Supramolecular Gels from Conjugates of Bile Acids and Amino Acids and Their Applications

Article abstract of DOI:10.1002/ejoc.201601616

Hydrogels composed of low molecular weight molecules are gaining increasing importance in biomedical applications. In this work, we report the formation of injectable hydrogels from bile acid–amino acid conjugates. The hydrogelation ability was dependent

Full text of DOI:10.1002/ejoc.201601616

A Journal of
Accepted Article
Title: Supramolecular gels from Bile acid-Amino acid conjugates and
their Applications
Authors: Uday Maitra and Mitasree Maity
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601616
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10.1002/ejoc.201601616
European Journal of Organic Chemistry
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Supramolecular gels from Bile acid-Amino acid conjugates and
their Applications
Mitasree Maity,[a] and Uday Maitra*[a]
explore the potential of hydrogelation of these molecules. We
synthesized a series of L and D amino acid conjugates of
lithocholic (LC) and deoxycholic (DC) acids (Figure 1) for a
systematic study of their structural properties, details of gel
formation and potential applications.
Abstract: Hydrogels from low molecular weight molecules are
gaining increasing importance in biomedical applications. In this
work, we report the formation of injectable hydrogels from bile acid-
amino acid conjugates. The hydrogelation ability was dependent on
multiple factors such as the bile acid backbone, linkage between the
bile acid and the amino acid, pH etc. We have shown that the
gelation properties depend on multiple non-covalent interactions and
the balance between hydrophobic and hydrophilic characters of the
molecules. Crystal structures of lithocholyl-Lphenylalanine,
lithocholyl-glycine, lithocholyl-Lvaline and lithocholyl-Lalanine were
also determined and compared. Finally, the rigid hydrogel
frameworks were utilized to produce inorganic-organic composite
materials with gold and ZnO nanoparticles.
Introduction
Small molecule derived supramolecular gels result from various
non covalent intermolecular interactions such as hydrophobic, π-
π stacking, van der Waals forces, hydrogen bonding and
aromatic interactions etc.1 Among many classes of molecules,
supramolecular gels derived from steroidal molecules have been
extensively studied.2,3 Bile acids, a special class of steroids, are
facially amphiphilic molecules with a unique molecular structure
of a polyhydroxylated tetracyclic steroid nucleus with an easily
modifiable carboxylic group attached to the side chain (Figure 1).
A number of bile acid conjugates are known in the literature with
applications in supramolecular chemistry, materials and
pharmacology.4 Sudhölter and co-workers showed that bile acid-
amino acid ester conjugates formed organogels in various
solvents.5 Incerti et al. revealed that amino acid conjugates of
lithocholic acid were antagonists of the EphA2 receptor,6 but
their aggregation or gelation properties were not reported. An
impressive number of N-terminal protected amino acids and
peptides have been shown to be hydrogelators with diverse
applications.7 Among these, the single amino acid derivatives
are typically protected at the N-terminus with a large aromatic
group, such as Fmoc, naphthalene, cinnamoyl, anthracene,
carbazole or pyrene, but some of these aryl groups have their
own cytotoxic properties.8
Figure 1. Bile acid-amino acid conjugates
Results and Discussion
Synthesis of bile acid-amino acid conjugates
Syntheses of bile acid-amino acid conjugates were carried out
as shown in scheme 1. Lithocholic acid and deoxycholic acid
were coupled with the methyl ester hydrochloride of the α-amino
acid using ethyl chloroformate (45-94% yield). The resulting
amides 1a−11a were hydrolyzed with NaOH to produce
compounds 1−11 with free carboxylic acid side chain (78-98%
yield).
Detailed
synthetic
procedures
and
structural
characterization data are provided in the experimental section.
