that bundles of nanofibers are present within both gels.
TEM images of 8OMeTAcG:2 and 8OMeTAcG:4 can be
4
seen in Figure S7 and also show the presence of fiber bundles.
A comparison of these images does suggest the formation of a
greater number of larger bundles in the polymer-containing
gels, although the evidence here is not definitive. AFM height
profiles indicate that the average height for the single fibers in
the 1:1 8OMeTAcG:1 gel is 2.97 ± 0.82 and 3.14 ± 0.52 nm
7
10
10
stress increase with the concentration of the polymer additive
(Figure 7d), while the 8OMeTAcG:3 cogel exhibits a behavior
7
in-between where the maximum modulus (8500 Pa) and yield
stress (142 Pa) are obtained at a 1:1 ratio of 8OMeTAcG:37
(Figure 7c).
for the 1:1 8OMeTAcG:3 gel (Figure S8).
4
Figure 6 shows a comparison of the stress sweep experiments
of all the 1:1 3 :8OMeTAcG cogels along with the
Thus, it appears that both the ratio of 8OMeTAcG to the 8-
methoxyguanosine repeat units as well as the ratio of
8OMeTAcG to polymer is important in determining the
mechanical properties of these cogels. Generally, higher
mechanical properties are observed either with high concen-
trations of polymers with a large number 8-methoxyguanosine
repeat units or with low concentrations of polymers with a low
number 8-methoxyguanosine repeat units. This interesting
behavior can be rationalized by understanding how much of the
8OMeTAcG is incorporated in to the gel phase. For example,
x
8
OMeTAcG gel. Here the mechanical properties of the gel
varies from the 3 :8OMeTAcG gel (268 Pa) to the
0
310
:8OMeTAcG gel (35 000 Pa) This data clearly shows that
for the 1:1 cogels (1) the poly(dimethylacrylamide) (30)
negatively impacts the mechanical properties of the gels and (2)
increasing the amount of guanosine repeat units results in
greater mechanical enhancement of the gel. Thus, the rheology
data matches the Tgel−sol data obtained from the DSC
experiments (vide supra) and is consistent with the 8-
1
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−1
Figure 7. Comparisons of stress sweeps (ω = 6.28 rad/s , T = 25 °C) of 1:1 8OMeTAcG cogels with 1, 3 , 3 , 3 , and 3 and 1 wt % 8OMeTAcG
0
4
7
10
in 100 mM NaCl at 25 °C. (a) Stress sweep experiments of 8OMeTAcG and (a) 3 , (b) 4 , (c) 3 , and (d) 3 gels in 100 mM NaCl.
4
4
10
7
at a ratio of 1:0.25 the amount of 8OMeTAcG incorporated in
the gel phase decreases upon increasing the number of 8-
methoxyguanosine repeat units in the polymer, e.g. 88% for the
34
observed at much higher polymer concentrations, e.g. at a 1:1.5
OMeTAcG:3 ratio 84% of 8OMeTAcG is incorporated in
gel and 79% for the 310 gel. However, the opposite trend is
8
x
the gel phase of the 3 gel while 89% is incorporated in the 3
4
10
gel (Table S2). It is important to note that even though the gel
content is similar in the 1:0.25 3 cogel and the 1:1.5 310 cogel
4
their mechanical properties, 11 000 vs 80 000 Pa, respectively,
are very different. Thus, there appears to be two main factors
that impact the properties of the gel. At low polymer
concentrations a key factor appears to be the solubility of the
8
OMeTAcG with the polymer with a larger number of 8-
methoxyguanosine side chains, resulting in higher solubility of
the gelator and thus weaker mechanical properties. As the
concentration of the polymer is increased, then the mechanical
properties are enhanced by the presence of more 8-
methoxyguanosine repeat units in the polymer, consistent
with supramolecular cross-linking occurring at the higher
polymer concentrations.
Frequency-sweep experiments were carried on 1 wt % of the
8
OMeTAcG gel and 1:1 8OMeTAcG cogels with 3 , 3 , 3 ,
0 4 7
310
, and 4 to further examine their properties (Figure 8). The
4
data indicate that for the 8OMeTAcG gel and all
OMeTAcG:3x cogels the moduli are independent of
Figure 8. Frequency sweeps of (a) 2 wt % 8OMeTAcG gel and 1:1
8
8
8
OMeTAcG:3 , 3 , 3 , and 3 gels in 100 mM NaCl; (b) 2 wt % 1:1
frequency over the entire sweep range, with G′ > G″ at all
0 4 7 10
OMeTAcG:3 and 4 gels in 100 mM NaCl. T = 25 °C, γ = 0.2%.
