Low molecular mass cationic gelators derived from deoxycholic acid: remarkable gelation of aqueous solvents

Article abstract of DOI:10.1016/j.tet.2007.03.118

During the past decade, the study of molecular self-assembly and network formation from small molecule gelators has become one of the most active areas of supramolecular chemistry. A serendipitous discovery of the gelation of a cationic bile salt (4) led

Full text of DOI:10.1016/j.tet.2007.03.118

Tetrahedron 63 (2007) 7309–7320
Low molecular mass cationic gelators derived from deoxycholic
acid: remarkable gelation of aqueous solvents
Shreedhar Bhata and Uday Maitraa,b,
*
aIndian Institute of Science, Department of Organic Chemistry, Bangalore 560012, India
bChemical Biology Unit, JNCASR, Bangalore 560012, India
Received 4 January 2007; revised 8 March 2007; accepted 16 March 2007
Available online 24 March 2007
Abstract—During the past decade, the study of molecular self-assembly and network formation from small molecule gelators has become
one of the most active areas of supramolecular chemistry. A serendipitous discovery of the gelation of a cationic bile salt (4) led us to inves-
tigate the aggregation properties of this new class of cationic hydrogelators. This article summarizes the recent efforts on the study of side
chain structure–aggregation property relationship of cationic bile salts. Bile acid analogs with a quaternary ammonium group on the side chain
were found to efficiently gel aqueous salt solutions. Some of the cationic bile salts gelled water alone and many of them gelled aqueous salt
solutions even in the presence of organic co-solvents (ꢀ20%) such as ethanol, methanol, DMSO, and DMF. These gels showed interconnected
fibrous networks. Unlike natural anionic bile salt gels (reported for NaDC and NaLC), the cationic gels reported here are pH independent.
Cationic gels derived from DCA showed more solid-like rheological response compared to natural NaDC gels studied earlier by Tato et al.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Gel formation in aqueous or non-aqueous media by poly-
mers and biopolymers is a well-documented phenomenon
in the literature. Besides the well-known class of synthetic
or natural macromolecules,8 which form reversible gels9
under appropriate concentration and solvent conditions,
another important class of (physical) gels is constituted by
small molecules.
The self-assembly process has enabled synthetic chemists to
create spectacular architectures ranging from dots and parti-
cles; rods and fibers; helices and grids; rotaxanes and cate-
nanes to molecular motors. Supramolecular gels result
from a complex self-assembly process leading to the forma-
tion of fibrous networks. Multiple supramolecular interac-
tions1 between the individual building blocks, for example,
hydrogen bonding, metal coordination, hydrophobic inter-
action, etc., are believed to be critical in the gel forming
process.
Low molecular weight hydrogelators offer several advan-
tages over polymer gels, despite the fact that such hydrogels
have complex intermolecular association modes that are
often difficult to define. Such small gelator molecules have
special chemical functionalities responsible for the auto-
associative behavior operative in certain concentration, tem-
perature, and solvent conditions. The ease of formation and
the reversibility of gel formation by small molecular weight
hydrogelators are attractive strategies for synthesizing new
materials. The properties of a material depend both on the
nature of its molecular constituents and the precise spatial
positioning of functional groups, which can be manipulated
in small molecules, but usually not in polymers.
Gels are classified in many ways. Depending on the nature of
the solvent that is being gelled, systems have been classified
as organogels and hydrogels. Hydrogels are important soft
materials that have been shown to have many applications
in material science and biomedical engineering.2 They are
useful as drug delivery vehicles, scaffolds for tissue engi-
neering,3 and for controlled release of other biological
agents.4 Gels are also useful in pollutant capture and re-
moval.5 Recent efforts are being made to construct hydrogel
based chemical sensors and enzyme inhibition assay kits.6 In
addition, gels have utility in the development of new mate-
rials that reversibly respond to various external stimuli.7
Since molecular gels can have diverse structures on the
microscopic and mesoscopic scales, there has been consider-
able interest in developing functional gels by fine tuning ge-
lator molecules and/or doping them with molecules with
additional properties.10 Examples of low molecular weight
compounds, which can gel water11 or organic liquids12 are
abundant in the current literature. But, there are few reports
* Corresponding author. Tel.: +91 80 2293 2690; fax: +91 80 2360 0529;
e-mail: maitra@orgchem.iisc.ernet.in
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.118

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S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
on the structure–property study of a given class of self-
assembled systems.
salts gave crystals, precipitates or solutions depending
upon the gelator concentration chosen. Compounds 4, 8, 9,
and 11 have been previously reported in detail.13 Compound
2 formed a stable gel in pure water at and above 0.8%. Com-
pound 12 at 1% w/v formed a stable gel in 0–4 M aqueous
NaCl solutions and the transparent gels progressively
became translucent with increasing NaCl concentration. In
contrast, bile salts 3, 6, and 9 showed an interesting aggrega-
tion behavior in 1 M NaCl: forming (a) gel in the concentra-
tion range of 0.2–1.0% w/v, (b) gel containing dispersed
gelator particles above 1.5%, and (c) microcrystals at >2%.
We have recently reported a few cationic analogs of bile salts
as potent hydrogelators.13 In this article we summarize the
detailed structure–property study made with this novel class
of cationic hydrogelators.
2. Results
2.1. Gelation studies
Gels formed at lower concentration of the gelator and NaCl
are transparent and turned progressively translucent when
the concentration of either of these is increased. There
were significant differences in the optical appearance of
the gels derived from cationic bile gelators. The gels made
of 3, 6, 9, and 12 were more translucent compared to those
All the final compounds listed in Schemes 1–3 were tested
for their gelling abilities in various solvents. Each of these
cationic bile salts exhibited unique aggregation behavior.
Compounds 2–4, 6, 8, 9, and 12 formed thermoreversible
gels in water in the presence of NaCl,14 whereas other bile
H2N
N
N ++
N+
I-
I-
N
N+
N+
12
I-
OH
OH
11
I-
1
OH
OH
OH
OH
OH
2
N
N
OH
NH2
N
10
N
HO
N+
N+
N
OH
N
N
I-
I-
OH
3
I
N
OH
OH
HO
9
N
OH
OH
O
N+
N
N
I-
N
N
HO
OH
HO
8
HO
N
OH
HO
N
4
HO
N+
I-
HO
HO
HO
N+
I-
HO
7
I-
O
N+
5
HO
6
I-
N+
N
HO
I-
N+
OH
Scheme 1. Synthesis of cationic bile salts from iododeoxycholane (the iodo derivative was prepared from deoxycholic acid by following the reported proce-
dure).13a

S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
7311
COOH
OH
a
OH
OH
OH
13
OH
b
I-
c
OTs
OH
N+
OH
14
OH
N
OH
15
d
a. TEA/Ethyl chloroformate in THF;
NaBH4 in water
I-
b. TsCl/Dry. Pyridine.
c. DABCO/Acetonitrile reflux;
Ion-exchange (Amberlite IRA 400)
N+
OH
d. N-methylpyrrolidine/Acetonitrile reflux
Ion-exchange (Amberlite IRA 400)
OH
16
Scheme 2. Synthesis of side chain elongated cationic bile salts.