We have recently shown bile acid-dipeptide conjugates as
efficient hydrogelators9 where the nature and position of the
amino acid residues in the dipeptides played important roles in
gelation. These results, and the recently published6 interesting
biological properties of single amino acids protected at the N-
terminal with a bile acid backbone prompted us to further
Scheme 1. Synthesis of bile acid-amino acid conjugates [Reagents: (a) ethyl
chloroformate, triethylamine, DMF, 1,4-Dioxane; (b) NaOH (H2O)/MeOH]
[a]
Department of Organic Chemistry
Indian Institute of Science, Bangalore, 560012
Karnataka, India
E-mail: maitra@orgchem.iisc.ernet.in
http://orgchem.iisc.ernet.in/faculty/um/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201601616
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Gelation studies
deoxycholic acid derivative DC-LF-OH (11) was synthesized
which has an additional hydroxyl group on the bile acid
backbone. Compound 11 did not form gel under identical
conditions due to its higher solubility (Table 1). These results
suggest that an appropriate balance of hydrophobic and
hydrogen bonding interactions in the gelator molecules is
Hydrogel formation was significantly dependent on the gelator
molecules, solvent, pH, temperature and other variables.10 ,11
Phenylalanine is a hydrophobic amino acid and covalently
attaching it with a bile acid moiety through an amide linkage
further enhances the hydrophobicity. The resulting molecule was
not soluble in phosphate buffer at pH 8 but formed a clear
solution on heating to 95 °C, which on sonication for a few
seconds led to the formation of a translucent hydrogel as shown
in Figure 2. The minimum gelation concentration of LC-LF-OH
(1) was 0.25% w/v at pH 8, indicating that the gelator molecule
was very efficient in forming hydrogels.
To identify the role of the bile acid, we tested gel formation of
tert-butyloxycarbonyl-L-phenylalanine (Boc-LF) under similar
gelation conditions. Boc-LF did not form gel, which indicates the
importance of LC in self-assembly and gelation. In addition,
lithocholic acid itself was not able to form gel under the tested
important for effective gelation by these compounds.
9
It was noteworthy to mention that LC-LA-OH (9) formed gel by
careful heating to 120 °C oil bath followed by sonication. In
addition, other bile acid-amino acid conjugate crystals
(crystallized from ethanol/water mixture) required higher
temperature to form gel, whereas the amorphous forms (as
synthesized and isolated by chromatography) formed gels under
standard (95 °C) conditions. This is likely to be related to the
slower dissolution rates of the crystalline forms.
Table 1. Gelation studies
condition. We also confirmed that
a physical mixture of
Phosphate buffer
Aqueous
NaOH
(pH 12)
G
TG
TG
TG
TG
TG
TG
MGC
(wt.-%)
lithocholic acid and phenylalanine did not produce a gel. This
further suggested that covalent linkage between bile acid and
phenylalanine was critical for gel formation.
Compound
(pH 6)
(pH 7)
TG
TG
TG
TG
TG
TG
TG
TG
P
(pH 8)
TG
TG
TG
TG
TG
TG
TG
TG
P
LC-LF-OH (1)
LC-DF-OH (2)
LC-LW-OH (3)
LC-DW-OH (4)
LC-LY-OH (5)
LC-DY-OH (6)
LC-LL-OH (7)
LC-LV-OH (8)
LC-LA-OH (9)
LC-G-OH (10)
DC-LF-OH (11)
P
P
P
P
P
P
P
P
P
P
S
0.25
0.16
0.25
1
0.5
1
1
1
-
TG
P
TG
S
TG
S
TG
S
0.5
-
G: transparent gel, TG: translucent gel, P: precipitate or suspension, S: clear
solution; Buffer concentration - 100 mM; *MGC at phosphate buffer pH 8;
**Compounds formed gels after heating at 95 °C followed by sonication;
***Compound 9 formed gel only after heating at 120 °C followed by sonication
Figure 2. Structure of the gelator LC-LF-OH (1) and photograph of hydrogel at
pH 8 (1 wt.-%)
We tested the effect of pH on the gelation of LC-LF-OH (1) within
the buffering range of phosphate buffer (5.8 to 8.0). Interestingly,
LC-LF-OH (1) did not dissolve in phosphate buffer at pH 6 even
after heating, but formed a milky solution and no gel formation
was observed after sonication. Further testing showed that 1
formed gels at pH 7, 7.5 and 8 but no gel formation was
observed at pH 5.8, 6 and 6.5 (phosphate buffer). Additionally,
LC-LF-OH (1) formed a hydrogel in the presence of NaOH (Table
1).
