Macroscopic self-assembly based on molecular recognition: Effect of linkage between aromatics and the polyacrylamide gel scaffold, amide versus ester

Article abstract of DOI:10.1021/ma302344x

The interactions of polyacrylamide- (pAAm-) based gels possessing cyclodextrin (CD) residues (CD-gels) with pAAm-based gels modified with aromatic residues through amide and ester linkages (ArA-gels and ArE-gels, respectively) were investigated to examine the effect of linkage (i.e., amide and ester) between aromatic residues and the pAAm gel scaffold. In the present study, benzyl (Bz), 2-naphthylmethyl (Np), 9-phenanthrylmethyl (Ph), and 1-pyrenylmethyl (Py) residues were chosen as a series of aromatic residues. αCD-gel did not interact notably with the ArA-gels and ArE-gels in water. βCD-gel interacted with the ArA-gels and ArE-gels possessing smaller aromatic residues (i.e., Bz and Np residues) in water to form gel assemblies. γCD-gel showed different tendencies of its interactions with the ArA-gels and with the ArE-gels; γCD-gel interacted with the ArA-gels carrying larger aromatic residues (i.e., Ph and Py residues), while γCD-gel formed stable gel assemblies only with NpE-gel among the ArE-gels examined. This is because γCD residues in γCD-gel included favorably dimeric aromatic residues in the ArA-gels and ArE-gels. Reflection fluorescence spectra for the ArA-gels and ArE-gels possessing fluorescent aromatic residues (i.e., Np, Ph, and Py residues) in the presence of 10 mM γCD were indicative of weak selectivities of γCD toward NpE, PhA, and PyA residues. Such weak selectivities may be largely enhanced in the macroscopic observation of interaction of CD-gels with the ArA-gels and ArE-gels presumably because of multivalency.

Full text of DOI:10.1021/ma302344x

Article
pubs.acs.org/Macromolecules
Macroscopic Self-Assembly Based on Molecular Recognition: Effect
of Linkage between Aromatics and the Polyacrylamide Gel Scaffold,
Amide versus Ester
Akihito Hashidzume,† Yongtai Zheng,† Yoshinori Takashima,† Hiroyasu Yamaguchi,†
and Akira Harada*,†,‡
†Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka
560-0043, Japan
‡Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST), 7 Gobancho,
Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
ABSTRACT: The interactions of polyacrylamide- (pAAm-)
based gels possessing cyclodextrin (CD) residues (CD-gels)
with pAAm-based gels modified with aromatic residues
through amide and ester linkages (ArA-gels and ArE-gels,
respectively) were investigated to examine the effect of linkage
(i.e., amide and ester) between aromatic residues and the
pAAm gel scaffold. In the present study, benzyl (Bz), 2-
naphthylmethyl (Np), 9-phenanthrylmethyl (Ph), and 1-
pyrenylmethyl (Py) residues were chosen as a series of
aromatic residues. αCD-gel did not interact notably with the
ArA-gels and ArE-gels in water. βCD-gel interacted with the
ArA-gels and ArE-gels possessing smaller aromatic residues (i.e., Bz and Np residues) in water to form gel assemblies. γCD-gel
showed different tendencies of its interactions with the ArA-gels and with the ArE-gels; γCD-gel interacted with the ArA-gels
carrying larger aromatic residues (i.e., Ph and Py residues), while γCD-gel formed stable gel assemblies only with NpE-gel among
the ArE-gels examined. This is because γCD residues in γCD-gel included favorably dimeric aromatic residues in the ArA-gels
and ArE-gels. Reflection fluorescence spectra for the ArA-gels and ArE-gels possessing fluorescent aromatic residues (i.e., Np, Ph,
and Py residues) in the presence of 10 mM γCD were indicative of weak selectivities of γCD toward NpE, PhA, and PyA
residues. Such weak selectivities may be largely enhanced in the macroscopic observation of interaction of CD-gels with the ArA-
gels and ArE-gels presumably because of multivalency.
chemical,24 and light stimuli.25 These previous studies have
demonstrated that the formation of gel assemblies is dependent
INTRODUCTION
■
Macroscopic self-assemblies based on noncovalent interactions
on the binding constant for CD and guest residues and on the
have been of increasing importance as smart materials which
possess stimuli-responsiveness and self-healing ability.1−3
concentration of CD and guest residues in gels.
Since the mobility of CD and guest residues should be also
Macroscopic self-assemblies have been reported based on
magnetic interaction,4−6 electrostatic interaction,7,8 hydro-
important for the interaction on the gel interface, the linkage
phile−lipophile balance,9−12 and capillary effect.13−16 In
between CD or guest residues and the pAAm gel scaffold may
be a considerable structural factor. The linkage between
biological systems, macroscopic self-assemblies based on
hydrophobic residues and the polymer backbone can be critical
molecular recognition are ubiquitous and sustain living
activities.17−19 In artificial systems, on the other hand, it has
in the self-organization behavior of hydrophobically modified
water-soluble polymers.26−31 For example, amide and ester
been still a great challenge to construct macroscopic self-
linkages between hydrophobic residues and the polymer
assemblies based on molecular recognition.
