Amphiphilic molecular gels from ω-aminoalkylated L-glutamic acid derivatives with unique chiroptical properties

Article abstract of DOI:10.1007/s00726-010-0480-z

Self-assembling amphiphiles with unique chiroptical properties were derived from L-glutamic acid through ω-aminoalkylation and double long-chain alkylation. These amphiphiles can disperse in various solvents ranging from water to n-hexane. TEM and SEM observations indicate that the improvement in dispersity is induced by the formation of tubular and/or fibrillar aggregates with nanosized diameters, which makes these amphiphiles similar to aqueous lipid membrane systems. Spectroscopic observations, such as UV-visible and CD spectroscopies indicate that the aggregates are constructed on the basis of S-and R-chirally ordered structures through interamide interactions in water and organic media, respectively, and that these chiroptical properties can be controlled thermotropically and lyotropically. It is also reported that the chiral assemblies provide specific binding sites for achiral molecules and then induce chirality for the bonded molecules. Further, the applicability of the amphiphiles to template polymerization is discussed. Springer-Verlag 2010.

Full text of DOI:10.1007/s00726-010-0480-z

Amino Acids (2010) 39:587–597
DOI 10.1007/s00726-010-0480-z
ORIGINAL ARTICLE
Amphiphilic molecular gels from x-aminoalkylated L-glutamic
acid derivatives with unique chiroptical properties
•
Yoshiko Kira Yutaka Okazaki Tsuyoshi Sawada
•
•
•
Makoto Takafuji Hirotaka Ihara
Received: 2 November 2009 / Accepted: 7 January 2010 / Published online: 27 January 2010
Ó Springer-Verlag 2010
Abstract Self-assembling amphiphiles with unique chir-
optical properties were derived from L-glutamic acid
through x-aminoalkylation and double long-chain alkyl-
ation. These amphiphiles can disperse in various solvents
ranging from water to n-hexane. TEM and SEM observa-
tions indicate that the improvement in dispersity is induced
by the formation of tubular and/or fibrillar aggregates with
nanosized diameters, which makes these amphiphiles
similar to aqueous lipid membrane systems. Spectroscopic
observations, such as UV–visible and CD spectroscopies
indicate that the aggregates are constructed on the basis of
S- and R-chirally ordered structures through interamide
interactions in water and organic media, respectively, and
that these chiroptical properties can be controlled thermo-
tropically and lyotropically. It is also reported that the
chiral assemblies provide specific binding sites for achiral
molecules and then induce chirality for the bonded mole-
cules. Further, the applicability of the amphiphiles to
template polymerization is discussed.
Introduction
Amino acids are one of the most powerful base materials
used for the fabrication of nanoscale organic structures.
This is because they are highly versatile biocompatible
chemical sources; in addition, the chirality afforded by the
asymmetric carbon is essential for the formation of accu-
rate structures with unique functions. A polypeptide, which
is a polymer of amino acids, folds into its characteristic
three-dimensional structure and behaves as a functional
molecule. The formation of regulated conformations of
polypeptides, such as a-helical and b-structural polymers,
generates secondary chirality, which can be estimated by
chiroptical spectroscopies, such as circular dichroism (CD)
and optical rotatory dispersion. On the other hand, the
chiroptical property based on molecular self-assembly is
attracting considerable attention because of the tunability
and amplification of chiroptical signals. Therefore, amino
acids might potentially find applications in molecular
devices, such as sensors and switches (Hembury et al.
2008). In the last two decades, molecular gel systems with
chiral low-molecular-weight compounds have been widely
investigated. These gels have attracted interest because of
the fact that gelation is induced by the formation of a three-
dimensional network with nanofibrillar aggregates that are
based on highly ordered structures with a unique chirality.
Therefore, low-molecular-weight gelators often possess an
asymmetric carbon derived from amino acids, sugars,
cholesterol, and so on (Terech and Weiss 1997; van Esch
and Feringa 2000; Estroff and Hamilton 2004; Ihara et al.
2004; Xue et al. 2004; Shimizu et al. 2005; Oda 2006). The
enhancement of the chiroptical properties in molecular gel
systems is generally sensitive to external stimuli, such as
temperature, light, and electricity. Precise control and
accurate detection of the chiroptical property of molecular
Keywords Amino acid-derived amphiphilic lipid Á
Self-assembly Á Organic nanotube Á
Chirally ordered structure Á Induced chirality
Electronic supplementary material The online version of this
article (doi:10.1007/s00726-010-0480-z) contains supplementary
material, which is available to authorized users.