1-Anilino-8-naphthalene sulfonate (ANS) has been used for
probing the formation of hydrophobic surfaces during pro-
tein folding since its fluorescence quantum yield increases
in non-polar environments.16 We have also used ANS fluo-
rescence for probing gelation.17 For estimating critical gel
concentrations (CGC) ANS anisotropy was preferred since
the fluorescence intensity enhancement was less sensitive
to CGC.18 Compounds 2, 3, 6, and 12 showed critical gel
concentration values of 7.0, 2.0, 2.5, and 4.5 mM, respec-
tively, in 0.5 M NaCl. CGC decreased upon increasing the
NaCl concentration, and the minimum concentration re-
quired to gel a given solvent mixture increased with an in-
crease in the amount of the organic solvent. Additionally,
lower CGC was associated with compounds with lower
aqueous solubility.
+
CH3CN
I
N
N
-
I
Reflux
Y
Y
X
X
X = Y = H
17. X = Y = H
18. X = Y = OH
N
X = Y = OH
N
OH
OH
Scheme 3. Mono and trihydroxy cationic bile salts.
of 2 and 8. Also, the gels of 3, 6, and 9 are more rigid and
resistant to mechanical shaking than are the other. A striking
difference in the macroscopic cohesion of the gels was also
observed. When subjected to mechanical agitation, gels of 2,
8, and 12 (typically from 0.5% to 1% w/v) broke into a slurry
of small globular aggregates, while the gels derived from 3,
6, and 9 broke into larger lumps. Thus, gels of 8 and 12 ex-
hibited lower cohesion at a macroscopic level. At this stage,
therefore, the unique macroscopic aspects and behaviors that
characterize each of the systems were noticeable.
2.2. Tgel measurements
In general, the thermal stability of gels as often characterized
by Tgel (melting temperature of the gel) is dependent on sev-
eral parameters like the surface of the gelation tube used,
concentration of NaCl chosen, diameter of the tube, and
the concentration of the gelator. However, under defined
experimental conditions unique gel melting values were
obtained.
For better gelation of aqueous solvents the use of NaCl with
bile salt is essential. NaCl provides an ionic environment in
addition to salting out effect. The concentration of NaCl
seemed to have a significant influence on the gel strength
of these bile salts. However, salt concentrations above 4 M
were found to destroy the gelation process.15 Gelation re-
quired at least 0.1 M NaCl, and for obtaining optically trans-
parent hydrogels 0.5–1 M NaCl was optimum. In addition to
aqueous NaCl solution, gels were obtained in 1 M aqueous
solutions of NaBr, Na2SO4, Na2CO3, NaN3, NaNO3, KCl,
LiCl, and BaCl2, but precipitated in aqueous 1 M NaI. Com-
pounds 3, 6, and 9 formed a gel in the presence of organic
co-solvents such as MeOH, EtOH, DMF, DMSO, etc. (up
to 20% organic co-solvent). Higher percentage of organic
co-solvents resulted in fluids indicating the importance of
the hydrophobic effect in the gelation process. Compounds
1, 5, 7, and 10 were found to be non-gelators.
The thermal stability of the gels increased with the concen-
tration of the gelator, in a nearly linear fashion (data not
shown). Higher thermal stability was also observed when
the salt concentration was increased.19 A comparison of
Tgel for cationic bile salt gels is shown in Figure 1. No trivial
relation between the melting temperature of the gels (Tgel
)
and the melting point of the pure cationic solids (on the order
of 265–290 ꢁC; see Section 5) was observed. The enthalpy of
disaggregation was estimated from plots of ln (C) versus 1/
€
Tgel using the Schroeder van Laar equation {ln (C)¼(DH/
RTgel+constant, where R is the universal gas constant and

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S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
carboxylic group with glycine or taurine, although increas-
ing the length of the side chain, causes little change in the
At gelator concentration 0.6% w/v in 0.5 M NaCl
90
80
70
60
50
40
30
20
10
0
CMC values.21
To study the effect of side chain length on the aggregation of
the cationic bile salts, the side chain was elongated by one
carbon unit for two of the representative members of the
class (compounds 15 and 16) as shown in Scheme 2, and
the aggregation properties were studied.22 Preliminary
investigation showed lowering of the CMC values upon in-
creasing the side chain length by one carbon. For compounds
15 and 16 lowering in CMC values from 2.5 to 1.0 mM and
2.0 to 1.5 mM were found, respectively, after one carbon
elongation at the side chain. However, noticeable differences
were observed on the gelation properties of the cationic bile
salts upon side chain elongation. The gelation abilities com-
pletely changed upon adding a carbon to the side chain.
Compound 16 formed a gel in 1 M NaCl solution at a bile
salt concentration greater than 0.3%. But compound 15 did
not gel salt solutions under identical conditions. This sug-
gested that C23-cationic bile salts are stronger gelators
than C24 analogs under these conditions.
2
3
4
6
8
9
12
Compound Number
Figure 1. Comparison of Tgel values versus side chain structure of DC salts
(experiments were performed at 0.6% w/v gelator concentration and 0.5 M
NaCl).
C is the concentration of the gelator}. The estimated DH (gel
melting) values were 50–80 kJ molꢂ1
.
Tgel values decreased with an increase in the percentage of
the organic co-solvent, owing to the increased solubility of
the gelator in the presence of higher amounts of the organic
solvent. The gels of DMSO/H2O were found to have higher
Tgel values than those of the gels formed in EtOH/H2O.
2.5. Electron microscopy study
Electron microscopic investigations of the xerogels of the
cationic bile salts were attempted. The xerogel showed dif-
ferent types of fibrillar networks (shown in Fig. 2A–D).
The diameter of the gel strands was of the order 3–10 mm.
2.3. Microviscosity measurements
The present experimental data do not allow us to postulate an
exact theoretical model for the supramolecular aggregation
of these molecules to form a self-aggregated gel network
as could be seen from the SEM. No pronounced helicity of
the fibers was observed even though the gelator molecules
are all homochiral.23
Polarization experiments carried out with 1,6-diphenylhexa-
triene (DPH) were made to determine the microviscosity of
the cationic bile salt aggregates using the following expres-
sion20
h ¼ 2P=ð0:46 ꢂ PÞ
ð1Þ
2.6. Rheological properties
where, P is the polarization of DPH inside the gel.
