Supramolecular hydrogels based on L-phenylalanine derivatives with a positively charged terminal group

Article abstract of DOI:10.1002/hlca.200900137

A new hydrogelator based on l-phenylalanine with a long hydrophobic chain and positively charged terminus was synthesized, and its gelation behavior in H₂O was investigated. Polarized optical microscopy (POM), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD) results indicate that the hydrogelator self-assembles into fibres-like aggregates which then lead to the formation of a hydrogel. ¹H-NMR and CD spectra of hydrogels and aqueous solution revealed that intermolecular H-bonding between the amide groups was the driving force for gelation. A luminescence study, in which ANS (8-anilinonaphthalene-1-sulfonic acid) was used as a probe, indicated that the hydrophobic interactions between long chains were the driving force for gelation. Consequently, it was proved that the hydrogelator self-assembles into fibre-like aggregates and then forms supramolecular hydrogels through the H-bonding and hydrophobic interactions.

Full text of DOI:10.1002/hlca.200900137

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Supramolecular Hydrogels Based on l-Phenylalanine Derivatives with a
Positively Charged Terminal Group
by Xin-Jian Fu*a)b), Hua Zhangb), Si-Kai Zhoub), Shao-Bing Liub), Fu-Quan Guob), Hong Wanga),
and Ya-Jiang Yang*a)
a) Department of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, Wuhan 430074, P. R. China
b) Department of Materials Engineering, Luoyang Institute of Science and Technology, Luoyang 471023,
P. R. China (phone: þ 86-379-65928203; fax: þ 86-379-65928196; e-mail: fuxinjian@126.com)
A new hydrogelator based on l-phenylalanine with a long hydrophobic chain and positively charged
terminus was synthesized, and its gelation behavior in H2O was investigated. Polarized optical
microscopy (POM), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction
(XRD) results indicate that the hydrogelator self-assembles into fibres-like aggregates which then lead
to the formation of a hydrogel. 1H-NMR and CD spectra of hydrogels and aqueous solution revealed that
intermolecular H-bonding between the amide groups was the driving force for gelation. A luminescence
study, in which ANS (8-anilinonaphthalene-1-sulfonic acid) was used as a probe, indicated that the
hydrophobic interactions between long chains were the driving force for gelation. Consequently, it was
proved that the hydrogelator self-assembles into fibre-like aggregates and then forms supramolecular
hydrogels through the H-bonding and hydrophobic interactions.
Introduction. – In the past decades, organogels, in which organic solvents are gelled
by low-molecular-weight compounds (organogelators), have attracted much attention
owing to their unique features and potential applications for new organic soft materials
[1]. Organogelators can self-assemble into supramolecular aggregates through
intermolecular interactions, such as H-bonding, p – p stacking, Van der Waals
attractions, coordination, and charge transfer. The solvent molecules are immobilized
by capillary forces in a three-dimensional network and lead to the gelation of organic
solvents. Although quite a few low-molecular-weight organogelators and their
applications have been reported, the study of low-molecular-weight hydrogelator
(LMWH) has been very limited [2]. Unlike traditional polymer hydrogels, which are
H2O-swollen crosslinked polymers, the LMWHs are preferred because of their rapid
response to external stimuli and thermoreversible nature, due to noncovalent
interactions. For the application of supramolecular hydrogels for biological use, their
biocompatibility and responsiveness are indispensable [3]. Furthermore, hydrogelators
formed in the presence of even a limited amount of additional organic solvent would
restrict their application in biological systems [4]. Hydrogelators can also gelate in H2O
which is interesting according to the essential criteria for biological use [5].
For an appropriate hydrogelator, the presence of both hydrophilic and hydrophobic
groups within the same molecule is an important requirement to aggregate in an
ordered fashion to induce immobilization of H2O [6]. Amphiphilic molecules,
ꢀ 2010 Verlag Helvetica Chimica Acta AG, Zꢁrich