Y. Kira Á Y. Okazaki Á T. Sawada Á M. Takafuji Á H. Ihara (&)
Department of Applied Chemistry and Biochemistry,
Kumamoto University, 2-39-1 Kurokami,
Kumamoto 860-8555, Japan
e-mail: ihara@kumamoto-u.ac.jp; ihara@gpo.kumamoto-u.ac.jp
123

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Y. Kira et al.
gels are important for the development of molecular
devices. Thus far, we have focused on L-glutamic acid as a
base unit for derivatization into a molecular gelator
because of its versatile chemical tunability through its two
carboxylic acid groups and one amino group.
In this study, we show that both the dispersity
improvement in aqueous and non-aqueous media and the
enhancement of chiroptical versatility can be achieved by
x-aminoalkylation into a double long-chain alkylated
L-glutamic acid as a self-assembling molecular gelator.
Furthermore, we demonstrate that the obtained molecular
gels contain nanotubules and nanofibrillar aggregates that
provide specific binding sites for achiral guest molecules to
induce secondary chirality, and these can be used for
template polymerization.
Materials and methods
Scheme 1 Chemical structures of L-glutamic acid-derived amphi-
philes and cyanine dye
Chemicals
All chemicals were reagent grade and purchased from
chemical suppliers. Tetrahydrofuran (THF) was distilled
before use, and all other solvents were used as received. The
cyanine dye NK-2012 and the photoinitiator 2-benzyl-2-
dimethylamino-1-(4-morpholinophenyl)-butanone-1 (Irga-
cure 369, koptimal = 321 nm) (Timpe et al. 1993; Artal et al.
2001) was used as received. The L-glutamic acid-derived
amphiphiles with a x-amino or an x-pyridinium head group
were synthesized as shown in Scheme 1. The chemical
structures of these compounds were identified by melting
point, FTIR, 1H-NMR and elemental analyses. FTIR spectra
were performed using a JASCO FT/IR-4000 spectrometer.
1H-NMR spectra were recorded by a JEOL JNM-EX400
spectrometer using tetramethylsilane as an internal standard.
7.3 Hz, 2H, CH2(CH2)2NH], 1.53 (tt, J 7.3, 7.3 Hz, 2H,
CH2CH2NH), 1.65 (tt, J 7.3, 7.3 Hz, 2H, HO2CCH2CH2),
2.35 (t, J 7.3 Hz, 2H, HO2CCH2), 3.20 (dt, J 7.3, 7.3 Hz,
2H, CH2NH), 4.77 (br s, 1H, NH), 5.09 (br s, 2H, CH2Ph),
7.31–7.36 (m, 5H, aromatics).
N-Benzyloxycarbonyl-3-aminopropionic acid was pre-
pared using 3-aminopropionic acid according to the similar
procedure. mp: 105–106°C. IR (KBr): 3,335, 1,685,
1,536 cm-1 1H-NMR (CDCl3, 400 MHz): d 2.61 (t, J
.
5.9 Hz, 2H, HO2CCH2), 3.47 (dt, J 5.9, 5.9 Hz, 1H,
CH2NH), 5.10 (br s, 2H, CH2Ph), 5.29 (br s, 1H, NH),
7.24–7.40 (m, 5H, aromatics).
Preparation of L-glutaminic acid didodecylamide
(G–NH2)
Preparation of N-benzyloxycarbonyl-x-aminoalkanoic
acid
N-Benzyloxycarbonyl-L-glutamic acid didodecylamide (G–
Z, Gopal et al. 2006) was prepared by a coupling reaction
of N-benzyloxycarbonyl-L-glutamic acid with dodecyl-
amine in THF using diethylphosphorocyanidate (DEPC),
and then L-glutaminic acid didodecylamide (G–NH2, Gopal
et al. 2006) was prepared by debenzyloxycarbonylation of
G–Z in 200 ml of ethanol with H2/Pd–carbon according to
the previously reported procedure.
6-Aminohexanoic acid (13.1 g, 100 mmol) was dissolved
in 200 ml of a 1 N NaOH, and then benzyloxycarbonyl
chloride (21.0 ml, 131 mmol), and 1 N NaOH were added
with cooling to 0°C. The mixture was stirred for 1 h at this
temperature. After being stirred for 12 h at room temper-
ature, the solution was washed with diethyl ether. Then,
1 N HCl was added into the water phase until the pH
became 1. The solution separated into water and oil phases.
The oil phase was extracted by washing with ethyl acetate.
The ethyl acetate phase was dried over Na2SO4, concen-
trated in vacuo. The residue was recrystallized from
n-hexane and dried in vacuo to yield a white solid powder.