Flow properties of the gels prepared in a glass vial of defined
size can be roughly characterized by visualizing their motion
as a cause of weight when inverted. However, for exact quan-
tification, rheological experiments are necessary. Linear and
non-linear rheological measurements were carried out to
characterize the flow behavior of the cationic bile salt gels.
Measured polarization and microviscosity values are listed
in Table 1.
Table 1. Fluorescence polarization and microviscosity values for cationic
bile salt aggregates obtained by probing with DPH (values are within
10% experimental error)
2.6.1. Amplitude sweep. Amplitude sweep measurements
were taken at an oscillating frequency of 1 Hz. G0 (storage
modulus) and G00 (loss modulus) values were determined
at different gelator concentrations. Gels made from com-
pounds 2, 3, 6, 8, 9, and 12 in NaCl solution were character-
ized by their high elastic modulus G0 and yield stress s*
values (the critical stress at which the gel flows or gel loses
its structure). G0 was found to be larger than G00 in the LVE
regime suggesting that the materials have solid-like
response. G0/G00 for various cationic gelator is shown in Fig-
ure 3. A plot of elastic modulus versus gelator concentration
(to verify G0fCm) is shown in Figure 4.
Bile
salt
Head group
Concn
(% w/v) (P)
Polarization Microviscosity
(h, Poise)
NaC COOH
NaDC COOH
2
3
6
12
10
10
1
0.1
0.55
1.25
2.4
7.2
3.1
0.17
0.25
0.35
0.28
0.26
DABCO
N-Methyl pyrrolidine 0.5
Quinuclidine
Dimazine
0.5
1.0
2.8
2.4. Side chain length and aggregation
The side chain of bile salts has been given primary impor-
tance in the self-assembly process of bile salts. It has been
reported by Roda et al. that the CMC values increased signif-
icantly as the side chain was shortened from C26 to C24 to
C23 for free bile salts. Conjugation of the side chain
The yield stress (s*) also increased with the gelator concen-
tration and the magnitude was proportional to the elastic
modulus (Fig. 3) and scaled as the power of the gelator con-
centration (s*fCn) (Fig. 5). The values of the exponents

S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
7313
Figure 2. SEM images of cationic gels: (A) 0.5% w/v of 3 in 0.5 M NaCl (scale bars A1¼200 mm and A2¼10 mm), (B) 0.5% w/v of 6 in 0.5 M NaCl (scale bars
B1 and B2¼3 mm: B1 and B2 are different zones of the same xerogel), (C) 1% w/v 2 in 0.15 M NaCl (scale bar 20 mm), and (D) 1% w/v 12 in 0.5 M NaCl (scale
bar 3 mm).
(m and n) listed in Table 2 are in good agreement with the
reported values for low molecular mass gelling agents.24,25
2.6.2. Frequency sweep. Frequency sweep measurements
were carried out on the cationic bile salt gels at applied stress
amplitudes, which fall in the linear viscoelastic domains
(LVE regime). The frequency sweep experiment showed
a gel-like behavior over the complete range of frequencies
studied. The typical rheograms obtained for the gels of
The results obtained from the amplitude sweep experiments
clearly indicate that individual cationic gels have unique
rheological response.

7314
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S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
Table 2. Rheological data obtained for some of the cationic bile salts (for
comparison, 0.4% w/v gelator concentration was found to be ideal to gel
all the four cationic bile salts)
10
8
Gelator C (% w/v, in 1 M NaCl) G0 (Pa) G0/G00 s* (Pa)
m
n
2
3
6
12
0.4
3500
5200
4100
460
6
10
9
7
35
20
2
2.1 2.5
2.2 2.7
2.6 2.4
2.3 2.2
0.4a
0.4a
0.4
6
4
a
20% MeOH was used to prepare the gel.
4
2
10000000
1000000
100000
10000
1000
0
2
3
6
8
9
12
Bile salt gel 0.6% w/v in 0.5 M NaCl
Figure 3. Comparison of G0/G00 versus side chain structure of DC salts (at
0.6% w/v gelator in 0.5 M aqueous NaCl).
100
10
Storage Modulus (G')
Loss Modulus (G")
Complex Viscosity (η*)
6
Compound 2
1
0.1
Compound 3
5.5
Compound 6
0.01
5
0.01
0.1
1
10
100
1000
Compound 12
Angular frequency (ω) 1/s
4.5
4
Figure 6. Frequency sweep experiment using cationic bile salt gels (0.4%
w/v gelator 2 in 1 M aqueous NaCl).
3.5
3
long time scales (small frequencies of the applied stress),
liquid-like flowing properties may prevail.
2.5
2
The complex viscosity decreased slowly and continuously
with frequency and the slope approached ꢂ1 in a log–log
representation (Fig. 7). This type of mechanical behavior
is generally a characteristic observed in soft viscoelastic
solids and hence many of the cationic bile salt derived mate-
rials can be rheologically classified as gels.26
-0.4 -0.3 -0.2 -0.1
0
0.1 0.2 0.3 0.4 0.5 0.6
log (C)
Figure 4. log–log representation of a plot of G0 (elastic modulus) versus
gelator concentration (0.4% w/v gelator in 1 M aqueous NaCl).
2.5
100000
2
1.5
10000
1
Compound 2
1000
0.5
Compound 3
Compound 2
Compound 3
Compound 6
Compound 12
Compound 6
0
Compound 12
100
-0.5
0.1
1
10
100
1000
-0.4
-0.2
0
0.2
0.4
0.6
Frequency /Hz
log (C)
Figure 7. Combined plot of G0 versus oscillating frequency for cationic bile
salt gels (0.4% w/v gelator in 1 M aqueous NaCl).
Figure 5. log–log representation of a plot of s* (yield stress) versus gelator
concentration (0.4% w/v gelator in 1 M aqueous NaCl).
2.6.3. Flow curves. Flow curves of cationic gels (at gelator
concentration 0.4% w/v) were measured in 1.0 M NaCl so-
lution, while 20% MeOH was used as a co-solvent to prepare
gels of 3 and 6. All the cationic gels studied showed a shear
thinning behavior with the exponent being nearly zero from
the power law fit (Fig. 8).
compound 2 in 1.0 M aqueous NaCl at 0.4% w/v gelator
concentration are presented in Figure 6. The elastic modulus
G0 was an order of magnitude higher than the loss modulus
G00 over the entire range of frequency, which characterizes
the materials as viscoelastic soft solids. Nevertheless, over

S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
7315
3
2.5
2
delight, these cationic bile salts (6 and 9) were found to be
excellent hydrogelators and gelled 1 M NaCl solution at
concentrations as low as 0.25% w/v. The gels obtained
were optically transparent up to 1% w/v. It was found that
compound 9 had higher Tgel and higher storage modulus
than compound 6. It is remarkable that a subtle variation
in the ring size causes such a noticeable change in their
gelation properties.