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especially amino acid derived hydrogelators, not only have such characteristics but also
are biodegradable and have low toxicity. Suzuki and co-workers synthesized some
amino acid derived hydrogelators, such as valine, isoleucine, and lysine derived with
positively charged pyridinium or imidazolium moieties as terminal groups, which can
gelatinize H2O [7]. Motulsky et al. reported the gelation of vegetable oil and H2O in the
presence of six different amphiphilic gelators based on l-alanine derivatives. They also
reported the formation of hydrogels through subcutaneous injections in rats and the
biodegradation and biocompatibility of hydrogels [6a]. In our previous research works,
we have synthesized an amphiphilic hydrogelator based on l-phenylalanine derivatives
and investigated the formation mechanism of supramolecular hydrogels [8].
In this work, an amphiphilic hydrogelator based on l-phenylalanine, N-octadecyl-
Na-[6-(pyridinium-1-yl)hexanoyl]-l-phenylalaninamide bromide (NP18PB), was syn-
thesized and characterized. The formation mechanism of supramolecular hydrogels in
the presence of NP18PB was investigated by using 1H-NMR, polarized optical
microscopy (POM), field emission scanning electron microscopy (FE-SEM), fluo-
rescence, X-ray diffraction (XRD), and circular dichroism (CD).
Results and Discussion. – 1. Synthesis of NP18PB (Scheme). The starting material
(4S)-4-benzyl-1,3-oxazolidine-2,5-dione (l-Phe-NCA) was synthesized according to a
reported procedure [9]. Octadecylamine and l-Phe-NCA were reacted in CH2Cl2 at
408. The product N-octadecyl-l-phenylalaninamide (designated as C18Phe) was then
reacted with 6-bromohexanoyl chloride in the presence of Et3N to yield Na-(6-
bromohexanoyl)-N-octadecyl-l-phenylalaninamide (designated as NP18). NP18 was
refluxed in pyridine leading to N-octadecyl-Na-[6-(pyridinium-1-yl)hexanoyl]-l-phe-
nylalaninamide bromide (NP18PB).
2. Hydrogelation Properties. The hydrogelation properties of NP18PB and its
minimum gel concentration (MGC) necessary for the gelation of H2O was investigated
and evaluated by the ꢂstable to inversion of a test tubeꢃ method [10]. Dissolution of
NP18PB in H2O by heating to 808 and subsequent cooling to room temperature led to
gelation at concentrations of 1.2 wt-% and above, and a visible and transparent
hydrogel was observed. Each hydrogelator molecule has thus the ability to immobilize
ca. 3100 H2O molecules at room temperature. Furthermore, the hydrogel formed by
NP18PB (1.2 wt-%) was observed to be stable at room temperature over a period of
one year.
Fig. 1 shows gel-to-solution transition temperatures (TGS) of supramolecular
hydrogels which formed with different concentrations of NP18PB, as determined by
dropping ball measurements. As shown in Fig. 1, while increasing the MGCs from 1.5 to
3.5 wt-%, TGS of hydrogels increased from 408 to 688 rapidly, whereas the TGS of
hydrogels increased from 688 to 738 slowly as the MGCs increased from 3.5 to 4.5 wt-%.
The higher stability of hydrogels at higher gelation concentration indicates that self-
assembly in the gel state is driven by strong intermolecular, noncovalent interactions
[5b].
3. Field Emission Scanning Electron Microscopy (FE-SEM) and Polarized Optical
Microscope (POM). To obtain visual insights into the aggregation mode of these
gelators in hydrogels, we took POM and FE-SEM images of the hydrogel. Fig. 2 shows
the FE-SEM and POM images of supramolecular hydrogels formed by NP18PB. As

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Scheme. Synthesis of NP18PB
Fig. 1. Variation of the TGS with the concentration of NP18PB

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shown in Fig. 2,a, NP18PB can self-assemble in deionized H2O and form nanofibers
with a diameter of ca. 30 – 50 nm (Fig. 2,a). Furthermore, typical fiber-like crystallites
Fig. 2. a) Field Emission Scanning Electron Micrographs (FE-SEM) of dried gels formed by 3.0 wt-% of
gelator in deionized H2O, magnified by 8000. b) Polarizing optical micrographs (POM) of hydrogels
formed by 4.0 wt-% of gelator in deionized H2O, magnified by 400.