Yield: 21.9 g (88%). mp: 54–56°C. IR (KBr): 3,344, 1,688,
1,545 cm-1. 1H-NMR (CDCl3, 400 MHz): d 1.37 [tt, J 7.3,
Preparation of N-(benzyloxycarbonyl-x-
aminoalkanoyl)-L-glutamic acid didodecylamide
(G–Cn–Z)
N-(Benzyloxycarbonyl-6-aminohexanoyl)-L-glutamic acid
didodecylamide (G–C5–Z) was prepared by following
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Amphiphilic molecular gels from x-aminoalkylated L-glutamic acid
589
procedure. G–NH2 (4.3 g, 8.9 mmol), N-benzylox-
ycarbonyl-6-aminohexanoic acid (3.1 g, 11.7 mmol), and
triethylamine (2.5 ml, 18.0 mmol) were dissolved in THF
(350 ml). The solution was cooled to 0°C, and DEPC
(2.0 ml, 13.2 mmol) was added to the solution. The mixture
was stirred for 1 h at this temperature. After being stirred for
12 h at room temperature, the solution was concentrated in
vacuo, and the residue was dissolved in chloroform. The
solution was washed with 0.2 N NaOH, 0.2 N HCl, and
water. The solution was dried over Na2SO4, concentrated in
vacuo and the residue was dissolved in N,N-dimethylform-
amide. Then, this solution was put into water and white
crystals appeared and were corrected by filtration. Yield:
5.20 g (88%). mp: 159–160°C. IR (KBr): 3,295, 2,920,
was concentrated in vacuo and the residue was dissolved in
chloroform. Then, this solution was put into acetonitrile
and white crystals appeared and were corrected by filtra-
tion. Yield: 2.0 g (82%). mp: 128–130°C. IR (KBr) 3,289,
1
2,919, 2,851, 1,636, 1,558, 1,542 cm-1. H-NMR (CDCl3,
400 MHz) d 0.88 (t, J 6.8 Hz, 6H, CH3), 1.25 [br s, 36H,
CH3(CH2)9], 1.41–1.57 [m, 6H, CH2(CH2)3CH2NH2], 1.66
(tt, J 7.3, 7.3 Hz, 4H, CH2CH2NHCO), 1.85–2.15 (m, 2H,
C*HCH2), 2.24 [t, J 7.8 Hz, 2H, NHCOCH2(CH2)4NH2],
2.16–2.54 (m, 4H, NHCOCH2 CH2C*), 2.69 (t, J 6.8 Hz,
2H, CH2NH2), 3.23 (dt, J 6.8, 6.8 Hz, 4H, CH2NHCO),
4.33 (dt, 5.9, 6.8 Hz, 1H, C*H), 6.07 (br s, 1H, NH), 6.82
(br s, 1H, NH), 7.26 (br s, 1H, NH). Anal. calcd for
C35H70N4O3: C, 70.1; H, 11.9; N, 9.42%. Found: H, 11.3;
C, 68.7; N, 8.67.
1
2,851, 1,687, 1,635, 1,548 cm-1. H-NMR: d 0.88 (t, J
6.8 Hz, 6H, CH3), 1.25 [br s, 36H, CH3(CH2)9], 1.42–1.56
(m, 6H, CH2(CH2)3CH2NHCO), 1.56–1.72 (m, 4H,
CH2CH2NHCO), 1.85–2.11 (m, 2H, C*HCH2), 2.22 (t, J
7.3 Hz, 2H, NHCOCH2CH2(CH2)4N), 2.16–2.54 (m, 2H,
3-Aminopropanoyl-L-glutamic acid didodecylamide (G–
C2–NH2) was also prepared from G–C2–Z according to the
similar synthetic procedure of G–C5–NH2. After recrys-
tallization from ethanol, white powders were obtained. mp:
136–138°C. IR (KBr): 3,292, 2,920, 2,851, 1,638,
NHCOCH2CH2C*), 3.19 (dt,
J
7.3, 7.3 Hz, 2H,
CH2NHCO), 3.23 (dt, J 7.3, 7.3 Hz, 4H, CH2NHCO), 4.31
(dt, J 7.3, 4.9 Hz, 1H, C*H), 4.86 (br s, 1H, NH), 5.10 (br s,
2H, CH2Ph), 6.01 (br s, 1H, NH), 6.80 (br s, 1H, NH), 7.28–
7.42 (m, 5H, aromatics). Anal. calcd for C43H76N4O5: C,
70.8; H, 10.5; N, 7.68. Found: C, 69.8; H, 10.3; N, 7.68.
Preparation of N-(benzyloxycarbonyl-3-aminopropa-
noyl)-L-glutamic acid didodecylamide (G–C2–Z) was pre-
1,555 cm-1 1H-NMR (CDCl3, 400 MHz) d 0.89 (t, J
.