backbone can demonstrate different properties in self-
Recently, we have first demonstrated macroscopic self-
assemblies based on molecular recognition using
organization of hydrophobically modified water-soluble poly-
mers, although amide and ester seem to be similar. Hydro-
phobically modified polyanions of amide linkage, i.e., statistical
polyacrylamide(pAAm)-based hydrogels possessing cyclodex-
trin (CD) and guest moieties.20 Gels carrying αCD, βCD, and
γCD moieties (αCD-gel, βCD-gel, and γCD-gel, respectively)
recognized gels carrying their respective counterparts.20−22
More recently, we have also realized macroscopic self-
assemblies responsive to external stimuli, i.e., temperature,23
Received: November 14, 2012
Revised: January 24, 2013
Published: February 19, 2013
© 2013 American Chemical Society
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Chart 1. Chemical Structures for CD-Gels (αCD-Gel, βCD-Gel, and γCD-Gel) and Guest Gels (ArA-Gels and ArE-Gels)
copolymers of sodium 2-acrylamido-2-methylpropanesulfonate
(NaAMPS) and N-dodecylmethacrylamide, show a strong
tendency of intrapolymer hydrophobic interaction to form
compact polymer micelles, in which the polymer main chain is
highly folded.32−35 Whereas, hydrophobically modified poly-
anions of ester linkage, i.e., statistical copolymers of NaAMPS
and dodecyl methacrylate, indicate a tendency of interpolymer
hydrophobic interaction to form larger polymer micelles.36
Furthermore, the linkage between guest residues and CD
residues in modified CDs can be also important in the
formation of supramolecular assemblies.37−40 Ueno et al.37
modified γCD with a 3-(1-pyrenyl)propyl residue on the
primary hydroxy side through amide and ester linkages, and
reported that the equilibrium constant of the formation of
dimeric inclusion complex for the modified γCD of amide
linkage (1.74 × 105 M−1) was an order of magnitude higher
than that for the modified γCD of ester linkage (1.53 × 104
M−1). Harada et al.38−40 modified αCD with a cinnamoyl
residue on the primary hydroxy side through amide and ester
linkages, and reported the supramolecular structure depending
on the linkage. The modified αCD of amide linkage forms a
supramolecular cyclic dimer in aqueous media,38,39 whereas the
modified αCD of ester linkage forms supramolecular oligomers
in aqueous media.40
RESULTS AND DISCUSSION
Preparation of the Guest Gels Carrying Aromatic
■
Amide and Aromatic Ester Residues (ArA-Gels and ArE-
Gels) and CD-Gels Used in This Study. In the present study,
benzyl (Bz), 2-naphthylmethyl (Np), 9-phenanthrylmethyl
(Ph), and 1-pyrenylmethyl (Py) residues have been chosen as
a series of aromatic residues because of ease of preparation of
the monomers and gels. The guest gels (ArA-gels and ArE-gels
in Chart 1) were prepared by radical terpolymerization of
acrylamide (AAm), N,N′-methylenebis(acrylamide) (MBA) (a
cross-linker), and an aromatic monomer in dimethyl sulfoxide
(DMSO) using ammonium peroxodisulfate (APS) as an
initiator at 60 °C. The feed ratios for the ArA-gels and ArE-
gels are listed in Table S1 in the Supporting Information. The
gels were soaked in water for several days before use. The CD-
gels were also prepared by radical terpolymerization of AAm,
MBA, and a CD monomer {5 mol % of mono(6-
deoxyacrylamido)-α-cyclodextrin (AαCD), mono(6-deoxyacry-
lamido)-β-cyclodextrin (AβCD), or mono(6-deoxyacrylami-
do)-γ-cyclodextrin (AγCD)} in water initiated by a redox pair
of APS and N,N,N′,N′-tetramethylethylenediamine (TMEDA)
at room temperature according to the procedure reported
previously (Chart 1).22−24 The gels obtained were washed with
water to remove the unreacted monomers and initiators. The
ArA-gels, ArE-gels, and CD-gels were dyed with different color
dyes for visual identification.
These previous studies motivated us to investigate the effect
of linkage on the macroscopic self-assemblies of CD-gels and
guest gels. Hence, we investigate the effect of linkage on the
formation of gel assemblies of CD-gels with gels modified with
aromatic residues through amide and ester linkages (ArA-gels
and ArE-gels, respectively). We first describe the interactions of
CD-gels with the ArA-gels and with the ArE-gels, and then
compare these interactions.
