Stimuli-responsive bile acid-based metallogels forming in aqueous media

Article abstract of DOI:10.1016/j.steroids.2014.10.001

The synthesis and gelation properties of a picolinic acid conjugated bile acid derivative in the presence of metal salts along with the stimuli-responsiveness of the systems are reported. The gels are formed in the presence of Cu²⁺ ions in the solvent systems composed of 30-50% of organic solvent (MeOH, acetonitrile, or acetone) in water. The gels respond to various stimuli: they can be formed upon sonication or shaking, and their gel-sol transformation can be triggered by a variety of chemical species. NMR, MS, and SEM techniques are exploited in order to gain a deeper insight on the self-assembled systems.

Full text of DOI:10.1016/j.steroids.2014.10.001

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Contents lists available at ScienceDirect
Steroids
journal homepage: www.elsevier.com/locate/steroids
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Stimuli-responsive bile acid-based metallogels forming in aqueous
media
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Q1 Virpi Noponen , Katri Toikkanen, Elina Kalenius, Riikka Kuosmanen, Hannu Salo, Elina Sievänen
University of Jyväskylä, Department of Chemistry, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
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Article history:
Received 1 July 2014
Received in revised form 9 September 2014
Accepted 1 October 2014
Available online xxxx
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The synthesis and gelation properties of a picolinic acid conjugated bile acid derivative in the presence of
metal salts along with the stimuli-responsiveness of the systems are reported. The gels are formed in the
presence of Cu ions in the solvent systems composed of 30–50% of organic solvent (MeOH, acetonitrile,
or acetone) in water. The gels respond to various stimuli: they can be formed upon sonication or shaking,
and their gel–sol transformation can be triggered by a variety of chemical species. NMR, MS, and SEM
techniques are exploited in order to gain a deeper insight on the self-assembled systems.
Ó 2014 Published by Elsevier Inc.
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Keywords:
Bile acid
Picolinic acid
Self-assembly
Metallogel
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Stimuli-responsive
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1. Introduction
achieved by either physical entrapment of noncoordinating metal-
lic elements or by exploiting the metal–ligand interactions as the
major driving force leading to the gel network formation. Metallo-
gels have been shown to be responsive to external stimuli and
exploit a range of spectroscopic, magnetic, catalytic, and redox
properties that are inherent to the metals they contain.
Steroids are widespread natural products having a large and
rigid steroidal nucleus combined with derivatizable functional
groups leading to an adjustable polarity profile, which makes them
attractive building blocks when designing novel low molecular
weight gelators [9]. Bile acids are a group of steroidal compounds
biosynthesized in the liver through several complementary path-
ways. Besides the rigid steroidal nucleus composed of four fused
rings they have a short aliphatic side chain containing a carboxylic
acid group in its end. In higher vertebrates the rings A and B of the
steroidal nucleus are in cis-fusion, causing curvature to the steroi-
dal nucleus, and thus making the molecules facially amphiphilic.
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The formation of highly ordered nanostructures from small
molecules through non-covalent interactions, such as hydrogen
bonding,
p–p stacking, metal coordination, solvophobic forces,
and van der Waals interactions, has widely been utilized as a bot-
tom-up approach to design and prepare nanostructured materials
with interesting properties [1,2]. Of these materials, supramolecu-
lar gels derived from low molecular weight gelators have attracted
ever increasing attention because of the wide range of applications
in which they may find use in our daily life. These applications
range from e.g., optoelectronics, light harvesting, and hybrid mate-
rials to tissue engineering, regenerative medicine, and drug deliv-
ery, as exhaustively reviewed several times [3–7].
The interest in the investigation of metal complexes as supra-
molecular metallogelators has been increasing only in the past
decade. The availability and the diversity of metal ligand coordina-
tion that could readily induce or control the self-assembly process
leading to the gel formation and thereby influence the properties of
the formed gel has raised the interest towards metallogels. Espe-
cially the utilization of transition metal complexes as metallogela-
tors has been found to lead to materials exhibiting interesting
optical, catalytic, and magnetic properties [8]. In general, the incor-
poration of the metallic elements into the gel matrix can be
The hydroxyl groups are located on the concave
a-face, whereas
the convex b-face possesses the three methyl groups [10]. Besides
as low molecular weight gelators, bile acids and their derivatives
have found use in pharmacology [11–13], asymmetric synthesis
and chiral discrimination [14], or as receptors for molecular and
ionic recognition [15,16], to name a few.