6.8 Hz, 6H, CH3), 1.26 (br s, 36H, CH3(CH2)9), 1.48 (br s,
4H, CH2CH2NHCO), 1.91–2.17 (m, 2H, C*HCH2), 2.23–
2.48 (m, 2H, NHCOCH2), 2.36 (t, J 6.3 Hz, 2H,
NHCOCH2CH2NH2), 3.04 (t, J 6.3 Hz, 2H, CH2NH2), 3.22
(dt, J 6.8, 6.8 Hz, 4H, CH2NHCO), 4.38 (dt, J 6.3, 7.8 Hz,
1H, C*H), 6.02 (br s, 1H, NH), 7.09 (br s, 1H, NH), 7.93
(br s, 1H, NH). Anal. calcd for C32H64N4O3: C, 69.6; H,
11.6; N, 10.2%. Found: C, 68.0; H, 11.3; N, 9.45.
pared from
a coupling reaction of G–NH2 with
N-benzyloxycarbonyl-3-aminopropionic acid according to
the similar synthetic procedure of G–C5–Z. After recrys-
tallization from ethanol, white powders were obtained. mp:
174–175°C. IR (KBr): 3,295, 2,921, 2,851, 1,698, 1,637,
Preparation of N-(3-pyridinium propionyl)-L-glutamic
acid didodecylamide bromide (G–C2–Py)
1
1,550 cm-1. H-NMR (CDCl3) d 0.88 [t, J 7.3 Hz, 6H,
CH3), 1.26 (br s, 36H, CH3(CH2)9], 1.49 (tt, J 7.3, 7.3 Hz,
4H, CH2CH2NHCO), 1.86–2.10 (m, 2H, C*HCH2), 2.46 (t, J
5.8 Hz, 2H, NHCOCH2CH2N), 2.19–2.52 (m, 2H,
3-Bromopropionyl-L-glutamic acid didodecylamide (G–
C2–Br, Gopal et al. 2006) was prepared by a coupling
reaction of G–NH2 with 3-bromopropionic acid, and then
N-(3-pyridinium propionyl)-L-glutamic acid didodecyla-
mide bromide (G–C2–Py, Gopal et al. 2006) was prepared
by quaternization of pyridine with G–C2–Br according to
the previously reported procedure.
NHCOCH2CH2C*), 3.23 (dt,
J
5.8, 6.8 Hz, 2H,
CH2NHCO), 3.40–3.56 (br m, NHCOCH2CH2N), 4.33 (dt, J
5.8, 6.8 Hz, 1H, C*H), 5.14 (br s, 2H, CH2Ph), 5.09 (br s,
1H, NH), 5.82 (br s, 1H, NH), 6.82 (br s, 1H, NH), 7.13 (br s,
1H, NH), 7.29–7.45 (m, 5H, aromatics). Anal. calcd for
C40H70N4O5: C, 70.1; H, 10.3; N, 8.08. Found: H, 10.3; C,
69.9; N, 8.15.
Polymerization of styrene in G–C2–Py
G–C2–Py (10 mg, 14.3 mmol) was dissolved in 2.5-ml
water and vortexed for 1 min at room temperature. The
solution was sonicated using a SONICATOR XL2020
(Misonix Inc.) equipped with a Ti microprobe tip (/
1.6 mm, 20 W) for 5 min at about 60°C and allowed to
stand for 30 min at room temperature. Methanol (0.1 ml)
containing Irgacure 369 (0.5 mg) and styrene (10 mg) were
added into the G–C2–Py aqueous solution and vortexed for
1 min at room temperature. The mixture was diluted by
water to become the concentration of G–C2–Py to 0.5 mM
Preparation of x-aminoalkanoyl-L-glutamic acid
didodecylamide (G–Cn–NH2)
6-Aminohexanoyl-L-glutamic acid didodecylamide (G–C5–
NH2) was prepared by following procedure. G–C5–Z
(3.0 g, 4.1 mmol) was dissolved in 200 ml of ethanol at
60°C, and Pd–carbon (0.6 g) was added to the solution.
After H2 gas was bubbled slowly into the solution for 8 h at
70°C, Pd–carbon was removed by filtration. The solution
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Y. Kira et al.
and furthermore vortexed for 1 min at room temperature.
Remaining oxygen was removed by bubbling a nitrogen
gas. The solution was replaced into a 1-mm quarts cell,
aged at 10°C for 1 h and irradiated by UV light ranging of
280–370 nm with the IR cut filter and UV transmittance
filter. Photo-induced polymerization was carried out using
OPM-9500 (ultrahigh-pressure mercury lamp of 0.75 kW
power) from Ushio Inc. and spectroscopically monitored
by the absorption decrease at 247 nm.