4
4
points. Interestingly, however, the 3 and 4 cogels behave very
0
4
differently with G′ and G″ approaching similar values at low
frequency (long time scales). This behavior indicates that in
these cogels the entangled fiber assembles can disentangle/
disengage at long time scales, resulting in much weaker gel
4
0
characteristics. The most interesting comparison to be made
here is between the cogels of 3 and 4 (Figure 8b), which
shows the significant weakening effect that removal of the 8-
4
4
1
815
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Article
methoxy substituent on the polymeric guanosine moiety has on
CONCLUSIONS
■
the properties of the cogels. The fact that 8OMeTAcG:3 is a
4
We have synthesized a new series of 8-methoxyguanosine/
dimethylacrylamide (DMA) copolymers and shown that
addition of these nongelling copolymers to the low molecular
weight gelator 8OMeTAcG results in significant changes to the
mechanical properties of the supramolecular hydrogel. The data
show that the 8-methoxyguanosine repeat units in the polymer
backbone are incorporated into the helical assemblies formed
by the gelator, suggesting the possibility of the polymers acting
as supramolecular cross-linking units. In addition, the polymers
also impact the solubility of the low molecular weight gelator
strong gel at all the frequencies studied while the
8
OMeTAcG:4 gel exhibits significant weakening at longer
4
time scales is consistent with the 8-methoxyguanosine repeat
units in 3 being bound more tightly into the 8OMeTAcG fiber
4
network than the guanosine repeat units in 44.
One key property of supramolecular gels is their shear
sensitivity which allows them, for example, to be injectable. A
key property of any injectable gel is its ability to reform after
the injection. Shear recovery experiments were therefore
performed on 1:1 cogels (along with the 2 wt % 8OMeTAcG
gel) to examine how long it takes these gels to recover after
being exposed to shear. Gels were monitored at 0.1 Pa for 2
min, a stress of 250 Pa was applied for 30 s to force the gels to
yield, and then the gels were monitored at a stress of 0.1 Pa for
(
8OMeTAcG) which, particularly at lower polymer concen-
trations, can negatively impact the properties of the gels. The
mechanical properties of the cogels were shown to be related to
a number of factors, such as the ratio of the two components
(gelator to polymer), the ratio of the polymeric 8-
methoxyguanosine and DMA repeat units (relative to the
gelator), and the chemical structure of the guanosine derivative
on the polymer. Thus, by judicious choice of the previously
mentioned factors the modulus of the 1 wt % 8OMeTAcG
hydrogel (2000 Pa) can be varied from 95 to 80 000 Pa (at
similar gelator concentration) through simple addition of the
nongelling polymer. Furthermore, all these gels are shear
sensitive. Exposure to shear results in a free following liquid
which re-forms the gel relatively quickly upon removal of this
stimulus, allowing these gels to be injectable.
15 min to measure their recovery time (Figure 9a). The data
indicate that while for all the gels G′ becomes greater than G″
by 50 s, full recovery of the mechanical strength of the gels does
take longer with the polymer additives and is dependent on the
number and type of the guanosine repeat units. For example,
the 2 wt % 8OMeTAcG gel fully recovers within 50 s, while the
1
8
:1 8OMeTAcG:34 gel recovers in ca. 69 s, the 1:1
OMeTAcG:4 gel at ca. 97 s, and a 1:1 8OMeTAcG:3 gel
7
10
in ca. 310 s. This difference in behavior can be visualized in
Figure 9b which shows images of 8OMeTAcG and 1:1.5
ASSOCIATED CONTENT
Supporting Information
■
8OMeTAcG:310 (chosen as it has the highest G′ value) gels
*
S
that have been injected through an 18-gauge needle and onto a
glass slide. While both gels can be injected and both re-form the
images clearly show that the 8OMeTAcG gel forms
immediately after being injected, while the 310 gel is more
spread out on the slide consistent with it taking a little longer to
fully recover.
Experimental section including VT-NMRs, TEM images,
polymerization data and compositions, rheology data, AFM,
NMR, and MALDI of guanosine derivatives. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail stuart.rowan@case.edu (S.J.R.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from
Kent H. Smith Charitable Trust, the Lubrizol Corporation, and
the National Science Foundation under Grant DMR-1204948,
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