Herein, we report the synthesis and gelation properties of an
aminoethyl amide derivative of deoxycholic acid conjugated via
amide bond with picolinic acid (4, Scheme 1) in the presence of
⇑
Corresponding authors. Tel.: +358 408053710; fax: +358 142602501.
E-mail addresses: virpi.noponen@gmail.com (V. Noponen), elina.i.sievanen@jyu.
2+
Cu salts, along with the stimuli-responsiveness of the formed
metallogels. Gelation properties of a series of aminoalkyl amides
fi (E. Sievänen).
http://dx.doi.org/10.1016/j.steroids.2014.10.001
0
039-128X/Ó 2014 Published by Elsevier Inc.
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of different bile acids have been systematically studied by us lately
[17]. N-(2-aminoethyl)-3 ,12 -dihydroxy-5b-cholan-24-amide (3,
dihydroxy-5b-cholan-24-amide (5.3 mmol, 1 eq.) in dichlorometh-
ane (30 mL) was added dropwise to the freshly prepared picolinic
acid anhydride. The stirring was continued for 20 h on an oil bath
(40 °C). The crude product obtained after the evaporation of the vol-
atiles was dissolved in CHCl3 (100 mL) and washed with water
(2 Â 75 mL), 0.1 M HCl solution (2 Â 75 mL), water (75 mL), and
finally with brine (2 Â 75 mL). Then the yellowish organic layer
was dried (Na SO ), filtered, and the volatiles evaporated under
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a
a
Scheme 1) employed as a starting compound in the current study
was not, however, shown to be capable of self-assembly leading
to gelation in the conditions studied. Inspired by the free amino
group of the compound we were prompted to synthesize a new
conjugate with the desire of gaining better gelation capabilities.
Because picolinic acid (2, Scheme 1) is known to act as a chelating
agent in the human body, it was chosen as the compound to be
conjugated with the deoxycholyl derivative. Indeed, compound 4
2
4
reduced pressure. The crude product was purified by column chro-
matography (silica gel, CH Cl :MeOH 90:10) to yield the compound
2
2
2+
was shown to form gels in the presence of Cu ions in the solvent
systems composed of 30–50% of methanol, acetonitrile, or acetone
in water. Moreover, the gels were shown to respond to various
stimuli: they could be formed upon sonication or shaking, and
their gel–sol transformation could be triggered by a variety of
chemical species.
as a white solid.
1
1
1
1
00
01
02
03
2.3. Compound 4
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Yield 84%. 1H NMR (CDCl
2-CH), 8.38 (m, 1H, NH), 8.18 (d, 1H, 29-CH), 7.86 (m, 1H, 30-
, 500 MHz, ppm): d 8.56 (d, 1H,
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154
3
3
CH), 7.44 (m, 1H, 31-CH), 6.46 (m, 1H, NH), 3.94 (m, 1H, 12b-H),
3.61 (m, 3H, 3b-H + 26-CH ), 3.49 (m, 2H, 25-CH ), 2.24 (m, 1H,
/b-H), 2.09 (m, 1H, 23 /b-H), 1.890.96 (m, 26H), 0.95 (d, 3H,
21-CH ), 0.89 (s, 3H, 19-CH
26 MHz, ppm): d 174.3 (C-24), 165.6 (C-27), 149.5 (C-28), 148.1
1
1
04
05
2. Experimental
2
2
2
3a
a
1
3
2.1. Materials
3
3 3 3
), 0.63 (s, 3H, 18-CH ). C NMR (CDCl ,
1
1
1
1
1
1
1
1
06
07
08
09
10
11
12
Pyridine-2-carboxylic acid (1) (99%) was purchased from
Aldrich. Other reagents used in the synthetic steps as well as the
solvents used in chromatography and gelation studies were of ana-
lytical grade. Triethylamine and ethyl chloroformate were distilled
prior to use. The mixed anhydride method used in the preparation
of the target molecule and slightly modified by us has been
reported previously [18–20].