Table 1 Dispersity of L-glutamic acid-derived amphiphiles in vari-
ous solvents
G–NH2 G–C2–Py G–C2–NH2 G–C5–NH2
Water
S
S
S
I
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Methanol
Ethanol
Acetonitrile
N,N-
S
Dimethylformamide
Chloroform
Tetrahydrofuran
Acetone
S
S
S
I
S
I
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Measurements
I
Transmission electron microscope (TEM) images were
recorded using a JEOL JEM-2000FX. The samples were
spotted on polyvinyl-formal-coated copper grids. Following
the excess of the samples was removed by a tissue paper and
the samples were air-dried, the samples were stained with a
1 wt% uranyl acetate aqueous solution. Scanning electron
microscope (SEM) images of the xerogels were recorded
using a JEOL JCM-5700. The critical gelation concentra-
tion (CGC) was determined by an inversion fluid method
using an / 10-mm test tube (Ihara et al. 2002). The xerogel
was prepared from organogel using freeze drying, and
osmium was coated onto the xerogel using Filgen Osmium
Plasma Coater OPC60A. UV–visible and CD spectra were
recorded using JASCO V-560 and JASCO J-725 spec-
trometers, respectively, and the temperature was controlled
by a Peltier thermostated cell holder. Differential scanning
calorimetry (DSC) analysis of the L-glutamic acid-derived
amphiphiles was carried out using DSC6200 and EXTRA
6000 from Seiko Instruments Inc. The aqueous solution of
these amphiphiles (20 mM, 50 ll) was sealed in 70-ll sil-
ver pan and measured between 10 and 80°C at heating and
cooling rate of 2°C/min.
Ethylacetate
Benzene
I
S
S
I
S
S
I
Toluene
Cyclohexane
Diethyl ether
n-Hexane
I
I
I
I
S and I indicate to be a clear solution and hard to solve in 1 mM at
10°C, respectively
typical aggregation morphologies of these fibrils. It is
clearly observed that G–C2–NH2 and G–C5–NH2 form
tubules and/or fibrils with nanosized diameters (outer
diameter 15–25 nm) in water and organic media, such as
acetonitrile, benzene, and cyclohexane, whereas G–NH2
without x-aminoalkylation forms only globular aggregates,
such as vesicles in water (Fig. 1a). However, no results
with organic solvents have been obtained thus far because
of the poor solubility of the samples in these solvents.
G–C2–NH2 and G–C5–NH2 exhibit gelation ability with
an increase in concentration, as shown in Fig. 3. This is
distinctly observed in organic media because the increase
in concentration promotes the development of fibrillar
structures into a three-dimensional network. This can be
confirmed by the SEM observation of the xerogels prepared
by freeze drying the organogel. Figure 3 shows a typical
example of the G–C2–NH2 benzene gel and its SEM image.
From the SEM image, the diameters of the fibrils are
observed to be 100–500 nm; these values are considerably
larger than the values of 15–25 nm obtained from TEM
observations. This is probably attributable to bundle for-
mation during the drying process.
Results and discussion
Unique dispersity of x-aminoalkyloyl L-glutamic acid
amphiphiles
The improvement of the dispersity of molecular tools in
solvents expands the possible applicability of amphiphiles.
The dispersity of L-glutamic acid-derived amphiphiles was
examined in various solvents ranging from water to
n-hexane. As summarized in Table 1, the dispersity of the
double long-chain alkylated L-glutamic acid (G–NH2) was
remarkably improved by x-aminoalkylation when com-
pared with that of G–C2–NH2 and G–C5–NH2, as shown in
Scheme 2.
Gels in an organic medium exhibit a CGC. These con-
centrations are evaluated by using a conventional inversion
method. In the case of G–C2–NH2, a sol to gel transfor-
mation is observed in the concentration range of 0.1–0.5,
0.5–1.0, and 0.5–1.0 mM at 10°C in acetonitrile, benzene,
and cyclohexane, respectively. These phenomena are
known as low-molecular-mass gelation in organic media
(Terech and Weiss 1997; van Esch and Feringa 2000; Ihara
et al. 2004; Oda 2006; Simalou et al. 2009). Similarity of
TEM observation indicates that the enhanced dispersity
can be explained by the formation of fibrillar aggregates in
both water and organic media. Figures 1 and 2 show the
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Amphiphilic molecular gels from x-aminoalkylated L-glutamic acid
591
Scheme 2 Synthetic procedure
of L-glutamic acid-derived
amphiphiles
Fig. 1 TEM images of G–NH2 (a) and G–C2–Py (b–d) aggregates in various solvents. Initial concentration: 0.2–0.5 mM. Scale bars 100 nm
gelation property is obtained in G–C5–NH2, but not in
G–C2–Py. Table 2 summarizes the CGC of G–C2–NH2 and
G–C5–NH2 in various solvents at 10°C.
Interestingly, as shown in Fig. 1c and d, nanotubular
structures are produced even in organic media, such as
acetonitrile and benzene. The thicknesses of these aggre-
gates are ca. 5–6 nm, indicating the bimolecular length of
G–C2–Py. Therefore, it can be concluded that the tubular
structures are based on bilayered structures similar to
aqueous lipid bilayered membranes (Kim and Needham
2002). It is known that organogel-forming low-molecular-
weight compounds can form nanofibrillar aggregates;
however, G–C2–Py is the first example that can produce
bilayer-based nanotubular aggregates in both aqueous and
non-aqueous organic media.