To verify whether the gel formation was specific to LC-LF-OH (1),
other aliphatic (Leu, Val, Ala and Gly) and aromatic (Trp and
Tyr) amino acid derivatives were prepared and their gelation
properties were tested under identical conditions. Surprisingly,
all the amino acid conjugates of lithocholic acid except LC-LA-
OH (9) formed gels at pH 8 (Table 1). Keeping in mind the
12
, 13
potential antimicrobial activity of D-amino acids,
diastereomeric LC derivatives of (D) aromatic amino acids were
also tested. No major variation in gelation was observed with the
change in chirality (except morphology) on the amino acid,
suggesting that hydrogel formation was not specific to D or L
configuration.14 Other LC-amino acid derivatives (2-10) showed
similar gelation behaviour as 1 at different pH (Table 1). To
check the effect of the bile acid backbone on gelation properties,
Figure 3. SEM images of LC-amino acid derivatives at pH 8 (a) 1, (b) 2, (c) 3,
(d) 4, (e) 5, (f) 6, (g) 7, (h) 8 and (i) 10
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Characterization of the gels by Scanning Electron
Microscopy (SEM) and Atomic Force Microscopy (AFM)
transformation of the hydrogel into a sol on the application of
high stress and immediate gel formation (G' > G") after the
removal of shear stress. Rheological studies of LC-LF-OH (1) gel
in different concentrations and LC-G-OH (10) gel also showed
shear thinning properties under identical conditions (supporting
information).
Scanning electron microscopic images of the xerogels provided
evidence of the three dimensional organization present in the
gels. Depending on the gelator molecules, the SEM images
showed different types of entangled network morphology (Figure
3). Gelators LC-LW-OH (3) and LC-LY-OH (5) showed
aggregated spherical structures fused with each other whereas
other gelators showed fiber like morphology. The diameter of gel
fibers was 20–100 nm depending on the derivatives. AFM
images showed morphologies similar to those observed by SEM
for the same gel (supporting information).
Rheological studies of the hydrogels
Rheology is the study of flow properties of matter, and is
important for practical applications of soft materials.15,16 Gels are
known as viscoelastic soft solids and can both store and
dissipate energy.17 Frequency sweep (fixed stress of 0.5 Pa)
and stress sweep (fixed frequency of 0.5 Hz) on the LC-LF-OH
(1) derived hydrogel (1 wt.-%, at pH 8) showed storage modulus
(G') value of 2290 Pa, and yield stress (σ*) value of 147 Pa
(Figure 4). In contrast the diasteriomeric LC-DF-OH (2) derived
hydrogel was much weaker mechanically (G' of 364 Pa, σ* of 28
Pa, Figure 4). Rheological experiments of LC-G-OH (10)
hydrogel also showed typical soft gel behavior (supporting
information).
Figure 5. (a) Shear thinning property of LC-LF-OH (1) gel at pH 8 and (b) time
sweep of LC-LF-OH (1) gel at pH 8 (1 wt.-%, pH 8) at two different oscillatory
stresses of 0.5 Pa and 200 Pa
Single crystal X-ray diffraction studies
The subject of molecular organization in a gel fiber, and that in
the crystalline state have been the subject of considerable
discussion.20 We felt that intermolecular interactions observed in
the crystalline state should provide important insight into the
possible molecular packing of molecules in the nanofibers.21
Single crystals of LC-LF-OH (1), LC-LV-OH (8) and LC-LA-OH (9)
were obtained from EtOH/H2O and that of LC-G-OH (10) was
obtained from MeOH/H2O and their crystal structures were
determined (Figure 6, supporting information). It was observed
that the steroidal ring systems, being rigid have very similar
conformations in all structures whereas the conformations of the
side chains were different. Conformation of the bile acid side
chain can be described based on the combination of the dihedral
angles C13–C17–C20–C22, C17–C20–C22–C23, C20–C22–
C23–C24 and C22–C23–C24–N (Table 2, for assignments of
the steroidal directions see Nakano et al.22 and Miyata et al.23).