(C-32), 137.5 (C-30), 126.4 (C-31), 122.4 (C-29), 73.1 (C-12), 71.8
(C-3), 48.3 (C-14), 47.1 (C-17), 46.5 (C-13), 42.1 (C-5), 40.4 (C-
25), 39.4 (C-26), 36.5 (C-4), 36.0 (C-8), 35.3 (C-1), 35.2 (C-20),
34.1 (C-10), 33.7 (C-9), 33.4 (C-23), 31.6 (C-22), 30.5 (C-2), 28.7
(C-11), 27.4 (C6/7), 27.1 (C6/7), 26.2 (C-16), 23.6 (C-15), 23.1 (C-
+
19), 17.4 (C-21), 12.7 (C-18). ESI TOF MS: [M+Na] m/z = 562,
+
[M+K] m/z = 578. M.W. (C32
49
H N
3
O
4
) = 539.75. Elemental analy-
sis: Found C, 70.73; H, 9.13; N, 7.67. Calc. for C32
.25H O: C, 70.62; H, 9.17; N, 7.72.
49 3 4
H N O
0
2
1
13
2.2. Synthesis of compound 4
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Compound 4 was synthesized by conjugating the freshly pre-
pared pyridine-2-carboxylic acid anhydride (2) with N-(2-amino-
2.4. NMR spectroscopy
155
ethyl)-3a,12
a-dihydroxy-5b-cholan-24-amide
(3)
prepared
1H, 13C, 13C DEPT-135, and 2D PFG 1H,13C HMQC and HMBC
NMR spectra used for characterization of the prepared compound
was recorded with a Bruker Avance DRX 500 MHz spectrometer
equipped with a 5 mm diameter broad band inverse detection
probehead operating at 500.13 MHz in H and 125.77 MHz in
experiments, respectively. The H NMR chemical shifts are refer-
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according to the previously reported synthetic protocol (Scheme 1)
[17].
Synthesis of compound 4 was performed in N -atmosphere. In a
2
1
13
round-bottomed three-necked 250 mL flask picolinic acid (pyri-
C
1
dine-2-carboxylic acid; 5.3 mmol, 1 eq.) and dry dichloromethane
(40 mL) were cooled on an ice-water bath to +10 °C, after which tri-
ethylamine (6.9 mmol, 1.3 eq.) was added to the solution from a
dropping funnel, followed by a dropwise addition of ethyl chlorofor-
mate (6.9 mmol, 1.3 eq.) in dichloromethane (3 mL). The mixture
enced to the signal of residual CHCl (7.26 ppm from internal
3
1
3
TMS). The C NMR chemical shifts are referenced to the centre
peak of the solvent CDCl (77.0 ppm from internal TMS). A compos-
3
ite pulse decoupling, Waltz-16, has been used to remove proton
1
3
13
was stirred at rt for 40 min, after which N-(2-aminoethyl)-3
a,12a
-
couplings from C NMR spectra. Assignment of the C NMR
Scheme 1. Synthesis route leading to compound 4.
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chemical shifts has been made with the help of 2D NMR spectra
and literature [21].
the sample stub, and after drying in air for two days, coated with
gold in a JEOL Fine Coat Ion Sputter JFC-1100.
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1
69
2.5. Gelation studies
2.7. Mass spectrometry
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Gelation tests for compound 4 in the presence of metal salts
were performed as follows: 5 mg of compound 4 was weighed in
The mass spectrometric experiments were performed with a
QSTAR Elite ESI-Q-TOF mass spectrometer equipped with an API
00 TurboIonSpray ESI source from AB Sciex (former MDS Sciex)
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a test tube and 150–250
After that, depending on the solvent composition, 50–225
deionized water was added. Finally, depending on the desired
ligand:metal molar ratio (2:1, 1:1, or 1:2), 125 L or 250 L of
aqueous metal salt solution was added, resulting in the total sol-
vent volume of 500 L. The obtained suspension or solution was
lL of MeOH/MeCN/acetone was added.
2
lL of
in Concord, Ontario (Canada). The gel samples were prepared by
2+
utilizing ligand:Cu molar ratio of 3:1 and 1:1 and 30% of MeOH
l
l
as the co-solvent. In order to investigate the effect of the counter
anion both CuSO
4 2 3 2 2
5H O and Cu(NO ) 3H O were exploited as
l
the copper source. For the measurements, the gel-phase samples
then subjected to ultrasound for ca. 1 min and heated with a heat
gun until a clear mixture was obtained. The solution was allowed
to cool, after which observations with regard to gelation were
made. The test tube was inverted, and if the solid mass did not
move, it was defined as a gel. If the gel was formed already while
sonicating, no heating was performed.
were dissolved and diluted in MeCN to obtain 9.25
for measurements.
lM samples
3. Results and discussion
203
Compound 4 was synthesized with an overall yield of 84% by
modifying the well-known synthesis method reported in the liter-
ature (Scheme 1). Conventional analysis methods were utilized in
order to prove the identity, and the full data are provided in the
experimental section. Compound 4 itself did not form gels in any
of the common solvents tested; in general, it was shown to be
rather poorly soluble in aromatic solvents, but then apparently
too soluble in more polar solvents.