1.5
1
Compound 2
Compound 3
Compound 6
Compound 12
‘Opening’ the piperidine head group of 9 resulted in a non-
gelator 10 with higher aqueous solubility. On the other hand,
incorporation of an oxygen atom at the 4-position of the
piperidine head group of 9 resulted in compound 8, which
formed gels across wider concentration ranges (1 M NaCl,
0.5–3% w/v). The G0 value measured for 8 was less than
compounds 6 and 9 but similar to compound 2. This is indic-
ative of better solvation of the head group of compound 8,
presumably because of the additional heteroatom. The gels
obtained were less strong than with a single heteroatom con-
taining head group (9). Finally, ‘opening up’ the morpholine
ring of 8 at the oxygen end resulted in highly water soluble,
non-gelling compound 7.
0.5
0
-3
-2
-1
0
1
2
3
4
log (shear rate)
Figure 8. Power law fit for flow curves (log–log plot of shear stress versus
shear rate) of cationic bile salt gels (at 0.4% w/v gelator in 1 M aqueous
NaCl).
Cationic bile salt gels show a finite minimum stress point
required to flow (yield value). This flattening of the shear
viscosity at zero shear followed by a sharp decay with
a slope approaching ꢂ1 is typical of semi-dilute solu-
tions of rods exhibiting a solid-like behavior of complex
fluids.27
Further modification of the head group was made by intro-
ducing bicyclic head groups such as quinuclidine. This com-
pound (3) was found to be an excellent gelator compared to
all other cationic bile salts in this group. This gel had the
highest G0 value (see Fig. 3 or Fig. 7). Therefore, it seems
clear that a rigid cationic head group leads to stronger
gels. The introduction of a second nitrogen atom on the bi-
cyclic quinuclidine head group of 3 resulted in gelator 2
with higher water solubility yielding softer gels than 3.
The gels made from 2 were similar in strength, but more
transparent than the gels from compound 8. Thus the results
indicated that the bile salts containing two heteroatoms at the
head group form softer gels than those with a single hetero-
atom.
3. Discussion
3.1. Bile salt structure and gelation property
Although gels derived from bile acids have been known for
a long time,28 they have not been systematically studied.29
The current work presents a first attempt to analyze and
correlate structure–aggregation properties derived from
cationic bile salts. A large collection of cationic side chain
modified bile salts was screened for gelation. Our studies
highlight the importance of the deoxycholate (DC) backbone
for their gelling property—presumably for having the ‘right’
hydrophilic–hydrophobic balance the details of which are
still not understood. Cationic bile salts derived from cholic
acid (compound 18) were found to be more water soluble,
hence they remained in solution under normal circum-
stances.30 On the contrary, cationic bile salts derived from
lithocholic acids (compound 17) were found to be highly
water insoluble with no gelation properties.30 The gelling
properties of the DC salts were found to be somewhat
dependent on the side chain structure, although, most of
them have high mechanical strength, thermal stability, and
optical clarity.
Gels prepared from compounds 2 and 8 showed identical gel
melting points and the G0/G00 values obtained were also very
similar. Furthermore, an open cyclic analog of 2 resulted in
a non-gelator molecule 1, even though it has a CMC compa-
rable to compound 2. Finally, we made compound 12 repre-
senting two N-atoms directly linked to each other and this
turned out to be a good gelator. The properties of the gels
prepared from 12 were also similar to compounds 2 and 8,
except they were somewhat translucent.
3.2. Comparison of aggregation properties of quater-
nary bile salts derived from open chain and cyclic bases
Unlike their cyclic counterparts, most of the bile salts pre-
pared from acyclic bases did not gel aqueous solvents, al-
though the CMC values for bile salts derived from both
open and cyclic bases were similar (Table 3). Bile salts
made from cyclic bases showed large fluorescence anisotro-
pies (ANS), up to 0.28, even at low bile salt concentrations
around 0.15% w/v, which is related to medium viscosity.
Under identical conditions, ANS fluorescence anisotropy
observed for bile salts derived from acyclic bases remained
below 0.15. These observations suggest the requirement of
a rigid bile salt side chain for the growth of the smaller
aggregates to obtain the gel.
The pyridinium salt (4) was the first compound studied in
this series. Subsequently, the deoxycholic acid side chain
was modified with various cationic head groups derived
from heterocyclic, aromatic, linear, and branched com-
pounds.
Replacement of the pyridinium cation (4) with a bipyridyl
unit (5) resulted in a poorly soluble, non-gelling compound,
as was observed with a quinolinium group also. Several non-
aromatic heterocycles such as N-methyl piperidine and
N-methylpyrrolidines were subsequently explored. To our

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S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
Table 3. Comparison of the CMC values obtained for bile salts bearing acyclic and cyclic quaternary head groups
Bile salt
Cyclic head group
CMC (mM)
Bile salt
Corresponding open
cyclic head group
CMC (mM)
2
9
8
DABCO
N-Methyl piperidine
N-Methyl morpholine
2.5ꢃ0.2
2.3ꢃ0.2
2.3ꢃ0.2
1
10
7
TMEDA
N-Methyl dipropylamine
N-Methyl diethanolamine
3.0ꢃ0.2
2.5ꢃ0.2
2.5ꢃ0.2
4. Conclusions
All new cationic bile salts were synthesized by using proce-
dures reported by Sangeetha et al. (from author’s laboratory)
and characterized by spectroscopic methods.13 General
method for the syntheses of 24-nor-3a-12a-dihydroxy-5b-
cholane salts: to a suspension of 24-nor-23-iodo-3a-12a-
dihydroxy-5b-cholane in CH3CN heated to reflux, corre-
sponding organic base was added in excess (2 equiv) and
refluxing was continued further for a period of 6 h. Solvent
was removed in vacuo and the crude product was chromato-
graphed over silica gel column using 25–85% EtOH/CHCl3
as an eluent or purified by precipitating the cationic bile salts
by adding diethyl ether to an ethanolic solution.
Although a wide variety of derivatives display gelation be-
havior, a small change in molecular structure can have a dra-
matic effect on the gelling capacity. Current research has
provided a detailed understanding of the aggregation behav-
ior of various cationic bile salt analogs. Importance of the
side chain structure and length on the gelation of cationic
bile salts has been qualitatively evaluated in the present
work. The results supported the earlier postulate of strong
correlation of aggregation versus side chain structure.
Most of the DC-hydrogels have been shown to behave as cel-
lular solids. But the cationic bile salts are strong gels charac-
terized by high elasticity and yield stress values. Flow
behaviors of the cationic bile salt gels were characterized
and the gels were shown to have a strong side chain structure
dependent flow. Interestingly, each of the cationic systems
exhibits a unique behavior in terms of its thermal, optical,
and mechanical properties. Elastic moduli of gels obtained
from these cationic derivatives are 2–3 orders of magnitude
larger than those observed for sodium deoxycholate at com-
parable concentrations. The elastic moduli are however
comparable to those reported for organogels derived from
compounds of low mass.