Macroscopic Observation of the Interaction of CD-
Gels with ArA-Gels. The interaction of CD-gels with the ArA-
gels was investigated by shaking in water with a glass Petri dish
of 9 cm in diameter, as can be seen in Figure 1. When αCD-gel
pieces were shaken with BzA-gel, NpA-gel, PhA-gel, and PyA-
gel pieces in water, no gel assemblies were formed, indicative of
no significant interaction of αCD-gel pieces with any pieces of
the ArA-gels (Figure 1a). As reported in our previous study,
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Table 1. Interaction of CD-Gels with ArA-Gels
a
CD-gel
ArA-gel
assembly
stress at rupture/Pa
b
αCD-gel
αCD-gel
αCD-gel
αCD-gel
BzA-gel
NpA-gel
PhA-gel
PyA-gel
BzA-gel
NpA-gel
PhA-gel
PyA-gel
BzA-gel
NpA-gel
PhA-gel
PyA-gel
N
N
N
N
A
800 110
720 30
630 70
740 90
b
βCD-gel
βCD-gel
βCD-gel
βCD-gel
1360 330
1600 90
760 110
530 200
310 20
A
N
N
N
N
A
c
b
γCD-gel
γCD-gel
γCD-gel
γCD-gel
580 80
1970 240
c
A
2930 310
a
b
A and N denote assembly and no assembly, respectively. Date from
c
ref 23. Data from ref 24.
gel (7.6 × 102 Pa) > βCD-gel/PyA-gel (5.3 × 102 Pa). γCD-gel
indicated the largest stress at rupture with PyA-gel (2.9 × 103
Pa), and the stress decreased in the order of γCD-gel/PyA-gel >
γCD-gel/PhA-gel (2.0 × 103 Pa) > γCD-gel/NpA-gel (5.8 ×
102 Pa) > γCD-gel/BzA-gel (3.1 × 102 Pa). On the basis of
these data, it is likely that the stress at rupture of ca. 103 Pa is
the lower limit for the formation of gel assemblies in water in
the cases of CD-gels/ArA-gels.23
Macroscopic Observation of the Interaction of CD-
Gels with ArE-Gels. The interaction of CD-gels with the ArE-
gels was also investigated, as can be seen in Figure 2. When
αCD-gel pieces were shaken with BzE-gel, NpE-gel, PhE-gel,
and PyE-gel pieces in water, no gel assemblies were formed,
indicative of no considerable interaction of αCD-gel pieces with
any pieces of the ArE-gels (Figure 2a).
Figure 1. Pictures of the interaction of αCD-gel (gentian blue) (a),
βCD-gel (red) (b), and γCD-gel (bottle green) with the ArA-gels
{BzA-gel (orange), NpA-gel (green), PhA-gel (purple), and PyA-gel
(yellow)} (c).
αCD-gel interacted favorably with gels possessing a linear
aliphatic guest, but it did not with gels possessing a cyclic or
branched aliphatic guest.20,21 More recently, it was demon-
strated that αCD-gel interacted with BzA-gel carrying 10 or 15
mol % Bz moieties only at lower temperatures.23 The formation
of no gel assemblies was ascribable to the lower binding
constants for the model systems composed of native αCD and
pAAm modified with aromatic residues through an amide
linkage (Table S2 in the Supporting Information). When βCD-
gel pieces were shaken with pieces of the four ArA-gels in water,
βCD-gel pieces interacted with BzA-gel and NpA-gel pieces to
form a gel assembly, whereas they did not interact with PhA-gel
or PyA-gel pieces (Figure 1b). These observations are
consistent with the binding constants for the model systems
(Table S2 in the Supporting Information); βCD interacted
preferably with BzA and NpA residues (8.4 × 101 and 2.7 × 102
M−1, respectively). When γCD-gel pieces were shaken in water
with pieces of the four ArA-gels, γCD-gel pieces did not interact
considerably with BzA-gel or NpA-gel pieces while they
interacted with PhA-gel and PyA-gel pieces to form a gel
assembly (Figure 1c). These observations are also in a good
agreement with the binding constants for the model systems;
γCD did not interact notably with the smaller aromatic
residues, i.e., BzA and NpA residues, whereas it interacted with
the larger aromatic residues, i.e., PhA and PyA residues (Table
S2 in the Supporting Information).
To investigate semiquantitavely the strength of interaction of
the CD-gels with the ArA-gels, stress−strain measurements
were carried out to evaluate the stress at rupture. The values
obtained are listed in Table 1. All the pairs of αCD-gel/ArA-
gels exhibited relatively small stresses of (6.3−8.0) × 102 Pa.
βCD-gel showed the largest stress at rupture with NpA-gel (1.6
× 103 Pa), and the stress decreased in the order of βCD-gel/
NpA-gel > βCD-gel/BzA-gel (1.4 × 103 Pa) > βCD-gel/PhA-
Figure 2. Pictures of the interaction of αCD-gel (gentian blue) (a),
βCD-gel (red) (b), and γCD-gel (bottle green) with the ArE-gels
{BzE-gel (sky blue), NpE-gel (blue), PhE-gel (yellow), and PyE-gel
(orange)} (c).
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The formation of no gel assemblies was also caused by the
lower binding constants for the model systems composed of
native αCD and pAAm modified with aromatic residues
through an ester linkage (Table S3 in the Supporting
Information). When βCD-gel pieces were shaken with pieces
of the four ArE-gels, βCD-gel pieces interacted with BzE-gel
and NpE-gel pieces to form a gel assembly, whereas they did
not interact with PhE-gel or PyE-gel pieces (Figure 2b). These
observations also agree with the binding constants for the
model systems (Table S3 in the Supporting Information).