When compared with other crystal structures, the side chain
conformation of LC-LA-OH (9) showed a nearly orthogonal turn
(87° between the planes through C10-C13-C20 and C23-C24-
C25) on the lipophilic back-side of the steroidal skeleton (Figure
7). Similar structures were previously observed for N-(2-
hydroxyethyl)- and N-(3-hydroxypropyl)-ursodeoxy-cholamides.24
The packing of the molecules in the crystals are shown in Figure
8. All the crystals of the bile acid-amino acid conjugates showed
bilayered assemblies in head-to-tail pattern, which is typical for
bile acids.25 As shown in Figure 8, the hydrogen bonding donors
Figure 4. Rheology study of LC-LF-OH (1) and LC-DF-OH (2) hydrogels (1 wt.-
%, pH 8): (a) frequency sweep and (b) stress sweep
Interestingly, these amino acid based gels exhibited rapid
recovery of their mechanical properties after a large-amplitude
oscillatory breakdown, known as shear thinning behaviour.18 The
rapid recovery of the hydrogel upon mechanical agitation makes
the hydrogel injectable. As shown in Figure 5a, the hydrogel of 1
can be easily pushed through a syringe with a 21 gauge needle
and the resulting fluid quickly reformed a gel after injection. This
property of the hydrogel is useful for potential application in a
drug delivery system.19 Rheological studies on the gel of LC-LF-
OH (1) at pH 8 were done under the application and release of
shear stress in a cyclic manner. A shear stress of 200 Pa was
applied for 5 minutes to transform the gel into the corresponding
sol state, and then the recovery of the gel from the sol state was
studied at 0.5 Pa stress for 10 minutes. Figure 5b shows the
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10.1002/ejoc.201601616
European Journal of Organic Chemistry
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and acceptors form intermolecular hydrogen bonds (cyan dotted
bond) with neighbouring molecules. Such interactions might play
an important role in the self-assembly process to produce the
hydrogels.21c
Table 2. Dihedral angles of the side chain
LC-LF-OH
(1)
LC-LV-OH
LC-G-OH
(10)
LC-LA-OH
(9)
Dihedral angles (°)
(8)
C13-C17-C20-C22
C17-C20-C22-C23
C20-C22-C23-C24
C22-C23-C24-N
-173.28
-166.95
-171.84
-90.43
-173.25
-166.31
-173.22
-86.47
171.58
-169.59
159.39
132.25
175.86
-174.99
56.56
-128.68
X-ray powder diffraction studies
In order to investigate the molecular packing of compounds LC-
LF-OH (1) and LC-G-OH (10) in the xerogel, the 1% (w/v) gel
samples were prepared and lyophilized. A comparison of the
simulated XRD pattern of the single crystal with the pXRD
pattern obtained from the xerogels (Figure 9) showed significant
differences between crystal and xerogel, indicating that the
molecular arrangements in the gel were not similar to those of
the crystals. 26 The difference in pXRD could be, in most
likelihood, due to the participation of water molecules in the gel
network as water being good hydrogen bond donor as well as an
acceptor. The xerogels, however, showed multiple sharp peaks
indicating considerable ordered structures in the fibers.