5.1.1. N,N-Dimethyl,N-(dimethylaminomethyl),N-(3a-
12a-dihydroxy-24-nor-5b-cholan-23-yl) iodide (1). Yield:
70%. Mp 251–252 ꢁC. [a]D25 49.0 (c 1.0, EtOH). IR (KBr,
cmꢂ1): 3444, 2922, 2865, 2772, 1632, 1466, 1447, 1383,
1042, 918. 1H NMR (300 MHz, DMSO-d6) d: 0.61 (s, 3H),
0.85 (s, 3H), 0.96 (d, J¼5.4 Hz, 3H), 1.01–1.80 (m, steroidal
CH, CH2, 31H), 2.99 (br s, 6H), 3.11 (br s, 6H), 3.768 (s,
1H), 4.29 (d, J¼3.6 Hz, 1H), 4.51 (d, J¼3.6 Hz, 1H). 13C
NMR (75 MHz, DMSO-d6) d: 12.29, 17.30, 23.06, 23.38,
26.07, 26.93, 26.03, 27.68, 28.55, 30.17, 32.89, 33.53,
33.78, 35.09, 35.60, 36.24, 41.53, 45.09, 45.84, 45.99,
47.49, 50.24, 50.398, 52.29, 59.54, 62.02, 69.87, 70.85,
79.20. LRMS: calcd for C29H55N2O2: 463.43, obsd
463.50. Anal. Calcd for C29H55N2O2I: C 58.97, H 9.39, N
4.74. Found: C 58.98, H 9.51, N 4.71.
SEM analysis of the dried gels shows fibers of finite radii
with indeterminate lengths either bundled or intertwined,
revealing no specialized branching. The gels obtained
from cationic bile salts melt at moderate temperatures (50–
70 ꢁC), which opens up various commercial applications.
5.1.2. N-(3a-12a-Dihydroxy-24-nor-5b-cholan-23-yl) di-
azabicyclo[2.2.2]octyl iodide (2). Yield: 75%. Mp 296–
297 ꢁC. [a]D25 39.0 (c 1.0, EtOH). IR (KBr, cmꢂ1): 3454,
1
We believe that some of these cationic bile salts may be
biologically active (antimicrobial, DNA binding, etc.). Ad-
ditionally, the gels derived from them may find use in novel
material and hybrid material synthesis. The gels can also
serve as a useful chiral medium for manipulating chemical
reactions and product selectivity. Some of these possibilities
are being explored in our laboratory and the results from
these experiments will be reported in due course.
2920, 2860, 1462, 1383, 1091, 1055, 1043, 843. H NMR
(300 MHz, DMSO-d6) d: 0.60 (s, 3H), 0.84 (s, 3H), 0.96
(d, J¼5 Hz, 3H), 1.01–1.81 (steroidal, CH, CH2, 27H),
3.00 (br s, 6H), 3.30 (br s, 6H), 3.77 (s, 1H), 4.34 (d,
J¼3.6 Hz, 1H), 4.55 (d, J¼3.6 Hz, 1H). 13C NMR
(75 MHz, DMSO-d6) d: 12.35, 17.33, 23.08, 23.40, 26.09,
26.87, 26.95, 27.03, 28.54, 30.19, 32.91, 33.55, 33.80,
35.12, 35.62, 36.25, 41.56, 44.73, 45.74, 46.03, 47.50,
51.32, 61.44, 69.87, 70.85, 79.30. LRMS: calcd for
C29H51N2O2: 459.39, obsd 459.46. Anal. Calcd for
C29H51N2O2I: C 59.38, H 8.76, N 4.78. Found: C 59.58, H
8.80, N 4.91.
5. Experimental section
5.1. Materials
5.1.3. N-(3a-12a-Dihydroxy-24-nor-5b-cholan-23-yl)
quinuclidinium iodide (3). Yield: 70%. Mp 277–278 ꢁC.
[a]2D5 42.0 (c 1.0, EtOH). IR (KBr, cmꢂ1): 3430, 2936,
Bile acids were purchased from Fluka for synthesizing
different cationic bile salts. All the organic bases used
were purchased from Aldrich (99%+). Pyrene (Aldrich)
was used after recrystallization in methanol. Solvents used
were of analytical grade or double distilled prior to use. Dou-
ble distilled water was used for all spectral measurements.
Glassware was rinsed with acids and cleaned with soap
water prior to use.
1
2860, 1620, 1462, 1382, 1043, 943. H NMR (300 MHz,
DMSO-d6) d: 0.60 (s, 3H), 0.84 (s, 3H), 0.95 (d,
J¼3.3 Hz, 3H), 1.02–2.05 (m, steroidal CH, CH2, 26H),
3.06–3.19 (br m, 15H), 3.77 (br s, 1H), 4.31 (d, J¼3.6 Hz,
1H), 4.52 (d, J¼3.6 Hz, 1H). 13C NMR (75 MHz, DMSO-
d6) d: 12.33, 17.29, 19.09, 23.06, 23.37, 26.09, 26.95,

S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
7317
27.20, 28.55, 30.19, 32.90, 33.55, 33.78, 35.12, 35.61, 36.24,
41.56, 45.74, 46.00, 47.48, 53.46, 61.33, 69.88, 70.85.
LRMS: calcd for C30H52NO2: 458.39, obsd 458.40. Anal.
Calcd for C30H52NO2I+H2O: C 59.69, H 9.02, N 2.30.
Found: C 59.78, H 8.93, N 2.21.
23.06, 23.40, 26.08, 26.94, 27.11, 27.29, 28.52, 30.19,
32.89, 33.70, 33.78, 35.09, 35.60, 36.24, 41.55, 45.90,
46.04, 47.50, 59.17, 61.89, 69.88, 70.89. LRMS: calcd for
C30H56NO2: 462.43, obsd 462.50. Anal. Calcd for
C30H56NO2I+3.5H2O: C 64.54, H 10.80, N 2.51. Found: C
64.62, H 10.11, N 2.45.