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1
84
2.6. Microscopy
1
1
1
1
1
85
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89
Scanning electron micrographs were taken with Bruker Quan-
tax400 EDS microscope equipped with a digital camera. Samples
of the xerogels were prepared either by placing a hot, clear solution
of the gelator and metal salt in an appropriate solvent mixture or a
scooped sample of the preformed gel on carbon tape placed over
Table 1
2
+
Q2 Gelation properties of varying 4:Cu molar ratios in aqueous solvents.
Solvent composition
Ligand:Cu2+ molar ratio
2:1
1:1
1:2
CuSO
4
Á5H
2
O
MeOH 50%
MeOH 40%
MeOH 30%
Gs
Gs
Gs
S
Gs
Gs
S
Gs
Gs
MeCN 50%
MeCN 40%
MeCN 30%
S
S
G
S
S
G
S
S
G
t
Acetone 50%
Acetone 40%
Acetone 30%
Gs ? C
Gs
Gs
PG ? C
Gs
Gs
PG ? C
Gs ? C
Gs
Cu(NO )
3 2
Á3H
2
O
MeOH 50%
MeOH 40%
MeOH 30%
S
S
PG
S
S
S ? PG
Gs
S ? PG
Gs
MeCN 50%
MeCN 40%
MeCN 30%
S
S
S
S
S
S
S
S
S
Acetone 50%
Acetone 40%
Acetone 30%
S
S
S
S
S
G
S
S
Gs
t
G = gel formed after heating and cooling; Gs = gel formed after sonication; G = transparent gel formed after heating and cooling; PG = partial gel; PG = partial weak gel;
S = clear blue solution; C = crystallizes.
4
Fig. 1. Sonication-induced gel formation of compound 4 with CuSO (1:1) in aqueous MeOH (30%).
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Compound 4 bearing a pyridine group, a classic metal-coordina-
co-solvent according to the Table 1. Since hydrogels are better
compatible with bodily tissues than organogels, the water content
of the solvent mixture was increased to as high a level as possible.
If it exceeded 70% precipitation instead of gel formation occurred.
The counter anion of the copper salt was shown to play a role in
the gel formation. The different properties of the copper salts, for
example in terms of complex formation in aqueous solutions,
may have affected on the behavior of the systems. In the studied
conditions, CuSO Á5H O was shown to promote gel formation more
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tion site, along with the aminoethyl amide linker between the
deoxycholic acid and picolinic acid was anticipated to be able to
interact with metal cations. Intrigued by the possibility of obtain-
2
+
+
2+
2+
ing metallogels Cu , Ag , and Zn cations were tested. Cu was
shown to give the most promising results in the preliminary tests,
and was thus chosen to be the metal cation concentrated on. Three
2
+
different Cu
salts, namely CuSO Á5H O, Cu(NO ) Á3H O, and
4
2
3 2
2
CuCl Á2H O were employed. The gels were shown to be formed
2
2
4
2
in water with 50–30% of methanol, acetonitrile, or acetone as the
often than Cu(NO ) Á3H O. Additionally, in the case of Cu(NO )
3
2
2
3 2-
Fig. 2. Appearance of the samples with ligand:Cu2+ molar ratio of 1:1 in aqueous solvents (30% of organic solvent).
Fig. 3. Illustration showing the gel–sol transformation of Cu2+-gels of compound 4 by the addition of external chemical stimuli.
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Fig. 4. Illustration of the reversible gel–sol transformation induced by the reduction of Cu2+ to Cu+ by ascorbic acid and subsequent oxidation back to Cu2+ by HNO
.
3
2+
Fig. 5. SEM images of the xerogels of compound 4 formed in the presence of Cu . Samples prepared by scooping a portion of the preformed gel on a sample stub. (A)
Ligand:Cu2 2:1 gel in MeOH, (B) Ligand:Cu 2:1 gel in MeCN, (C) Ligand:Cu 2:1 gel in acetone, (D) Ligand:Cu 1:1 gel in MeOH, (E) Ligand:Cu 1:1 gel in MeCN, (F)
+
2+
2+
2+
2+
Ligand:Cu2 1:1 gel in acetone. Scale bars: (A) 10
+
lm, (B) 20
l
m, (C) 2
l
m, (D) 2
l
m, (E) 20
lm, (F) 2
l
m.