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with a diameter of ca. 30 mm were observed as shown in Fig. 2,b. These results indicate
that the hydrogelator can self-assemble into fiber-like aggregates and H2O molecules
are entrapped in the three-dimensional network by capillary forces, which lead to the
formation of stable hydrogels.
4. 1H-NMR Spectra. To obtain information on the intermolecular H-bonding
1
interaction between the amide groups, the H-NMR study was performed by using
1.5 wt-% NP18PB in (D6)DMSO with an increasing amount of H2O. As shown in
Fig. 3, the signal of amide NHa moved downfield from 7.24 to 7.42 ppm as the H2O
content increased up to 30%, and then shifted upfield from 7.42 to 7.34 ppm as the water
content increased further to 40%. A similar phenomenon was observed for NHb, the
signal of which moved downfield from 8.00 to 8.26 ppm as the H2O content increased
up to 20%, and then shifted upfield from 8.26 to 7.95 ppm as the H2O content increased
to 40%. The changes in the chemical shifts of the amide H-atoms to lower fields for up
to a 30% H2O content and then upfield over 30% reveal that the H-bonding with
(D6)DMSO (S¼O ··· HꢀN) replaces that with H2O (H2O ··· HꢀN), and then with
intermolecular H-bonding between the amide groups. On the other hand, the chemical
shift of the pyridinium Hc H-atoms shifted upfield (from 8.30 to 8.04 ppm) at 0 to 30%
H2O content, and then to lower field (from 8.04 to 8.14 ppm) at 40% H2O. It indicated
that the hydration of the charged pyridinium residues leads to the upfield shift when the
H2O content is below 30% and the dehydration of the charged pyridinium leads to the
downfield shift if the H2O content is higher than 30% [11]. Consequently, the
Fig. 3. 1H-NMR Spectra of NP18PB in (D6)DMSO with increasing H2O content

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intermolecular H-bonding interaction is one of the main driving forces for the self-
assembly of NP18PB molecules.
5. Fluorescence Study. To further characterize the hydrophobic interaction, the
luminescence spectra were studied with 8-anilino-1-naphthalenesulfonic acid (ANS) as
a probe. Fig. 4,a shows the typical luminescence spectra of ANS in aqueous solutions
containing various concentrations of NP18PB, and Fig. 4,b, shows the dependence of
the luminescence maxima (lmax) and the relative luminescence intensities (I/I0, where
I0 and I represent the luminescence intensities of ANS at lmax in the absence and in the
presence of NP18PB, resp.) on the concentration of NP18PB. At concentrations of
Fig. 4. a) Luminescence spectra of ANS (1.0 ꢁ 10ꢀ5 m) in aqueous solutions containing various
concentrations of NP18PB at room temperature. Inset: enlargement of the low concentration range. b)
&
*
Dependence of the luminescence maxima (lmax
; ) and relative luminescence intensities (I/I0; ) on the
concentration of NP18PB. [NP18PB] (ꢁ100 wt-%): a: 0; b: 0.5; c: 1.0; d: 1.2; e: 1.4; f: 1.6; g: 1.8; h: 2.0.