5.1.4. N-(3a-12a-Dihydroxy-5b-24-nor-cholan-23-yl)
4,40-bipyridinium iodide (5). Yield: 65%. Mp 216–
217 ꢁC. [a]D25 31.0 (c 1.0, EtOH). IR (KBr, cmꢂ1): 3414,
2959, 2924, 2861, 1634, 1471, 1458, 1380, 1300, 1251. 1H
NMR (300 MHz, DMSO-d6) d: 0.61 (s, 3H), 0.92 (s, 3H),
1.08 (d, J¼6.3 Hz, 3H), 1.14–2.00 (m, steroidal CH, CH2,
27H), 3.81 (br s, 1H), 4.31 (d, J¼3.8 Hz, 1H), 4.50 (d,
J¼3.8 Hz, 1H), 8.04 (d, J¼5.7 Hz, 2H), 8.62 (d, J¼6.6 Hz,
2H), 8.87 (d, J¼5.7 Hz, 2H), 9.26 (d, J¼6.6 Hz, 2H). 13C
NMR (75 MHz, DMSO-d6) d: 12.37, 17.22, 23.07, 23.41,
26.05, 26.93, 27.09, 28.59, 30.20, 32.90, 33.59, 33.78,
35.09, 35.60, 36.22, 37.56, 41.54, 46.06, 47.50, 58.72,
69.90, 70.91, 79.234, 121.92, 125.41, 140.81, 145.35,
150.96, 152.12. LRMS: calcd for C33H47N2O2: 503.36,
obsd 503.4. Anal. Calcd for C33H47N2O2I+1.5H2O: C
60.36, H 7.52, N 4.26. Found: C 60.40, H 7.56, N 4.12.
5.1.8. N-(3a-12a-Dihydroxy-24-nor-5b-cholan-23-yl) hy-
drazyl iodide (12). Yield: 65%. Mp turns brown at 160 ꢁC
and melts near 175 ꢁC. [a]2D5 36 (c 1.0, EtOH). IR (KBr,
cmꢂ1): 3471, 3389, 3249, 3139, 2934, 2862, 1621, 1448,
1
1378, 1044. H NMR (300 MHz, DMSO-d6) d: 0. 59 (s,
3H), 0.83 (s, 3H), 0.95 (d, J¼6.3 Hz, 3H), 1.00–1.80 (m, ste-
roidal CH, CH2, 24H), 3.16 (s, 6H), 3.77 (s, 6H), 4.27 (d,
J¼3.6 Hz, 1H), 4.50 (d, J¼3.6 Hz, 1H). 13C NMR (75 MHz,
DMSO-d6) d: 12.37, 17.40, 23.13, 23.45, 26.14, 26.99,
27.08, 28.06, 30.23, 32.98, 33.32, 33.84, 35.13, 35.68,
36.28, 41.60, 45.77, 46.05, 47.59, 55.05, 55.37, 57.38, 66.16,
69.96, 70.94. LRMS: calcd for C25H47N2O2: 407.44, obsd
407.39. Anal. Calcd for C25H47N2O2+3H2O: C 4.34, H
8.94, N 5.05. Found: C 54.30, H 8.57, N 4.89.
5.1.9. 3a-12a-24-Trihydroxy-5b-cholane (13). Toastirring
solution of deoxycholic acid (1 g, 2.55 mmol) in THF
(20 mL), triethylamine (0.6 mL, 8.2 mmol) was added
followed by the slow addition of ethyl chloroformate
(0.4 mL, 3.24 mmol). The reaction mixture was stirred for
2 h and a solution of sodium borohydride (1.2 g, 32 mmol)
in water (7 mL) was added slowly. The resulting mixture
was allowed to stir at room temperature (28 ꢁC) for 12 h.
HCl (20 mL of 1 M solution) was added to the reaction mix-
ture and was stirred for an additional 30 min. Inorganic ma-
terials were removed out by filtration and the reduction
product was extracted using ethyl acetate (2ꢄ50 mL). The
EtOAc layer was washed with water, dried over anhyd
Na2SO4, and the white powder obtained was finally crystal-
lized from ethyl acetate to yield 0.8 g of the crystalline solid
product with mp 102–103 ꢁC. [a]2D5 54.0 (c 2.0, EtOH). IR
(neat, cmꢂ1): 3398, 2938, 2863, 1063. 1H NMR
(300 MHz, CDCl3) d: 0.68 (s, 3H), 0.90 (s, 3H), 0.98 (d,
J¼6.0 Hz, 3H), 1.05–1.86 (m, steroidal CH, CH2, 39H),
2.78 (m, 2H), 3.99 (br s, 1H), 3.60 (m, 1H). 13C NMR
(75 MHz, CDCl3) d: 10.91, 12.72, 17.61, 23.15, 23.68,
26.16, 27.17, 27.58, 28.61, 29.58, 29.79, 30.36, 31.85, 33.50,
34.11, 35.29, 35.41, 35.99, 36.36, 42.07, 45.81, 47.38, 48.12,
62.96, 71.31, 72.86. LRMS: calcd for C24H42O3+Na:
401.30, obsd 401.4. Anal. Calcd for C24H42O3+0.5H2O: C
74.35, H 11.18. Found: C 74.11, H 10.87.
5.1.5. N-(3a-12a-Dihydroxy-24-nor-5b-cholan-23-yl) N-
methylpyrrolidinium iodide (6). Yield: 70%. Mp 253–
254 ꢁC. [a]D25 39 (c 1.0, EtOH). IR (KBr, cmꢂ1): 3419,
2932, 2858, 1635, 1469, 1375, 1042, 930. 1H NMR
(300 MHz, DMSO-d6) d: 0.60 (s, 3H), 0.84 (s, 3H), 0.98
(d, J¼5 Hz, 3H), 1.02–1.81 (m, steroidal CH, CH2, 34H),
2.07 (br s, 4H), 2.97 (s, 3H), 3.78 (s, 1H), 4.34 (d, J¼
3.6 Hz, 1H), 4.56 (d, J¼3.6 Hz, 1H). 13C NMR (75 MHz,
DMSO-d6) d: 12.50, 17.20, 20.04, 21.13, 23.12, 23.52,
26.12, 26.99, 27.31, 28.64, 30.25, 32.25, 32.95, 33.84,
35.09, 35.68, 36.28, 41.61, 46.04, 46.41, 47.52, 63.34,
63.52, 69.97, 71.08, 79.20. LRMS: calcd for C28H50NO2:
432.38, obsd 432.34. Anal. Calcd for C28H50NO2I+0.5H2O:
C 59.14, H 9.04, N 2.46. Found: C 59.14, H 9.04, N 2.50.
5.1.6. N-(3a-12a-Dihydroxy-5b-24-nor-cholan-23-yl) N-
methyldiethanolammonium iodide (7). Yield: 65%. Mp
226–227 ꢁC. [a]2D5 35.0 (c 1.0, EtOH). IR (KBr, cmꢂ1):
3406, 2965, 2934, 2913, 2858, 1630, 1448, 1370, 1090,
1043, 925. 1H NMR (300 MHz, DMSO-d6) d: 0.60 (s, 3H),
0.84 (s, 3H), 0.97 (d, J¼5.1 Hz, 3H), 1.00–2.07 (m, steroidal
CH, CH2, 31H), 3.09 (s, 3H), 3.79 (br s, 5H), 5.49 (br s, 2H),
4.31 (d, J¼3.6 Hz, 1H), 4.54 (d, J¼3.6 Hz, 1H). 13C NMR
(75 MHz, DMSO-d6) d: 12.34, 17.31, 18.57, 23.10, 23.45,
26.11, 26.99, 27.15, 27.48, 28.59, 30.21, 32.93, 33.83,
33.87, 35.16, 35.65, 36.27, 41.61, 46.07, 47.54, 49.02,
54.77, 61.18, 63.12, 69.92, 70.92, 79.30. LRMS: calcd for
C28H52NO4: 466.38, obsd 466.4. Anal. Calcd for
C28H52NO4I: C 56.65, H 8.83, N 2.36. Found: C 56.62, H
8.92, N 2.24.