On the other hand, G–C2–NH2 and G–C5–NH2 do not
exhibit a distinct CGC in water even at a high concentra-
tion, such as 20 mM, although the viscosity increases
remarkably. In addition, the viscosity of G–C5–NH2 is
higher than that of G–C2–NH2. This is related to the fact
that G–C2–NH2 can mainly form short tubules in water,
whereas G–C5–NH2 can form developed, twisted, ribbon-
like aggregates (Fig. 2).
Formation of nanotubular structures in both aqueous
and non-aqueous media
Chiroptical property in aqueous system
G–C2–Py is a derivative of G–NH2 with a pyridinium head
group. The dispersity of G–C2–Py in organic solvents is not
improved because of its ionic property; however, the
aggregation behavior changes significantly. This suggests
that the alkyl spacer chain between a L-glutamic acid
moiety and a hydrophilic group promotes the transforma-
tion of the aggregation morphology to a fibrillar form.
In aqueous media, G–C2–NH2 provides negative and strong
Cotton effects ([h]max = ca. –1.0 9 105 deg cm2 d mol–1
at 10°C) in the absorption band around 200 nm, which is
assigned to the amide bonds (Fig. 4c). This CD intensity is
almost 20 times larger than that in the molecular-based
chirality of G–C2–NH2. On the other hand, the tempera-
ture-[h]max plots of Fig. 4d show that the CD strength is
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Y. Kira et al.
Fig. 2 TEM images of G–C2–NH2 (a–d) and G–C5–NH2 (e–h) aggregates in various solvents. Initial concentration: 0.05–0.5 mM. Scale bars
100 nm
Table 2 Critical gelation concentration of L-glutamic acid-derived
amphiphiles in various solvents at 10°C
G–C2–Py
G–C2–NH2
G–C5–NH2
Water
[20 mM
[20 mM
[20 mM
[20 mM
[20 mM
[20 mM
[20 mM
[20 mM
Methanol
Ethanol
7.5–10 mM
10–20 mM
Acetonitrile
0.1–0.5 mM 1.0–2.5 mM
N,N-Dimethylformamide [20 mM
[20 mM
[20 mM
1.0–2.5 mM
Chloroform
Tetrahydrofuran
Acetone
[20 mM
[20 mM
–
7.5–10 mM 2.5–5.0 mM
2.5–5.0 mM 2.5–5.0 mM
1.0–2.5 mM 1.0–2.5 mM
0.5–1.0 mM 1.0–2.5 mM
–
Fig. 3 a Typical example of the concentration-dependent gel to sol
transformation of G–C2–NH2 in benzene. The gel–sol transition is
observed in the concentration ranging in 0.5–1.0 mM in benzene at
10°C. b SEM image of a xelogel prepared from freeze drying of a
G–C2–NH2 benzene gel (20 mM)
Ethylacetate
Benzene
–
[20 mM
Toluene
10–20 mM 0.5–1.0 mM 1.0–2.5 mM
Cyclohexane
Diethyl ether
n-Hexane
–
–
–
0.5–1.0 mM 1.0–2.5 mM
0.5–1.0 mM 1.0–2.5 mM
0.1–0.5 mM 0.5–1.0 mM
remarkably temperature dependent and strongly suggest
that the critical temperature lies in the temperature range
around 35–40°C.
concentration (20 mM). The observed DSC thermogram
exhibits an endothermic curve with a peak at 37°C and a
shoulder at 30°C (Fig. 4b) and an exothermic curve, with a
The thermotropic phase transition behavior of G–C2–
NH2 in water can also be detected by DSC at a high
123

Amphiphilic molecular gels from x-aminoalkylated L-glutamic acid
593
Fig. 4 CD spectra (a, c, e) and
temperature-[h]max plots (b, d,
f) of G–C2–Py, G–C2–NH2 and
G–C5–NH2 in water (0.5 mM).
The dotted lines indicate DSC
thermograms obtained in the
heating process in water
(20 mM)
peak at 18°C in the heating and cooling process, respec-
tively. Because these phenomena are thermally reversible,
it is confirmed that the G–C2–NH2 assembly exhibits
thermotropic phase transition in water. This observation is
similar to the behavior of the gel to liquid crystalline phase
transition of lipid membranes in an aqueous solution
(Matsui and Kaneshima 2002). Therefore, the enhanced
CD intensity at a low temperature is attributed to the
induction of the secondary chirality with S-chiral stacking
(Simonyi et al. 2003) based on highly ordered aggregation.
Similar chiroptical properties with thermotropic phase
transitions are also observed in G–C2–Py and G–C5–NH2,
as shown in Fig. 4a, b, e, and f, whereas G–C2–Py exhibits
a specific temperature dependence around 25–45°C.
transition behavior at temperatures between 10 and 80°C.