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NP18PB lower than 1.2 wt-%, NP18PB is dispersed in the solution. The exciplex
through the interaction between NP18PB molecules is hardly formed resulting in a
lower fluorescence intensity (Fig. 4,a). At a concentration of NP18PB at 1.2 wt-% or
higher, the lmax is blue-shifted from 535 nm to 468 nm with increasing NP18PB
concentration. Further addition increases the relative luminescence intensity
(Fig. 4,b), but produces only a slight change in the lmax value (Fig. 4,b). Such
luminescence behavior is frequently observed when the ANS molecules are incorpo-
rated into a hydrophobic environment; namely, the interior of the strands of self-
assembled nanofibers is hydrophobic [12]. Therefore, this result indicated that one of
the driving forces for the self-assembly of NP18PB into nanofibers is a hydrophobic
interaction, which is in agreement with a previous report for amino acid based
hydrogelators [13].
6. Circular Dichroism (CD). Circular dichroism (CD) spectra of NP18PB are
shown in Fig. 5. A negative Cotton effect was observed in the amide absorption region,
i.e., at 220 – 225 nm, that could be attributed to the p ! p transition of the amide bond
along with a shoulder at longer wavelength from the n ! p transition of the same bond
[14]. These transitions are extremely sensitive to coupling with neighboring amides.
The observed supramolecular chirality that emerged through noncovalent organized
packing of molecular components was supported by a variable temperature CD study
of NP18PB at 1.2 wt-%. The intensity of the CD peak at 220 – 225 nm gradually
decreased with an increase in temperature from 258 to 808 as a result of the gel-to-
solution state transition, leading to the destruction of the three-dimensional aggregate
structure (Fig. 2,a). The results indicated that the chirality of the gelator is responsible
for the formation of the self-assembled aggregates and supramolecular hydrogel.
Fig. 5. CD Spectra of NP18PB at 1.2 wt-% in plain H2O with varying temperature from 208 to 808,
hydrogels (solid line) at 258; loose hydrogels (dashed line) at 608; homogeneous solution (dotted line) at
808.

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7. X-Ray Diffraction (XRD) Measurements. The xerogel of NP18PB obtained from
H2O by a freezing method resulted in spongelike aggregates, instead of a typical
crystalline solid. The X-ray diffraction patterns of the xerogel of NP18PB prepared
from H2O show periodical reflection peaks (Fig. 6), an indication that NP18PB indeed
assembles into a lamellar organization. The obtained long spacings of the xerogel of
NP18PB are 2.90 nm, 1.46 nm and 0.97 nm, corresponding to the ratio of 1:1/2 :1/3 :1.
The 2.90 nm length is smaller than twice that of the extended molecular length of 1
(2.45 nm, by CPK molecular modeling), but larger than the length of one molecule.
Thus, the aqueous gel of NP18PB should maintain an interdigitated bilayer structure
with a thickness of 2.90 nm (Fig. 6). Long spacing of 2.90 nm was also observed for the
aqueous gel state of NP18PB at high concentration. In addition, the wide-angle region
of the X-ray diagram for the aqueous gel of NP18PB revealed a series of sharp
reflection peaks, supporting the view that presumably long alkyl chain groups form
highly ordered layer packing through the interdigitated hydrophobic interaction [9].
Thus, a rational model of the energy-minimized structure of NP18PB self-assembled in
water is shown in Fig. 7.
Fig. 6. XRD Spectra of the gelator xerogel from hydrogel (6.0 wt-%)
Conclusions. – We have synthesized a hydrogelator based on l-phenylalanine with a
long hydrophobic chain and a positive charged terminus, i.e., a pyridinium group. After
dissolution of the hydrogelator in H2O, it can gelate H2O, and form a transparent
hydrogel. 1H-NMR, POM, FE-SEM, CD, XRD, and fluorescence studies demonstrated
that the formation of the hydrogels by this hydrogelator involves two self-assembling
processes, i.e., the formation of some aggregates and their growth into nanofibers.
Moreover, the driving forces for the self-assemblies are the hydrophobic interactions
between the alkyl groups and H-bonding between the amide groups.