When γCD-gel pieces were shaken with pieces of the four ArE-
gels, γCD-gel pieces did not interact notably with BzE-gel, PhE-
gel, or PyE-gel pieces while they interacted strongly only with
NpE-gel pieces to form a gel assembly (Figure 2c). (Only a
PyE-gel piece attached to a γCD-gel piece in the gel assembly at
an agitation speed of ca. 400 rpm, but the interaction was very
weak.) These observations are not consistent with the binding
constants for the model systems; γCD did not interact
considerably with BzE or NpE residues because of the poor
matching of size and shape, whereas it interacted preferably
with PhE and PyE residues (Table S3 in the Supporting
Information).
the γCD-gel/PyE-gel was also overestimated presumably
because of the stickiness of PyE-gel in air.
Comparison between the Interactions of CD-Gels
with the ArA-Gels and with the ArE-Gels. Here we
compare the interactions of CD-gels with the ArA-gels and with
the ArE-gels. αCD-gel and βCD-gel exhibited the same
tendencies of their interactions with the ArA-gels and with
the ArE-gels. αCD-gel did not form any gel assemblies with the
ArA-gels and ArE-gels presumably because of the smaller cavity
size of αCD residue. βCD-gel interacted with the ArA-gels and
ArE-gels possessing smaller aromatic residues (i.e., Bz and Np
residues) in water to form gel assemblies because of the middle
cavity size of βCD residue. On the other hand, γCD-gel showed
different tendencies of its interactions with the ArA-gels and
with the ArE-gels. γCD-gel interacted with the ArA-gels
carrying larger aromatic residues (i.e., Ph and Py residues).
Whereas, γCD-gel formed stable gel assemblies only with NpE-
gel among the ArE-gels examined, but did not interact notably
with BzE-gel, PhE-gel, or PyE-gel. To confirm these differences,
additional examinations of the formation of gel assemblies were
carried out, as can be seen in Figure 3. When γCD-gel pieces
To investigate semiquantitatively the strength of interaction
of the CD-gels with the ArE-gels, stress−strain measurements
were also performed to estimate the stress at rupture, as listed
in Table 2. All the pairs of αCD-gel indicated smaller stresses at
Table 2. Interaction of CD-Gels with ArE-Gels
a
CD-gel
ArE-gel
assembly
stress at rupture/Pa
αCD-gel
αCD-gel
αCD-gel
αCD-gel
βCD-gel
βCD-gel
βCD-gel
βCD-gel
γCD-gel
γCD-gel
γCD-gel
γCD-gel
BzE-gel
NpE-gel
PhE-gel
PyE-gel
BzE-gel
NpE-gel
PhE-gel
PyE-gel
BzE-gel
NpE-gel
PhE-gel
PyE-gel
N
N
N
N
A
780 140
670 130
720 10
960 180
2510 170
2580 150
820 170
1630 420
910 70
A
N
N
N
A
1430 50
1200 290
2170 420
N
N
a
A and N denote assembly and no assembly, respectively.
rupture of (6.7−9.6) × 102 Pa. βCD-gel indicated larger
stresses with BzE-gel and NpE-gel {(2.5−2.6) × 103 Pa}, with
which βCD-gel formed gel assemblies in water. βCD-gel
indicated a smaller stress with PhE-gel. Although βCD-gel did
not form any gel assemblies with PyE-gel, βCD-gel/PyE-gel
exhibited a larger stress at rupture of 1.6 × 103 Pa. The stress−
strain measurements were carried out in air, while the
formation of gel assemblies was examined in water. Since
PyE-gel was sticky in air, the stress at rupture evaluated by the
stress−strain measurements might be overestimated presum-
ably because of the stickiness of PyE-gel. γCD-gel indicated
smaller stresses at rupture with BzE-gel (9.1 × 102 Pa) and
PhE-gel (1.2 × 103 Pa), with which γCD-gel did not form any
gel assemblies in water. Whereas γCD-gel indicated a larger
stress at rupture with NpE-gel (1.4 × 103 Pa), with which γCD-
gel interacted significantly. Although γCD-gel/PyE-gel did not
interact significantly in water, the pair showed a larger stress at
rupture (2.2 × 103 Pa). It is likely that the stress at rupture for
Figure 3. Pictures of the interaction of γCD-gel (bottle green) with
NpA-gel (green) and NpE-gel (blue) (a), with PhA-gel (purple) and
PhE-gel (yellow) (b), and with PyA-gel (yellow) and PyE-gel (orange)
(c).
were shaken with NpA-gel and NpE-gel pieces in water, γCD-
gel pieces formed a gel assembly only with NpE-gel pieces
(Figure 3a). When γCD-gel pieces were shaken with PhA-gel
and PhE-gel pieces in water, γCD-gel pieces formed a gel
assembly only with PhA-gel pieces (Figure 3b). In the case of
γCD-gel with PyA-gel and PyE-gel pieces, γCD-gel pieces
formed a gel assembly only with PyA-gel pieces at a higher
agitation speed (ca. 500 rpm) (Figure 3c). These observations
indicate that γCD-gel can discriminate the linkage between
aromatic residues and the pAAm gel scaffold.