Figure 6. Crystal structures of LC-Amino acid conjugates: (a) LC-LF-OH (1),
(b) LC-LV-OH (8), (c) LC-LA-OH (9) and (d) LC-G-OH (10)
Figure 7. Overlay of crystal structures: (a) LC-LV-OH (8) with LC-G-OH (10)
and (b) LC-G-OH (10) with LC-LA-OH (9)
Figure 9. pXRD of crystal and xerogel of (a) LC-LF-OH (1) and (b) LC-G-OH
(10)
Synthesis of hydrogel-nanocomposites
Gel-nanocomposites have received increasing attention in
recent years owing to their tuneable and stimuli responsive
properties. 27 There are several reports for preparations of
inorganic nanoparticles such as silica, TiO2, CdS nanoparticles
etc. within a gel matrix.28 Although there are different ways to
fabricate nanoparticles in the gel fibers,27a,29 we chose to design
an in situ27d,30 synthesis of ZnO and gold nanoparticles in the gel
matrix.
(A) Synthesis of ZnO nanoparticles
Zinc oxide (ZnO) is an environment friendly, wide band-gap
semiconductor (energy gap is 3.37 eV at room temperature) and
it has a large exciton binding energy (60 meV).31 It has been
studied widely for electronic and optical devices such as light
Figure 8. Crystal packing of LC-Amino acid conjugates of (a) LC-LF-OH (1),
(b) LC-LV-OH (8), (c) LC-LA-OH (9) and (d) LC-G-OH (10)
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emitting diodes, solar cells, transducers and photodetectors,32
photo-catalysts, functional devices, cosmetics, gas sensors,
industrial additives33 etc., and for antimicrobial activity.34
We have used a very mild condition for the preparation of ZnO
NP. Since the gel was formed in the presence of NaOH at pH 12
(Table 1), we have taken advantage of this property for the in
situ formation of ZnO nanoparticles in LC-LF-OH (1) and LC-DF-
OH (2) gels. The ZnO nanoparticles in the LC-L/DF-OH gel were
synthesized by adding zinc acetate with the gelator molecule in
the presence of NaOH, and then the solution was heated at
95 °C for 5 minutes followed by sonication. After a few minutes
at room temperature, ZnO hydrogel nanocomposite was formed.
The absorption spectra of the composite gels showed peaks
around 360-365 nm (Figure 10) which are characteristic for the
35
band-edge absorbance of crystalline ZnO NP.
The
nanoparticles showed weak luminescence (Figure 11) with a
broad band centred at 620 nm (λex = 365 nm) due to surface
defect from the ZnO core36 (the sharp peak around 530 nm is
probably from the Raman scattering of the gel, as this peak was
also observed in the native gel, supporting information).
Figure 12. TEM images of (a) LC-DF-OH gel in NaOH, (b-f) ZnO NP doped
LC-DF-OH gel, (g-h) HRTEM images of ZnO NP and (i) SAED pattern of ZnO
NP
Figure 10. UV-Visible absorption spectra of ZnO NP in (a) LC-LF-OH (1) and
(b) LC-DF-OH (2) gel
Figure 11. Fluorescence spectra of ZnO NP in (a) LC-LF-OH (1) and (b) LC-
DF-OH (2) gel (λex = 365 nm)
Figure 13. (a) Synthesis of AuNP in LC-LW-OH gel and (b) UV-Visible
absorption spectrum of AuNP in LC-LW-OH gel
TEM studies were carried out to observe the size and shapes of
the ZnO NP synthesized in the LC-DF-OH gel matrix. The TEM
images showed spherical ZnO NP arranged on the fibers (Figure
12), with the average size of the nanoparticles being about 6 nm.
HRTEM showed d-spacing of 0.19 nm, corresponding to the
(102) lattice fringe of ZnO. However, crystallinity was clearly
poor as observed in the SAED pattern (Figure 12i).
(B) Synthesis of AuNP
Gold nanoparticles have numerous established applications in
chemistry and biology.37 It was therefore of interest to use the
gel nanofibers for in situ AuNP synthesis. For gold nanoparticle
synthesis, we chose the tryptophan derived38 (LC-LW-OH, 3) gel
prepared in phosphate buffer (pH 8). Aqueous NaBH3CN was
allowed to slowly diffuse through an Au(III) doped LC-LW-OH gel
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European Journal of Organic Chemistry
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measurements Shimadzu spectrometer was used and FL spectra were
recorded on Edinburgh FLS 980 with 455 nm filter. The gel was heated to
95 °C for 2-3 min to convert it to a sol and transferred into a cuvette for
absorbance and fluorescence measurements where the gel reformed.