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Fig. 7. Model of the energy-minimized structure of hydrogelator self-assembled in water
Experimental Part
General. l-Phenylalanine, bis(trichloromethy1) carbonate (BTC), N-octadecylamine, 6-bromohex-
anoyl chloride, and 8-anilinonaphthalene-1-sulfonic acid (ANS) were purchased from Aldrich and used
as received. THF was dried over Na and distilled before use. Pyridine was distilled over NaOH, and
CH2Cl2 was distilled over P4O10. Other solvents were distilled before use.
Synthesis of the Hydrogelator NP18PB (Scheme). (4S)-4-Benzyl-1,3-oxazolidine-2,5-dione (l-Phe-
NCA). The starting material (4S)-4-benzyl-1,3-oxazolidine-2,5-dione (l-Phe-NCA) was synthesized
according to a reported procedure [9]. M.p. 95 – 968 (dec.). FT-IR (KBr): 3285 (NꢀH, amide A), 1853,
1
1760 (C¼O, ester), 1634 (C¼O, amide I), 1554 (NꢀH, amide II). H-NMR (400 MHz, CDCl3): 7.46 –
7.21 (m, 5 arom. H); 6.42 (s, 1 NH); 4.55 – 4.32 (m, 1 HꢀC); 3.36 – 2.88 (m, CH2ꢀAr). Anal. calc. for
C10H9NO3 (191.19): C 62.83, H 4.75, N 7.32; found: C 62.73, H 4.73, N 7.34.
N-Octadecyl-l-phenylalaninamide (C18Phe). Octadecylamine (3.57 g, 13.3 mmol) and l-Phe-NCA
(2.54 g, 13.3 mmol) were dissolved in 50 ml of CH2Cl2 and cooled in an ice bath. The mixture was stirred
for 1 h and then at 408 for 72 h. The product N-octadecyl-l-phenylalaninamide (C18Phe) was precipitated
from Et2O. FT-IR (KBr): 3284 (NꢀH, amide A), 1634 (C¼O, amide I), 1554 (NꢀH, amide II).
1H-NMR (400 MHz, CDCl3): 8.24 (s, NH2); 8.20 (d, J ¼ 7.6, NH2 or NH); 7.33 – 7.19 (m, 5 arom. H (Ph));
3.88 – 3.69 (m, PhCH2CH); 3.29 – 3.16 (m, CH2CH2NH); 3.13 – 2.90 (m, PhCH2); 2.20 – 2.13 (m,
CH2Me); 1.31 – 1.16 (m, 30 H, CH2(CH2)15CH2Me); 0.84 (t, J ¼ 6.8, CH2CH2Me). Anal. calc. for
C27H48N2O (416.69): C 77.83, H 11.61, N 6.72; found: C 77.74, H 11.64, N 6.76.
Na-(6-Bromohexanoyl)-N-octadecyl-l-phenylalaninamide (NP18). After drying in vacuum, C18Phe
(4.87 g, 11.3 mmol) and 4.68 ml of Et3N were added to 300 ml of anh. THF. 6-Bromohexanoyl chloride
(2.66 g, 12.5 mmol) was added dropwise to the mixture at 08. The reaction was carried out at 08 for 1 h
and then at r.t. for 4 h. After evaporating THF, Na-(6-bromohexanoyl)-N-octadecyl-l-phenylalanin-
amide (NP18) was obtained and recrystallized from EtOH, and dried in vacuum. FT-IR (KBr): 3284
1
(NꢀH, amide A), 1634 (C¼O, amide I), 1554 (NꢀH, amide II). H-NMR (400 MHz, CDCl3, TMS):
7.48 (d, J ¼ 7.8, NH); 7.35 – 7.17 (m, 5 arom. H (Ph)); 6.60 (t, J ¼ 5.6, NH); 3.4 (t, J ¼ 4.4, CH2Br); 3.28 –
3.15 (m, CH2CH2NH); 3.14 – 3.06 (m, PhCH2CH); 3.03 – 2.88 (m, PhCH2); 2.57 – 2.48 (m, NHCOCH2);