To understand the mechanism of the different tendencies of
gel interactions of γCD-gel with the ArA-gels and with the ArE-
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Figure 4. Reflection fluorescence spectra for NpA-gel (a), NpE-gel (b), PhA-gel (c), PhE-gel (d), PyA-gel (e), and PyE-gel (f) in the absence (solid
line) and presence of 10 mM γCD (dotted line) excited at 280 (a and b), 295 (c and d), and 335 nm (e and f), respectively.
gels, reflection fluorescence spectra were recorded for the ArA-
gels and ArE-gels possessing fluorescent aromatic residues (i.e,
Np, Ph, and Py residues) as can be seen in Figure 4. The
fluorescence spectrum for NpA-gel contained a marked band
ascribable to Np excimer around 400 nm in the absence of γCD
(Figure 4a). In the presence of 10 mM γCD, the spectrum
exhibited a modest increase in the intensity of excimer
fluorescence (Figure 4a), indicative of only a weak interaction
of γCD with NpA residues in NpA-gel. On the other hand, the
fluorescence spectrum of NpE-gel indicated a broad shoulder
ascribable to Np excimer in the absence of γCD (Figure 4b). In
the presence of 10 mM γCD, the spectrum indicated a band
assignable to Np excimer enhanced remarkably (Figure 4b),
indicating that γCD molecules added induced the formation of
dimer of NpE residues. These observations are thus indicative
of a marked interaction of γCD with NpE residues in NpE-gel.
In the fluorescence spectra of PhA-gel and PhE-gel (Figure 4,
parts c and d), there were no bands assignable to excimer
fluorescence in the absence and presence of 10 mM γCD
because it is known that phenanthrene and most of its
derivatives do not emit excimer fluorescence.41,42 In both cases,
there were no remarkable differences between the spectra in the
absence and presence of 10 mM γCD. However, it should be
noted here that the fluorescence spectrum of PhA-gel in the
presence of 10 mM γCD indicated a slight broadening and a
small red shift (Figure 4c). This observation is suggestive of a
weak selectivity of γCD toward PhA residues. Both the
fluorescence spectra for PyA-gel and PyE-gel indicated
comparable bands of monomer and excimer fluorescence in
the absence of γCD, and exhibited a markedly enhanced
excimer fluorescence in the presence of 10 mM γCD (Figures
4e and 4f). It is noteworthy that the maxima of excimer
fluorescence indicated blue shifts and the blue shift for PyA-gel
(16 nm) was larger than that for PyE-gel (8 nm) presumably
because of the less polar environment inside the cavity of γCD
residues.43,44 This observation also indicates a weak selectivity
of γCD toward PyA residues. All the cases examined here
showed small differences in fluorescence spectra, indicative of
weak selectivities of γCD. However, such weak selectivities may
be largely enhanced in the macroscopic observation of
interaction of CD-gels with guest gels presumably because of
multivalency.
As reported in our previous paper, the formation of gel
assemblies based on molecular recognition depends not only
on the binding constant of CD residues with guest residues but
also on the concentrations of CD and guest residues in gels.22
In addition, in the gel interaction of γCD-gel, the aggregation
properties of guest moieties are also important because γCD
residues on the γCD-gel surface prefer dimeric aromatic
residues.24
Here we compare the basic characteristics of the ArA-gels
and ArE-gels used in this study (Table 3). The ArA-gels and
ArE-gels were prepared by radical terpolymerization of AAm,
MBA, and an aromatic monomer in DMSO using APS as an
initiator. After polymerization, the solvent was replaced with
water. Upon the replacement of the solvent, the ArA-gels and
ArE-gels indicated different swelling properties depending on
their hydrophobicity. Table 3 contains the swelling ratio (V/V0,
where V0 and V denote the volumes of gel before and after
replacement of the solvent, respectively) for the ArA-gels and
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Table 3. Properties for pAAm-Gel, ArA-Gels, and ArE-Gels
swelling
a
stress at
rupture/
kPa
ArA-gel or mol % content
ratio
strain at
rupture/%
ArE-gel
of Ara or ArE (V/V0) concn/M
pAAm-gel
BzA-gel
BzE-gel
NpA-gel
NpE-gel
PhA-gel
PhE-gel
PyA-gel
PyE-gel
−
10
4.9
2.2
0.7
3.0
2.0
1.7
1.2
2.0
1.2
0
1.7
4.7
320
160
310
250
140
210
500
220
110
0.091
0.286
0.017
0.025
0.029
0.042
0.020
0.033
10
15
2.5
2.5
2.5
2.5
2.0
2.0
3.6
5.1
6.2
17.6
6.3
16.5
a
Swelling ratios were recorded after soaking gels in water for one week
at room temperature (23 °C).
ArE-gels. For each aromatic residue, the V/V0 value of an ArA-
gel was larger than that of the corresponding ArE-gel, although
the contents of AAm, MBA, and an aromatic guest were the
same; 2.2 and 0.7 for BzA-gel and BzE-gel, 3.0 and 2.0 for NpA-
gel and NpE-gel, 1.7 and 1.2 for PhA-gel and PhE-gel, and 2.0
and 1.2 for PyA-gel and PyE-gel, respectively. These
observations indicate that the ArE-gels were more hydrophobic
than the ArA-gels. In the present study, the content of cross-
linker (i.e., MBA) was fixed at 2 mol % for the ArA-gels and
ArE-gels carrying Bz, Np, and Ph residues and at 3 mol % for
the PyA-gel and PyE-gel (see Table S1 in the Supporting
Information). The stress at rupture of the ArA-gels and ArE-
gels would depend on V/V0 because an ArA-gel or ArE-gel of a
larger V/V0 contained a lower density of cross-links. It is
noteworthy that PhE-gel and PyE-gel exhibited higher stresses
at rupture although these V/V0 values were modest (=1.2).