Dynamic rheological measurements were performed on AR 1000
rheometer (TA instruments) using parallel-plate geometry. The gels were
scooped up, placed over the rheometer plate, and allowed to equilibrate
under experimental condition for 1 h.
(Figure 13a). The gel turned wine red, and the normalized
absorption spectrum showed a characteristic surface plasmon
resonance peak around 530 nm (Figure 13b, supporting
information). TEM images of AuNP doped LC-LW-OH hybrid gel
showed the formation of uniform gold nanoparticles of average
size of 10-15 nm (Figure 14). Clearly, this is an easy and
efficient way for the in situ formation and stabilization of Au NPs
in a hydrogel.
Synthesis: Lithocholic acid/deoxycholic acid (0.65 mmol) was taken in
an oven dried round bottom flask (50 mL) and dissolved in 1,4-dioxane (5
mL) at 10 °C. Then Et3N (0.14 mL, 0.98 mmol) was added to it followed
by ethyl chloroformate (0.093 mL, 0.98 mmol) and stirred at room
temperature for 30 min. Amino acid methyl ester hydrochloride (0.98
mmol) was taken in another round bottom flask and dissolved in DMF (2
mL) at 0 °C. Et3N (0.14 mL, 0.98 mmol) was added to it and stirred at
room temperature for 30 min. The free amino acid methyl ester in DMF
was directly added to the freshly prepared mixed anhydride. Stirring was
continued for 12-24 h. After completion of the reaction (monitored by
TLC), the reaction mixture was diluted with ethyl acetate (50 mL), the
organic layer was washed with water (100 mL), and then with brine (50
mL). After removing the volatiles, the crude products were purified by
column chromatography on silica gel, using ethyl acetate/petroleum ether
as the eluent, which yielded products (1a-11a) typically as white solids.
Compounds (1a-11a) were dissolved in MeOH, and then aqueous NaOH
(2M, 1.5 eq) was added slowly and the mixtures were stirred at room
temperature. After completion of the reaction, MeOH was evaporated.
The reaction mixture was diluted with water and acidified with HCl (1M)
drop wise to pH 2. The precipitate formed was filtered and washed with
water. The crude products were purified by column chromatography on
silica gel, using MeOH/ CHCl3 as the eluent, which yielded solid products
(1-11).
Figure 14. TEM images of (a-d) AuNP doped LC-LW-OH hybrid gel, (e)
HRTEM images of AuNP and (f) SAED pattern of AuNP
Conclusions
Bile acid-amino acid conjugates as low molecular weight
gelators (LMWG) are easier to synthesize on a large scale and
are stable compounds, giving them some advantages over other
peptide and dendrimer derived LMWG. Our studies indicated
that the hydrophobicity of the bile acid unit as well as the amino
acid residues played crucial roles in gelation. SEM and AFM
images of the xerogels showed fibrous or spherical morphology
of the aggregates. Molecular arrangements of the gelators in the
crystalline state were studied by single crystal x-ray diffraction,
and were found to be different in the xerogels. We believe that
the injectable properties of the hydrogel, and the ease of in situ
formation of ZnO/Au nanoparticles in the hydrogels would
encourage further research in such systems leading eventually
to practical applications.