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+
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Á3H O, in most of the cases ligand:Cu molar ratio of 1:1 or 1:2
solution first became opaque and finally after some shaking a light
green solution with some green precipitate was obtained. In order
to oxidize Cu again to Cu 20 lL of 0.5 M nitric acid was added to
the solution, which was then heated until a clear light blue solution
was obtained. This solution was finally sonicated in order to revive
the gel.
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301
2
was required for the gel formation, and the gels were only formed
in rather high water concentrations. However, the gels formed
+
2+
with Cu(NO ) Á3H O were shown to be rather stable, whereas the
3
2
2
gels formed with CuSO Á5H O were shown to crystallize/precipi-
4
2
tate moderately quickly. CuCl Á2H O, however, was only able to
2
2
give clear blue solutions; no gels were detected. Interestingly, very
recently D zˇ oli c´ and Cametti with their co-workers reported the
metallogel formation of pyridyloxalamide in alcohols in the pres-
In order to gain some visual insight into the microscopic mor-
phology of the dried self-assembled materials the scanning elec-
tron microscopy (SEM) was utilized. The xerogel samples for the
SEM were prepared by following two different protocols: in the
first one, the gel was formed by the heating–cooling cycle and
allowed to stabilize, after which the preformed gel was scooped
onto a sample stub covered with the carbon tape (Fig. 5). In the
second experiment, the clear hot solution containing the ligand,
copper salt, and the aqueous solvent was pipetted onto the sample
stub and allowed to dry (Fig. 6). As expected, and observed by us
previously [23], as well, differences in the sample appearance
between these two sample preparation protocols were observed.
The sample appearance, however, was shown to be independent
2+
ence of only CuCl of several other Cu salts tested [22].
2
Gelation was shown to take place by solely sonication (denoted
Gs in Table 1, Fig. 1) in several cases. If sonication was not suffi-
cient enough in order to promote gelation, the traditional heating
and cooling-cycle was performed. Interestingly, in some cases the
gelation of the clear blue solution obtained after the heating and
cooling/settling period of one day could be induced by vigorous
shaking. Similar phenomenon of mechanically triggered gel forma-
tion has been recently reported by us [23] and others [24]. In most
of the cases rather translucent light blue gels were obtained
(Fig. 2).
The gel systems obtained were shown to act as responsive soft
materials responding to a variety of chemical stimuli (Figs. 3 and
4). Three drops of pyridine, triethylamine, or aqueous ammonia
or small amount of solid disodium salt of EDTA (ethylenediamine
tetra-acetic acid) were added onto the surface of a preformed gel.
The upper layer of the gel was immediately shown to transform
into a solution, and after a gentle agitation, the gel–sol transforma-
tion was completed. In the case of pyridine an intensively blue-col-
ored clear solution was obtained (Fig. 3).
2
+
of the ligand:Cu molar ratio, or the solvent system used.
2
+
From the gelation experiments it was clear that Cu was essen-
tial for gel formation. The role of the cation in the process was fur-
1
ther investigated by utilizing NMR spectroscopy. First, the H NMR
spectra were measured in absence of the cation for samples of
0.17%, 0.5%, and 1% in concentration (4 in MeOD:D O mixture).
2
The spectra appeared well-resolved. Increasing of the concentra-
tion did not have any particular effect on the values of the chemical
shifts. From the spectra it was obvious that there was no gel
formed. When the cation was added to the sample solutions, the
overall resolution and S/N in all spectra dramatically decreased.
The most probable explanation is, naturally, the paramagnetic nat-
The gels also showed reversible redox-responsiveness (Fig. 4):
the Cu ions could be reduced to Cu by adding ascorbic acid
(1 eq. in 20 L of water) onto the preformed gel. The upper layer
of the gel was immediately shown to transform into solution. In
order to speed up the process the sample was heated until a clear
yellow solution was obtained. When cooled down, the clear yellow
2
+
+
2
+
l
ure of Cu .