This observation indicates that aggregates of PhE and PyE
residues by hydrophobic or π−π interactions acted as
pseudocross-links in PhE-gel and PyE-gel. Furthermore, the
morphologies of the ArA-gels and ArE-gels freeze-dried were
observed by scanning electron microscopy (SEM). Figure 5
demonstrates a typical example of SEM images of the ArA-gels
and ArE-gels. This figure also contains an image of SEM for a
pAAm gel sample for comparison. All the images show network
structures. Similar to previous reports on SEM observation of
gels,45−49 white parts seem to correspond to the polymer
networks. These images indicate that a gel of a lower V/V0
tends to have a network structure with smaller pores. Thus, the
network structure in SEM images may depend on the
hydrophobicity of gel.
The differences of properties of the ArA-gels and ArE-gels
may be caused by differences of amide and ester linkages, which
may be not only the difference in the hydrophobicity and
rigidity of amide and ester but also the difference in the radical
polymerization behavior of acrylamides and acrylates. Since the
propagation rate constants of acrylates in radical polymerization
are usually larger than those of the corresponding acryl-
amides,50 the distributions of aromatic residues inside the gel
may be different for the ArA-gels and ArE-gels. The difference
in the distribution of aromatic residues may be responsible
partly for the difference in hydrophobicity of the ArA-gels and
ArE-gels. However, it is very difficult to investigate the
distribution of aromatic residues inside the gels at present.
Since αCD and βCD residues on the gel surface prefer
monomeric aromatic guest residues, the formation of gel
assemblies of αCD-gel and βCD-gel is dependent dominantly
on the binding constant for the model systems and on the
Figure 5. SEM images for freeze-dried samples of pAAm-gel (a), BzA-
gel (b), BzE-gel (c), NpA-gel (d), NpE-gel (e), PhA-gel (f), PhE-gel
(g), PyA-gel (h), and PyE-gel (i) at a magnification of 2 × 103. The
bars correspond to 10 μm.
concentrations of the CD and guest residues on the gel
surfaces. In the present study, the difference in the
concentration of aromatic guest residues was not critical, and
αCD-gel and βCD-gel thus demonstrated the same tendencies
in their interactions with the ArA-gels and with the ArE-gels.
On the other hand, since γCD residues on the gel surface prefer
dimeric aromatic residues, the formation of gel assemblies may
depend on the aggregation behavior of aromatic residues on the
gel surface, as well as on the binding constant for the model
systems and the concentrations of the residues. Since the ArE-
gels possessed a higher concentration of aromatic residues than
did the ArA-gels because of their smaller swelling ratios (i.e., V/
V0), it is likely that the ArE-gels contained higher degrees of
aggregation of aromatic residues. This may be responsible for
the different tendencies of the formation of gel assemblies of
γCD-gels with the ArA-gels and with the ArE-gels. γCD
residues of γCD-gel may prefer aggregated NpE residues
whereas they may not interact favorably with aggregated PhE or
PyE residues presumably because of the size and stability of
aggregates. As can be seen in Figure 4b, the fluorescence
spectrum of NpE-gel exhibited only a broad shoulder assignable
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Article
morphology of gel surface was scanned at an accelerating voltage of
1.50 kV with the LEI mode.
to excimer of NpE residues even though NpE-gel contained
more concentrated NpE residues. This observation is suggestive
of the formation of nonfluorescent aggregates of NpE residues
(larger than dimer). We are thus investigating the aggregated
state of aromatic residues on the gel surface.
On the basis of the data described above, the linkage between
guest residues and the pAAm gel scaffold can be an important
structural factor in macroscopic observation of gel interaction,
especially for γCD-gel. This is because the selectivity of
macroscopic interaction can be largely enhanced by multisite
interaction even though the difference between linkages is
small.
Materials. 1-Pyrenemethylamine hydrochloride was purchased
from Sigma-Aldrich Co., Ltd. 2-Naphthalenemethanol, 2-naphthylme-
thylamine hydrochloride, 9-phenanthrenecarbaldehyde, carbon tetra-
bromide (CBr4), and sodium azide (NaN3) were purchased from
Wako Pure Chemical Industries, Ltd. Benzyl alcohol, benzylamine, 1-
pyrenemethanol, and acryloyl chloride were obtained from Tokyo
Chemical Industry Co., Ltd. Triethylamine (Et3N), sodium borohy-
dride (NaBH4), triphenylphosphine (PPh3), magnesium sulfate, AAm,
APS, TMEDA, MBA, DMSO, acetone, methanol, 2-propanol, ethyl
acetate, diethyl ether, sodium bicarbonate, and sodium hydroxide were
purchased from Nacalai Tesque Inc. N,N-Dimethylformamide,
dichloromethane (DCM), and tetrahydrofuran (THF) were purified
by utilizing a Glass Contour solvent dispending system. Water was
purified by a Millipore Milli-Q system. αCD, βCD, and γCD were
purchased from Junsei Chemical Co., Ltd. and recrystallized twice
from water prior to use. Other reagents were reagent grade and used
without further purification.