Methyl N-(3α-hydroxy-5β-cholan-24-oyl)-L-phenylalaninate (LC-LF-
OMe, 1a)
Yield- 48%, m.p. 185-186 °C (lit. 185-187 °C)
6
1H NMR (CDCl3, 400 MHz) δ: 7.29-7.23 (m, 3H), 7.08 (d, J = 8 Hz, 2H),
5.85 (d, J = 8 Hz, 1H), 4.92-4.87 (m, 1H), 3.73 (s, 3H), 3.66-3.58 (m, 1H),
3.18-3.06 (m, 2H), 2.31-1 (m, steroidal H), 0.92 (s, 3H), 0.89 (d, J = 8 Hz,
3H), 0.63 (s, 3H). 13C NMR (CDCl3, 100 MHz) δ: 173.0, 172.1, 135.8,
129.2, 128.5, 127.0, 71.8, 56.4, 55.9, 52.8, 52.2, 42.67, 42.0, 40.3, 40.1,
37.8, 36.4, 35.8, 35.3, 35.2, 34.5, 33.3, 31.5, 30.5, 28.1, 27.1, 26.35,
24.1, 23.3, 20.7, 18.3, 12.0.IR (KBr, cm-1): 3475, 3290, 2941, 2931,
2866, 1754, 1645, 1526, 1442, 1374, 1281. HRMS [M + Na] calcd for
C34H51NO4Na, 560.3716; found, 560.3718.
N-(3α-Hydroxy-5β-cholan-24-oyl)-L-phenylalanine (LC-LF-OH, 1)
Experimental Section
Yield- 87%, m.p. 230-234 °C (lit. 225-228 °C)
6
Instruments and General considerations: All starting materials and
solvents were obtained from commercial sources and were used without
further purification. Melting points of the product were determined on a
Digimelt melting point apparatus. 1H NMR and 13C NMR spectra were
recorded on a Bruker NMR spectrometer at 400 MHz and 100 MHz,
respectively. The SEM images were recorded with FEI Sirion after a 10
nm thick gold-coating of the samples. AFM was recorded on a JPK Nano
wizard II. For TEM, Technai T20 and JEOL machine were used. For TEM
sample preparation, a small amount of the gel/gel-nanocomposite was
added on a carbon-coated copper grid to make a fine layer of the gel and
dried, and stained with 0.1% uranyl acetate. For UV-Vis absorption
1H NMR (MeOH-d4:CDCl3= 1:4, 400 MHz) δ: 7.29-7.16 (m, 5H), 4.78-
4.75 (m, 1H), 4.10 (m, H2O), 3.51-3.56 (m, 1H), 3.36 (MeOH), 3.3-3.19
(m, 1H), 3.09-3.04 (m, 1H), 2.26-0.97 (m, steroidal H), 0.92 (s, 3H), 0.90
(d, J = 8 Hz, 3H), 0.64 (s, 3H).13C NMR (MeOH-d4:CDCl3= 1:4, 100
MHz) δ: 174.7, 173.9, 136.6, 129.5, 128.6, 127.1, 77.7, 71.7, 56.8, 56.2,
53.4, 42.9, 42.4, 40.8, 40.4, 37.6, 36.2, 36.1, 35.7, 35.6, 34.8, 33.4, 31.9,
30.3, 28.3, 27.4, 26.7, 24.4, 23.5, 21.0, 18.4, 12.2. IR (KBr, cm-1): 3336,
2933, 2866, 2356, 2341, 1716, 1628, 1536, 1384, 1278, 1186. HRMS [M
+ Na] calcd for C33H49NO4Na, 546.3559; found, 546.3559.
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Methyl N-(3α-hydroxy-5β-cholan-24-oyl)-D-phenylalaninate (LC-DF-
OMe, 2a)
h, a wine red colored gel was obtained. AuNP (derived from 0.3 mM Au
salt) doped LC-LW-OH (0.5 wt.-%) hybrid gel was used for TEM imaging.
Yield- 45%, m.p. 234-236 °C (lit.6 228-231°C)
Supporting Information (see footnote on the first page of this article):
Characterization data (1H NMR and 13C NMR spectra, HRMS), SEM,
AFM, rheology, crystallographic data.