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2.20 – 2.13 (m, CH2CH2Me); 1.30 – 1.25 (m, CH2(CH2)15CH2Me, CH2(CH2)3CH2Br); 0.88 (t, J ¼ 6.8, Me).
Anal. calc. for C33H57BrN2O2 (593.73): C 66.76, H 9.68, Br 13.46, N 4.72; found: C 66.87, H 9.65, Br 13.41,
N 4.70.
N-Octadecyl-Na-[6-(pyridinium-1-yl)hexanoyl]-l-phenylalaninamide Bromide (NP18PB). NP18
(3.5 g, 5.5 mmol) was refluxed in 50 ml of pyridine under N2 atmosphere overnight. The mixture was
evaporated and recrystallized from a mixture of EtOH and acetone. The final product N-octadecyl-Na-
[6-(pyridinium-1-yl)hexanoyl]-l-phenylalaninamide bromide (NP18PB) was obtained. FT-IR: 3284
(NꢀH, amide A), 1634 (C¼O, amide I), 1546 (NꢀH, amide II). 1H-NMR (400 MHz, CDCl3): 9.54 (d,
J ¼ 5.6, 2 HꢀC(2Py)); 8.52 (t, J ¼ 7.8, HꢀC(4Py)); 8.31 (t, J ¼ 7.4, 2 HꢀC(3Py)); 7.73 (d, J ¼ 7.4, NH); 7.35 –
7.17 (m, 5 arom. H (Ph)); 6.76 (t, J ¼ 5.5, NH); 3.27 – 3.13 (m, PhCH2); 3.18 – 3.16 (m, CH2CH2NH);
3.03 – 2.88 (m, PhCH2CH); 2.23 – 2.03 (m, CH2CH2NþBrꢀ); 2.2 – 2.13 (m, CH2Me); 1.31 – 1.25 (m,
CH2(CH2)15CH2Me, CH2(CH2)3CH2Nþ); 0.88 (t, J ¼ 6.8, CH2Me). Anal. calc. for C38H62BrN3O2 (672.83):
C 67.84, H 9.29, Br 11.87, N 6.25; found: C 67.96, H 9.23, Br 11.84, N 6.23.
FT-IR Spectra: Fourier transform infrared (FT-IR) spectra were recorded on EQUINOX55
spectrophotometer (KBr pellets). 1H-NMR Spectra: solns. of NP18PB (1.5 wt-%) were prepared in
(D6)DMSO containing various ratios of H2O, namely, 0, 10, 20, 30, and 40%.
Polarized Optical Microscope. The optical analysis of the hydrogels was performed by using a
polarized optical microscope (POM, BH-2). The soln. containing 2 wt-% of NP18PB was dropped on a
pre-warmed glass plate and allowed to cool to r.t. The hydrogel samples were kept at r.t. for 4 – 6 h before
testing.
Field Emission Scanning Electron Microscopy (FE-SEM, Sirion 200, FEI). The gel samples were
frozen by liquid N2 and then freeze-dried. The fractured specimens were coated with Au. The electric
current was 15 mA, and the accelerating voltage was 5 kV.
Fluorescence Spectroscopy. The steady-state fluorescence spectra were recorded with an FP-6500
spectrofluorometer. Fluorescence spectra were measured at [ANS] ¼ 1.0 ꢁ 10ꢀ5 m and [gelator] ¼ 0 –
2.0 wt-% in a fluorescence spectroscopic cell (1 cm ꢁ 1 cm). The excitation wavelength was 360 nm,
corresponding to the absorption maximum.
Circular Dichroism (CD). The CD spectra of an aq. soln. of NP18PB at 1.2 wt-% with varying temp.
from 208 to 808 were recorded by using a quartz cuvette of 1 mm path length with a Jasco
spectropolarimeter.
X-Ray Diffraction (XRD). XRD measurements were taken with an XꢀPert PRO diffractometer, and
the source was CuKa radiation (a ¼ 0.15406 nm) with a voltage and current of 40 kV and 30 mA, resp.
Hydrogels of NP18PB were mounted on the aluminum holder and scanned from 2 to 408.
Gelation and Minimum Gelator Concentrations (MGC). The weighed hydrogelator NP18PB was
mixed with 3 ml of H2O in a test tube, and the mixture was heated until the solid was completely
dissolved. The soln. was allowed to cool to r.t. for 12 h and exhibited no gravitational flow upon inversion
of the test tube. A required minimum amount of NP18PB gelation is defined as minimum gelation
concentration (MGC) [10].
The Phase Dissociation Temp. (TGS). A small steel ball (250 mg, ˘ 4 mm) was placed on top of the
supramolecular hydrogel in a test tube (˘ 10 mm). Then the sample was slowly heated (28/min) in a
thermostatted water bath. When the ball fell to the bottom of the test tube, the temp. was defined as the
phase dissociation temp. (TGS) of the supramolecular hydrogels [15].
The work was financially supported by the National Natural Science Foundation of China (No.
20474022) and Scientific Research Program of the Education Department of Henan Province, China (No.
2008B430014).
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Received April 22, 2009