CONCLUSION
■
The interactions of CD-gels with the ArA-gels and with the
ArE-gels were investigated to examine the effect of linkage (i.e.,
amide and ester) between aromatic residues and the pAAm gel
scaffold. The examinations of the formation of gel assemblies by
shaking in water indicated that αCD-gel and βCD-gel exhibited
the same tendencies of interactions with the ArA-gels and with
the ArE-gels whereas γCD-gel showed different tendencies.
αCD-gel did not form any gel assemblies with the ArA-gels and
ArE-gels. βCD-gel interacted with the ArA-gels and ArE-gels
possessing Bz and Np residues. γCD-gel formed gel assemblies
with NpE-gel, PhA-gel, and PyA-gel. Reflection fluorescence
spectra for the ArA-gels and ArE-gels possessing Np, Ph, and
Py residues confirmed the weak selectivities of γCD toward
NpE, PhA, and PyA residues. Since γCD residues in γCD-gel
preferred dimeric aromatic residues, aggregation properties of
the aromatic residues in the ArA-gels and ArE-gels might be
important for the interaction of γCD-gel. On the basis of the
data described above, the linkage between guest residues and
the pAAm gel scaffold can be an important structural factor in
macroscopic observation of gel interaction, because that the
selectivity of macroscopic interaction can be largely enhanced
by multisite interaction even though the difference between
linkages is small.
9-Phenanthrenemethanol was prepared from phenanthrylmethyl-9-
carboxyaldehyde with NaBH4 according to the procedure of Forrester
et al.51 9-Bromomethylphenanthrene was prepared from 9-phenan-
threnemethanol using CBr4 in the presence of PPh3 by a slight
modification of the procedure of Drouet et al.52 9-Azidomethylphe-
nanthrene was prepared from 9-bromomethylphenanthrene and NaN3,
and then 9-phenanthrylmethylamine was prepared from 9-azidome-
thylphenanthrene with PPh3 by a slight modification of the procedure
of Ahlford et al.53
AαCD, AβCD, and AγCD were prepared as reported previously.54
Preparation of Aromatic Acylamides (ArA). N-Benzylacryla-
mide (BzA),23 N-(2-naphthyl)methylacrylamide (NpA),22 and N-(1-
pyrenyl)methylacrylamide (PyA)24 were prepared as reported
previously.
N-9-Phenanthrylmethylacrylamide (PhA). To a solution of 9-
phenanthrylmethylamine (622 mg, 3.0 mmol) and Et3N (622 μL, 4.5
mmol) in THF (15 mL), a solution of acryloyl chloride (354 μL, 4.5
mmol) in THF (5 mL) was added dropwise at 0 °C under an argon
atmosphere with stirring. The reaction was allowed to run at room
temperature overnight. The precipitate formed was removed by
filtration. After evaporation of the solvent, the product was purified by
silica gel column chromatography using a mixed solvent of ethyl
acetate and hexane (1/2, v/v) as eluent to give the product in 41%
yield. 1H NMR (500 MHz, DMSO-d6, 30 °C) δ 8.901−8.850 (m, 1H,
phenanthryl), 8.809 (d, J = 8.2 Hz, 1H, phenanthryl), 8.643 (d, J = 5.3
Hz, 1H, NH), 8.126 (dd, J = 8.0 and 1.4 Hz, 1H, phenanthryl), 7.949
(dd, J = 7.7 and 1.4 Hz, 1H, phenanthryl), 7.771 (s, 1H, phenanthryl),
7.744−7.607 (m, 4H, phenanthryl), 6.320 (dd, J = 17.1 and 10.2 Hz,
1H, vinyl), 6.178 (dd, J = 17.0 and 2.3 Hz, 1H, vinyl), 5.632 (dd, J =
10.1 and 2.3 Hz, 1H, vinyl), 4.863 (d, J = 5.7 Hz, 2H, methylene).
Positive ion MALDI−TOF−MS: m/z = 261.3, [M]+ 261.3, [M + Na]+
283.9, [M + K]+ 299.9.
Preparation of Aromatic Acrylates (ArE). To a solution of
aromatic methanol (10 mmol) and Et3N (15 mmol) in THF (40 mL),
a solution of acryloyl chloride (15 mmol) in THF (5 mL) was added
dropwise at 0 °C under an argon atmosphere with stirring. The
reaction was allowed to run at room temperature overnight. The
precipitate formed was removed by filtration. After evaporation of the
solvent, the mixture was purified by silica gel column chromatography
to give aromatic acrylate monomer.