1H NMR (CDCl3, 400 MHz) δ: 7.31-7.25 (m, 3H), 7.09-7.07 (m, 2H), 5.85
(d, J = 8 Hz, 1H, NH), 4.92-4.87 (m, 1H), 3.73 (s, 3H), 3.66-3.58 (m, 1H),
3.18-3.07 (m, 2H), 2.28-0.99 (m, steroidal H), 0.91-0.88 (m, 6H), 0.63 (s,
3H).13C NMR (CDCl3, 100 MHz) δ: 173.1, 172.3, 135.9, 129.4, 128.6,
127.2, 71.9, 56.6, 56.1, 52.9, 52.4, 42.8, 42.2, 40.5, 40.3, 38.0, 36.5,
35.9, 35.5, 35.4, 34.6, 33.5, 31.6, 30.6, 29.8, 28.3, 27.3, 26.5, 24.3, 23.5,
20.9, 18.4, 12.1. IR (KBr, cm-1): 3448, 2933, 2865, 2851, 1751, 1653,
CCDC-1451856 (LC-LF-OH, 1); 1451825 (LC-LV-OH, 8); 1475698 (LC-
LA-OH, 9); 1451855 (LC-G-OH, 10) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
1527. HRMS [M
+ Na] calcd for C34H51NO4Na, 560.3716; found,
560.3717.
Acknowledgements
N-(3α-Hydroxy-5β-cholan-24-oyl)-D-phenylalanine (LC-DF-OH, 2)
We thank the Department of Science & Technology, New Delhi
(grant no. SR/S1/OC-68/2011), for the support of this work. UM
also thanks the DST for the award of a J. C. Bose fellowship.
MM thanks the CSIR for a research fellowship. The Chemical
Science divisional TEM facilities and the AFMM centre are
thanked for the TEM and SEM imaging analysis, respectively.
We thank Ms. S. G. Shubha for her help in synthesising some of
the starting materials and Mr. Amar Hosamoni (SSCU, IISc,
Bangalore) for help with the X-ray structure analysis.
Yield- 98%, m.p. 182-183 °C (lit.6 155-158 °C)39
1H NMR (MeOH-d4, 400 MHz) δ: 7.87 (s, 1H, NH), 7.27-7.17 (m, 5H),
4.65-4.63 (m, 1H), 3.57-3.49 (m, 1H) 3.22-3.18 (m, 1H), 2.96-2.90 (m,
1H), 2.26-0.98 (m, steroidal H), 0.93 (s, 3H), 0.87 (d, J = 8 Hz, 3H), 0.63
(s, 3H). 13C NMR (MeOH-d4, 100 MHz) δ: 175.0, 137.2, 128.8, 127.9,
126.3, 78.0, 70.9, 56.4, 56.0, 42.4, 42.0, 40.4, 40.0, 37.0, 35.8, 35.7,
35.2, 35.0, 34.2, 32.4, 31.6, 29.7, 29.3, 27.7, 26.9, 26.2, 23.8, 22.5, 20.5,
20.5, 17.3, 11.2. IR (KBr, cm-1): 3449, 3300, 2934, 2864, 1716, 1612,
1545, 1445, 1363, 1318, 1289, 1220. HRMS [M
+ Na] calcd for
C33H49NO4Na, 546.3559; found, 546.3558. Elemental analysis calc. for
Keywords: Bile acids • Amino Acids • Supramolecular chemistry
C33H49NO4, C, 75.68, H, 9.43, N, 2.67; found, C, 75.39, H, 9.228, N, 2.33.
• X-ray Crystallography • Hydrogel
*Structural characterization data for compounds (3a-11a) and (3-11) are
provided in the supporting Information.
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A number of new bile acid-amino acid
conjugates exhibited pH dependent
hydrogelation. These gels were used
as templates for in situ synthesis of
Supramolecular chemistry, Hydrogel*
Mitasree Maity, Uday Maitra*
Page No. – Page No.
ZnO
and
Au
nanoparticle-gel
Supramolecular gels from Bile acid-
Amino acid conjugates and their
Applications
composites.
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