5.1.10. 3a-12a-Dihydroxy-5b-24-(4-toluenesulfonyloxy)
cholane (14). To a suspension of 3a-12a-24-trihydroxycho-
lane (1.0 g, 2.65 mmol) in THF (20 mL), triethylamine
(1.5 mL, 20.54 mmol) and p-toluenesulfonylchloride
(1.0 g, 5.24 mmol) were added. The reaction mixture was
stirred in an ice-water bath for 12 h. Crude mixture was
extracted into chloroform (50 mL), washed with water
(2ꢄ20 mL), and dried over anhyd Na2SO4. Finally, after re-
moval of volatiles the residue was chromatographed over sil-
ica column using 25% EtOAc/CHCl3 as an eluent, resulting
in 1 g (70%) of the product as white solid with mp 66–68 ꢁC.
[a]2D5 39.5 (c 2.0, EtOH). IR (neat, cmꢂ1): 3388, 2937, 2863,
5.1.7. N-(3a-12a-Dihydroxy-24-nor-5b-cholan-23-yl) N-
methyl-N,N-dipropylammonium iodide (10). Yield:
75%. Mp 247–248 ꢁC. [a]D25 39.0 (c 1.0, EtOH). IR (KBr,
cmꢂ1): 3361, 2935, 2860, 1644, 1471, 1448, 1371, 1091,
1045. 1H NMR (300 MHz, DMSO-d6) d: 0.61 (s, 3H), 0.85
(s, 3H), 0.98 (d, J¼5.1 Hz, 3H), 1.01–1.80 (m, steroidal
CH, CH2, 37H), 2.94 (s, 3H), 3.16 (m, 4H), 3.78 (br s,
1H), 4.24 (d, J¼3.6 Hz, 1H), 4.46 (d, J¼3.6 Hz, 1H). 13C
NMR (75 MHz, DMSO-d6) d: 10.55, 12.30, 15.16, 17.34,
1
1598, 1448, 1360, 1176, 1038, 916, 813, 737. H NMR
(300 MHz, CDCl3) d: 0.64 (s, 3H), 0.90 (s, 3H), 0.92 (d,

7318
S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
merged, 3H), 0.97–1.83 (m, steroidal CH, CH2, 29H), 2.45
(s, 3H), 3.61 (m, 1H), 3.96 (br s, 1H), 4.00 (m, 1H), 7.35
(d, J¼7.8 Hz, 2H), 7.76 (d, J¼7.5 Hz, 2H). 13C NMR
(75 MHz, CDCl3) d: 12.69, 17.41, 21.64, 23.13, 23.65,
25.67, 26.13, 27.14, 27.52, 28.63, 30.40, 31.36, 33.60,
34.11, 35.06, 35.24, 36.01, 36.39, 42.07, 46.43, 47.33,
48.19, 71.19, 71.69, 73.01, 127.86, 129.81, 133.25, 144.65.
LRMS: calcd for C31H48O5S+Na: 555.33, obsd 556.0. Anal.
Calcd for C31H48O5S: C 68.72, H 9.11. Found: C 69.00,
H 8.81.
(70%) as white solid. Mp 283–284 ꢁC. IR (KBr, cmꢂ1):
3395, 2949, 1465, 1080, 1050.1H NMR (300 MHz,
DMSO-d6) d: 0.61 (s, 3H), 0.86 (s, 3H), 0.92 (d, J¼
5.2 Hz, 3H), 1.03–1.91 (m, steroidal CH, CH2, 29H), 3.00
(br s, 6H), 3.24 (br s, 6H), 4.47 (br s, 1H). 13C NMR
(75 MHz, DMSO-d6) d: 11.83, 18.54, 20.39, 23.28, 23.79,
26.18, 26.74, 26.87, 27.59, 30.37, 33.43, 34.20, 35.12, 35.37,
36.29, 41.47, 42.33, 44.71, 51.39, 55.00, 56.08, 61.36, 69.83.
LRMS: calcd for C29H51N2O2: 443.1, obsd 443.5. Anal.
Calcd for C29H51N2O2I: C 61.04, H 9.01, N 4.91. Found:
C 59.72, H 8.86, N 4.45.
5.1.11. N1-(3a-12a-Dihydroxy-5b-cholan-24-yl) diaza-
bicyclo[2.2.2]octanyl iodide (15). To a refluxing suspension
of 3a-12a-dihydroxy-5b-24-(4-toluenesulfonyloxy) chol-
ane (1.0 g, 1.87 mmol) in CH3CN (10 mL), diazabi-
cyclo[2.2.2]octane (0.42 g, 3.75 mmol) was added and
refluxed for 6 h. The solvent was evaporated and the crude
product was purified by reprecipitating the alcoholic solu-
tion repeatedly with diethyl ether as mentioned earlier. Am-
berlite IRA 400 (iodide form) was used to exchange tosylate
by iodide. The pure product weighed 1.04 g (83%) as white
solid. Mp 299–300 ꢁC. [a]2D5 39.0 (c 1.0, EtOH). IR (KBr,
cmꢂ1): 3454, 2920, 2860, 1462, 1383, 1091, 1055, 1043,
5.1.14. N-(3a-7a-12a-Trihydroxy-24-nor-5b-cholan-23-
yl) diazabicyclo[2.2.2]octyl iodide (18). To a refluxing
suspension of 24-nor-23-iodo-3a-7a-12a-trihydroxy-5b-
cholane22b (1.0 g, 2.04 mmol) in CH3CN (10 mL), diazabi-
cyclo[2.2.2]octane (0.45 g, 4.0 mmol) was added and
refluxed for 6 h. The solvent was evaporated and the crude
product was purified by reprecipitating the alcoholic solu-
tion repeatedly with diethyl ether as mentioned earlier.
The pure product weighed 0.74 g (65%) as white solid.
Mp 293–294 ꢁC. [a]D25 19.0 (c 1.0, EtOH). IR (KBr,
cmꢂ1): 3398, 2948, 1669, 1636, 1466, 1078, 1051. 1H
NMR (300 MHz, DMSO-d6) d: 0.59 (s, 3H), 0.80 (s, 3H),
0.97 (d, J¼4.8 Hz, 3H), 1.01–2.26 (m, steroidal CH, CH2,
26H), 3.01 (br s, 6H), 3.42 (br s, 6H), 3.60 (br s, 1H), 3.76
(br s, 1H), 4.04 (br s, 1H), 4.17 (br s, 1H), 4.36 (br s, 1H).