TEM observation shows that only fragmented fibrils can be
found at a high temperature.
Host–guest function as chiral assembly
It is known that chiral assemblies with highly ordered
structures often provide specific binding sites for achiral
Chiroptical property in non-aqueous system
G–C2–NH2 and G–C5–NH2 in acetonitrile exhibit a posi-
tive cotton effect in the absorption band around 200 nm
that is several times stronger than that in water (Fig. 5a, b).
A similar positive cotton effect with a small negative band
at 220 nm was observed in cyclohexane (Fig. 5c, d). These
results indicate that R chirally (Simonyi et al. 2003)
ordered structures are mainly produced in organic solvents
through interamide bonding interactions, and the detailed
ordering state is dependent on the type of solvent used,
whereas S-chiral ordering is dominant in water.
Figure 5 shows the temperature dependence of CD
spectra. As in the case of water, the CD intensity decreases
with an increase in the temperature; however, there is no
distinct critical temperature. This observation is supported
by the fact that the DSC does not indicate any phase
Fig. 5 Temperature dependencies of CD spectra of G–C2–NH2 (a, c)
and G–C5–NH5 (b, d) in acetonitrile (0.2 mM) and cyclohexane
(0.5 mM)
123

594
Y. Kira et al.
molecules and then induce secondary chirality for the
bonded molecules (Hatano et al 1973; Nakashima et al.
1981; Shibata et al. 1993). Therefore, this phenomenon can
be useful as an informative evaluation method for L-glu-
tamic acid-based chiral assemblies. Figure 6 shows UV–
visible and CD spectra in the mixed system with a cyanine
dye NK-2012 as an anionic achiral molecule in water. kmax
in NK-2012 appeared at 543 nm in aqueous solution and
shifted to 507 and 503 nm in the G–C2–Py and G–C2–NH2
systems, respectively. These typical blue shifts are accom-
panied by the induction of large cotton effects (Fig. 6b, d).
These spectral patterns indicate that almost all the NK-2012
combines with the G–C2–Py and G–C2–NH2 assemblies,
probably due to electrostatic interaction, and this host–guest
system promotes an H-like aggregation of the NK-2012 as a
head-to-head stacking state (Shibata et al. 1993; Mishra
et al. 2000; Sagawa et al. 2004). In addition, the CD patterns
support the speculation that the aggregation is based on S-
chiral ordering. In contrast, the NK-2012 in the G–C5–NH2
system exhibits two absorption bands from kmax of 543 nm:
one is a blue-shifted band at kmax of 501 nm due to H-like
aggregation and the other is a red-shifted band at kmax of
576 nm due to J-like aggregation (Fig. 6e). Interestingly,
these kmax shifts are accompanied by inductions of S-chi-
rality and R-chirality, respectively (Fig. 6f). Therefore, the
G–C5–NH2 assembly posses two types of chirally specific
binding sites that are based on the C5 moiety.
ethanol solution of NK-2012 (0.2 mM, 200 ll) with a
cyclohexane solution of L-glutamic acid-derived amphi-
philes (0.5 mM, 4 ml) at 10°C. As shown in Fig. 7, the
induction of CD signals is observed around kmax (429 nm)
of NK-2012 in the G–C2–NH2 system at 10°C. These
results indicate that the G–C2–NH2 assembly can provide
chirally specific binding sites even in an organic medium.
NK-2012 probably forms H-like aggregates with R-chiral
arrangement in the G–C2–NH2 system. The solubility of
the dye-G–C2–NH2 assembly complex decreases above
55°C and precipitation is confirmed at 70°C. In this case,
an amino group of G–C2–NH2 does not form ammonium
ions. Therefore, it is considered that the NK-2012 binds to
the G–C2–NH2 assembly thorough polar interaction. In
contrast, almost no CD induction is detected around kmax of
NK-2012 in the G–C5–NH2 system regardless of the
existence of the H-like aggregate species that produces
kmax around 450 nm at 10°C. These results are probably
explained by the non-chirally stacked binding of NK-2012
on the G–C5–NH2 assembly. In this case, the C5 spacer
chain can become rather flexible by solvation, resulting in
the disordering of the NH2 moiety even if the L-glutamic
acid moiety is in an ordered state through an interamide
interaction. In the case of a dye-G–C5–NH2 assembly
complex, no precipitation is observed even at 70°C; how-
ever, monomeric species with kmax of 563 nm increase
with temperature. This is probably due to the disordered
state of G–C5–NH2 molecules at higher temperatures.
Figure 8 shows schematic representations of the binding
behaviors of NK-2012 on and/or in the chiral assemblies in
aqueous and non-aqueous systems.