EXPERIMENTAL SECTION
■
Measurements. 1H NMR spectra were measured on a JEOL
JNM-ECA500 spectrometer. Chemical shifts were referenced to the
solvent values (2.49 and 4.79 ppm for DMSO-d6 and D2O,
respectively). Positive-ion matrix-assisted laser desorption/ionization
time-of-flight mass spectrometry (MALDI−TOF−MS) experiments
were performed using a Bruker autoflex speed mass spectrometer
using 2,5-dihydroxy-benzoic acid as a matrix. Mass number was
calibrated by four peptides, i.e., Angiotensin II ([M + H]+ 1046.5418),
Angiotensin I ([M + H]+ 1296.6848), Substance P ([M + H]+
1347.7354), and Bombesin ([M + H]+ 1619.8223). Gel assembly
tests were carried out using an EYELA CM-1000 cute mixer. The
agitation speed was fixed at ca. 400 rpm unless otherwise indicated.
Stress−strain curves for gel assemblies were recorded using a Yamaden
RE-33005 Rheoner creep meter. Each sample was prepared in a quartz
mold of 5 × 3 mm2 in cross sectional area and measured at a rate of
0.1 mm s−1 at room temperature. Fluorescence spectra for solutions of
the model systems were obtained on a HITACHI F-2500
spectrophotometer using a 1 cm quartz cuvette. The slit widths for
both excitation and emission sides were kept at 2.5 nm during
measurement. Reflection fluorescence spectra for gel samples were
obtained using a HORIBA FluoroMax-4 spectrofluorometer. A quartz
plate with a thin gel film was fixed on the holder of the fluorometer to
obtain spectra. SEM images were obtained on a JEOL-7600F scanning
electron microscope. A piece of freeze-dried gel sample was set on the
holder using a double-sided adhesive and conductive tape. The
Benzyl Acrylate (BzE). BzE was obtained as an oil after purifying by
silica gel column chromatography using a mixed solvent of diethyl
1
ether and hexane (1/20, v/v) in 40% yield. H NMR (500 MHz,
DMSO-d6, 30 °C) δ 7.394−7.309 (m, 5H, phenyl), 6.359 (dd, J = 17.3
and 1.6 Hz, 1H, vinyl), 6.219 (dd, J = 17.3 and 10.3 Hz, 1H, vinyl),
5.961 (dd, J = 10.31 and 1.6 Hz, 1H, vinyl), 5.175 (s, J = 2H,
methylene).
2-Naphthylmethyl Acrylate (NpE). NpE was obtained as an oil after
purifying by silica gel column chromatography using a mixed solvent of
1
DCM and hexane (1/10, v/v) in 48% yield. H NMR (500 MHz,
DMSO-d6, 30 °C) δ 7.939−7.893 (m, 4H, naphthyl), 7.545−7.497 (m,
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3H, naphthyl), 6.390 (dd, J = 17.3 and 1.6 Hz, 1H, vinyl), 6.253 (dd, J
= 17.3 and 10.3 Hz, 1H, vinyl), 5.978 (dd, J = 10.3 and 1.6 Hz, 1H,
vinyl), 5.346 (s, 2H, methylene). Positive ion MALDI−TOF−MS: m/
z = 212.2, [M]+ 212.7, [M + Na]+ 235.7, [M + K]+ 251.7.
Agency (JST), the Core Research for Evolutional Science and
Technology (CREST).
REFERENCES
9-Phenanthrylmethyl Acrylate (PhE). PhE was obtained as a
colorless crystalline product after purifying by silica gel column
chromatography using a mixed solvent of DCM and hexane (1/6, v/v)
in 65% yield. 1H NMR (500 MHz, DMSO-d6, 30 °C) δ 8.863 (dd, J =
29.7 and J = 8.3 Hz, 2H, phenanthryl), 8.040 (dd, J = 43.7 and J = 7.3
Hz, 2H, phenanthryl), 7.952 (s, 1H, phenanthryl), 7.762−7.639 (m,
4H, phenanthryl), 6.377 (dd, J = 17.2 and 1.4 Hz, 1H, vinyl), 6.246
(dd, J = 17.3 and 10.3 Hz, 1H, vinyl), 5.969 (dd, J = 10.3 and 1.4 Hz,
1H, vinyl), 5.700 (S, 2H, methylene). Positive ion MALDI−TOF−
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1-Pyrenylmethyl Acrylate (PyE). PyE was obtained as colorless
powder after purifying by silica gel column chromatography using a
mixed solvent of DCM and hexane (1/10, v/v) in 40% yield. 1H NMR
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8.221−8.132 (m, 3H, pyrenyl), 8.098 (t, J = 7.6 Hz, 1H, pyrenyl),
6.373 (dd, J = 17.3 and 1.6 Hz, 1H, vinyl), 6.237 (dd, J = 17.3 and 10.3
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acrylate. A predetermined amount of AAm, MBA, and an aromatic
monomer were dissolved in DMSO (1.5 mL). After purging with dry
argon for 30 min, APS (3 mg, 13 μmol) was added to the monomer
solution. The reaction mixture was placed into a 1 cm quartz cuvette
and sealed. The cuvette was set in an oven thermostated at 60 °C for 6
h. The resulting gel was soaked in water for several days to remove the
solvent and the unreacted monomers and initiators. For visual
identification, the ArA-gels and ArE-gels obtained were dyed by
immersing into solutions of the different dyes.
■
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The authors declare no competing financial interest.
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The authors thank Dr. Akihiro Ito, Department of Chemistry,
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