13C NMR (75 MHz, DMSO-d6) d: 12.28, 17.38, 22.64,
22.74, 26.26, 26.93, 27.18, 28.54, 30.39, 33.69, 34.38,
34.93, 35.31, 41.49, 44.76, 45.75, 45.82, 51.38, 61.56,
66.18, 70.41, 70.92. LRMS: calcd for C29H51N2O3: 475.7,
obsd 475.4. Anal. Calcd for C29H51N2O3I+2H2O: C 54.54,
H 8.68, N 4.39. Found: C 54.92, H 8.36, N 4.15.
1
843. H NMR (300 MHz, DMSO-d6) d: 0.57 (s, 3H), 0.80
(s, 3H), 0.94 (d, J¼6 Hz, 3H), 1.12–1.69 (m, steroidal,
CH, CH2, 31H), 3.12 (m, 6H), 3.41 (m, 6H), 3.83 (br s,
1H). 13C NMR (75 MHz, DMSO-d6) d: 12.51, 17.19,
18.20, 23.12, 23.68, 26.00, 27.25, 28.62, 30.00, 32.00,
32.95, 33.83, 35.02, 35.26, 35.68, 36.42, 41.60, 43.59,
46.02, 46.25, 47.50, 50.67, 63.71, 69.93, 71.01. LRMS:
calcd for C30H53N2O2: 473.5, obsd 473.4. Anal. Calcd for
C30H53N2O2I+1.5H2O: C 57.38, H 8.99, N 4.46. Found: C
57.48, H 8.70, N 4.36.
5.1.12. N-[3a-12a-Dihydroxy-5b-cholan-24-yl]-N-meth-
ylpyrrolidinium iodide (16). To a refluxing suspension of
5.2. General
3a-12a-dihydroxy-5b-24-(4-tolunesulfonyloxy)
cholane
All steady state fluorescence experiments were performed
using a Perkin–Elmer LS-50B luminescence spectrometer
with a built-in system for polarization of the excited and
emitted lights and calculation of grating factor and were ex-
pressed as anisotropy. The grating factor G, which is the ratio
of the sensitivity of the detecting system for the vertically
and horizontally polarized lights, was calculated prior to
every individual anisotropy measurement. All fluorescence
spectra were recorded in 2 mm or 5 mm quartz cuvettes.
(0.5 g, 0.94 mmol) in acetonitrile (10 mL), N-methylpyrroli-
dine (0.2 mL, 2.3 mmol) was added and kept at 70 ꢁC for an
additional 5 h. The crude product was purified by column
chromatography using 20–80% ethanol/chloroform as an el-
uent giving 0.4 g (68%) of the pure salt as a white solid. Mp
258.8–259.0 ꢁC. [a]D25 41.0 (c 1.0, EtOH). IR (neat, cmꢂ1):
3411, 2934, 2862, 1636, 1466, 1044, 755. 1H NMR
(300 MHz, CDCl3) d: 0.60 (s, 3H), 0.83 (s, 3H), 0.96 (d,
J¼6 Hz, 3H), 1.01–1.75 (m, steroidal CH, CH2, 33H), 2.06
(br s, 4H), 2.96 (s, 3H), 3.79 (br s, 1H), 4.28 (br s, 1H),
4.50 (br s, H). 13C NMR (75 MHz, CDCl3) d: 12.51,
17.20, 20.04, 21.13, 23.12, 23.52, 26.12, 27.00, 27.31, 28.64,
30.25, 32.25, 32.95, 33.84, 35.09, 35.68, 36.28, 41.61, 46.04,
46.41, 47.52, 63.52, 63.39, 69.95, 71.08, 79.20. LRMS:
calcd for C29H52NO2: 446.74, obsd 447.0. Anal. Calcd for
C29H52 NO2I: C 60.72, H 9.14, N 2.45. Found: C 60.80, H
9.14, N 2.45.
5.3. CMC determination
Over 50 methods have been employed in the literature to de-
termine the CMC values of surfactant solutions. An invasive
method, which involved the use of hydrophobic dye pyrene
in to the bulk of the surfactant medium, was applied in the
current measurement.31 The concentration of the cationic
bile salt at which pyrene I3/I1 showed the first inflection
point was taken as CMC. In general, the cationic bile salts
showed CMC values between 2 and 4 mM in 0.5 M NaCl
solution depending on the head group, which was found to
be lower than sodium deoxycholate.32
5.1.13. N-(3a-Hydroxy-24-nor-5b-cholan-23-yl) diazabi-
cyclo[2.2.2]octyl iodide (17). To a refluxing suspen-
sion of 24-nor-23-iodo-3a-hydroxy-5b-cholane22b (1.0 g,
2.24 mmol) in CH3CN (10 mL), diazabicyclo[2.2.2]octane
(0.49 g, 4.4 mmol) was added and refluxed for 6 h. The sol-
vent was evaporated and the crude product was purified by
reprecipitating the alcoholic solution repeatedly with diethyl
ether as mentioned earlier. The pure product weighed 0.87 g
5.4. Gelation test
Gelation tests were carried out by dissolving a known
amount of the gelator in water or aqueous salt solution taken

S. Bhat, U. Maitra / Tetrahedron 63 (2007) 7309–7320
7319
in test tubes (d¼8 mm, l¼10 cm) upon heating. The clear
solution obtained was kept at room temperature without
any disturbance. Observations were recorded after 24 h.
viscoelastic regime of deformations and yield stress of the
gels was done by dynamic measurements at a frequency of
1 Hz of the oscillatory stress.
5.5. CGC measurements
Acknowledgements
Fluorescence anisotropy of ANS (1-anilino-8-naphthalene
sulfonate) was found to be more sensitive to gel formation.
Critical gel concentration (CGC) of the cationic bile salt
was determined by probing the fluorescence anisotropy of
ANS. Aqueous ANS solution (10 mM) was used to make
the gel/micelle. Fluorescence anisotropy was measured
against gelator concentration after thermostating the sam-
ples for at least 2 h. The CGC was determined by plotting
the fluorescence anisotropy of ANS as a function of bile
salt concentration. The concentration at which the ANS
anisotropy increased rapidly was taken as critical gel con-
centration (CGC). Binding of ANS fluorescent probe to mac-
romolecules depends on the surface cationic charge and
solution pH.33
The authors thank CSIR, New Delhi for financial support of
this work. Prof. A. K. Sood and Rajesh Ganapathy, Depart-
ment of Physics, IISc provided help and support with the
rheometer facility. The IISc Nanocenter initiative is thanked
for SEM facility. We also thank Dr. Pierre Terech (Grenoble,
France) for stimulating discussions.
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