The binding behavior of NK-2012 in an organic medium
is also examined by using a cyclohexane–ethanol mixture
(20:1) because of the limitation of the solubility for NK-
2012. The sample mixtures are prepared by mixing an
Fig. 6 UV–visible and CD
spectra of NK-2012 (0.01 mM)
in the presence of G–C2–Py,
G–C2–NH2 and G–C5–NH2
(0.5 mM) in water
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Amphiphilic molecular gels from x-aminoalkylated L-glutamic acid
595
The mixed samples of styrene with G–C2–Py are pre-
pared by injecting a given amount of styrene into an
aqueous G–C2–Py solution and ultrasonicating the sample.
The resultant mixtures are almost clear to slightly turbid
depending on the amount of styrene. Figure 9a shows a
TEM image of G–C2–Py before the addition of styrene. It
is found that the main products are nanotubes with outer
diameters of 15–17 nm and a length of ca. 100–500 nm.
The membrane thickness is 5–6 nm, which corresponds to
the bimolecular length of G–C2–Py. As shown in Fig. 9b,
the addition of styrene induces significant changes in the
aggregation morphology. For example, by the addition of
an equal amount of styrene to G–C2–Py, the outer diameter
and membrane thickness were found to increase from 18 to
22 and 7 to 9 nm, respectively. A further distinct change is
a one-dimensional one in that the length increases to more
than 1,000 nm. Therefore, it is proved that G–C2–Py
nanotubes can carry styrene, probably with a hydrophobic
moiety.
Fig. 7 UV–visible and CD spectra of NK-2012 (0.01 mM) in the
presence of G–C2–NH2 and G–C5–NH2 (0.5 mM) in a cyclohexane–
ethanol mixture (20:1)
Radical polymerization is carried out at 10°C by the
addition of 5 wt% of Irgacure 369 as a photo-radical ini-
tiator; then, the decrease in the absorption of styrene is
observed. It is confirmed that 90% of styrene is consumed
by photoirradiation for 4 h (Supplementary Fig. 1). Fig-
ure 9c shows TEM images after polymerization. Two types
of globular polymers can be found: a large one with a
diameter of 20–35 nm on and/or in the membranes, and a
small one with a diameter of 7–16 nm in the inner tube.
These observations indicate that the resultant polymer
domains are phase separated from the G–C2–Py membrane
during the polymerization. In this work, we cannot achieve
the exact control of the polymer size; however, further
investigation is expected to bring about successful results
for nanosize organization through polymerization.
Conclusions
Fig. 8 Schematic illustration of the binding behaviors of NK-2012 on
and/or in the chiral assemblies in aqueous and non-aqueous systems
In this study, we have shown that the dispersity of double
long-chain alkylated L-glutamic acid as a molecular gelator
in solvents ranging from water to n-hexane can be
remarkably improved by x-aminoalkylation as a simple
chemical modification. This high dispersity enables the
formation of highly ordered states both in aqueous and
non-aqueous systems. In addition, the obtained molecular
gels exhibit unique chiroptical properties based on highly
ordered structures through interamide interactions. We
have also demonstrated that these chiroptical properties can
be controlled thermotropically and lyotropically; the chiral
assemblies provide specific binding sites for achiral cya-
nine dye and then induce chirality for the bonded dyes.
Further, we have discussed the applicability of amphiphiles
to template polymerization.
Host–guest function for template polymerization
Nanostructures based on bilayer membranes such as vesi-
cles and rods are expected to function as a specific medium
for the preparation of nanostructures by polymerization
(Hotz and Meier 1998; Becerra et al. 2004; da Silva Gomes
et al. 2006; Dergunov and Pinkhassik 2008). Tekobo and
Pinkhassik (2009) used nanodisks prepared by a phospho-
lipids for the polymerization of styrene and reported that
disk-like polymers could be produced. In this work, photo-
induced radical polymerization of styrene is investigated in
the presence of aqueous G–C2–Py nanotubes.
123

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Y. Kira et al.
Fig. 9 TEM images of G–C2–
Py (0.5 mM) without (a) and
with styrene (0.35 mg/ml)
mixture before (b) and after (c)
UV irradiation (4 h) in water–
methanol mixture (250:1) at
10°C
Acknowledgments This work was supported by the Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology of Japan and Grant-in-Aid for JSPS
Fellows from Japan Society for the Promotion of Science.
Kim DH, Needham D (2002) Lipid bilayers and monolayers:
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Matsui H, Kaneshima S (2002) Molecular interactions between local
anesthetics and model biomembranes. In: Hubbard AT (ed)
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Mishra A, Behera RK, Behera PK, Mishra BK, Behera GB (2000)
Cyanines during the 1990s: a review. Chem Rev 100:1973–2012
Nakashima N, Fukushima H, Kunitake T (1981) Large induced
circular dichroism of methyl orange bound to chiral bilayer
membranes: its extreme sensitivity to the phase transition and
the chemical structure of the membrane. Chem Lett
10:1207–1210
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