2
+
The spectra were measured for ligand:Cu 1:1 samples of
0.17%, 0.5% (gel), and 1% (gel) in concentration (4:Cu2+ in
MeOD:D O mixture). Increasing of the concentration resulted in
2
Fig. 6. SEM images of the xerogels of compound 4 formed in the presence of Cu2+. Samples prepared from a hot clear solution. (A) Ligand:Cu2+ 2:1 in MeOH, (B) Ligand:Cu2+
2
+
1
:1 in MeOH, (C and D) Ligand:Cu 1:1 in acetone. Scale bars: (A) 2 lm, (B) 20 lm, (C) 20 lm, (D) 2 lm.
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Fig. 7. Samples with 1% of 4 in MeOD with varied ligand:Cu2+ molar ratios: 10:1 (undermost) ? 10:2 (5:1) ? 10:3 ? 10:4 (5:2) ? 10:5 (2:1) uppermost.
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gel formation, which was evident from the ever broadening signals
measurements, the gel samples were further dissolved and diluted
in MeCN to obtain 9.25 lM samples for measurements.
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1
of H NMR spectra due to restriction of molecular tumbling as a
2+
result of gel formation. The ligand:Cu molar ratio was further
increased to 1:2, and the measurements repeated for the same
set of concentrations. The appearances of the spectra were close
to identical, when compared with the ligand:Cu2 molar ratio of
1:1. When the temperature was increased from 30 °C to 60 °C in
Unfortunately, for compound 4 the results were not straightfor-
ward. For each sample, a collection of different ligand:metal
adducts were observed in the spectrum. While copper undoubtedly
is essential to gel formation, based on the mass spectrometric
experiments a certain ligand:copper complex inducing the gel for-
mation is not likely – rather several different adducts synergetical-
ly end in gelation.
+
2+
5 °C temperature steps for the 1% sample of ligand:Cu in a molar
ratio of 1:2, the breakdown of the gel network could be deduced
from the ever improving resolution of the 1H NMR spectra. The
pyridine protons became visible and were observed to move
towards the upper field region as the gel was melting (i.e., the tem-
perature was increased). This indicates pyridine playing an impor-
tant role in the process leading to gel formation, most probably by
4
. Conclusions
352
In this work we report the synthesis and structural analysis of
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2
+
an aminoethyl amide derivative of deoxycholic acid conjugated
via amide bond with picolinic acid. Furthermore, the compound
is shown to be able to form gels in the presence of Cu2+ ions in
the solvent systems composed of 30–50% of methanol, acetonitrile,
or acetone in water. The formed gels are shown to respond to var-
ious stimuli: they can be formed upon sonication or shaking, and
their gel–sol transformation can be triggered by a variety of chem-
ical species. The gels also show reversible redox-responsiveness:
chelating Cu ions. When the gel-forming network breaks down
2
+
the Cu cation does not interact with pyridine as intensively,
and consequently withdrawal of the pyridine electrons by the
positively charged cation diminishes. This results in decreased
deshielding, which can be seen as movement of the chemical shift
to the upper field.
Next, the molar ratio of copper to ligand was increased from
1:10 to 1:2. Due to the simultaneous effects of unfavorable NMR
properties of copper and restricted molecular tumbling due to gel
formation, not much but the decrease in resolution as a conse-
quence of the increasing amount of copper could be observed.
The measurements were further repeated in pure MeOD, a solvent
which did not induce gel formation. Paramagnetic properties of
2
+
+
the Cu ions can be reduced to Cu followed by gel–sol transition.
+
2+
Oxidation of the Cu ions again to Cu revives the gel. The
2+
essential role of the pyridine moiety chelating the Cu ions in
the process leading to gel formation was proved by NMR experi-
ments. Mass spectrometry and SEM techniques were used in fur-
ther characterization the systems.
2
+
Cu affected on the appearance of the spectrum, but when the
2
+
The important physiological roles of bile acids and picolinic acid
combined with the formation of stimuli-responsive gels in aqueous
media responding to several stimuli may in the future lead to fas-
cinating biomedical applications, especially due to the neuropro-
tective, immunological, and anti-proliferative activities of
picolinic acid.
amount of Cu was increased, the pyridine protons broadened
and moved to downfield direction (Fig. 7). This further confirms
the role of pyridine chelating copper as the key process in the gel
formation. A tendency of the 12 b proton on the steroidal skeleton
to move to downfield direction was observed as well.
In addition to the role we were also interested in the coordina-
tion degree of copper in the current gel-forming system. Encour-
aged by the work of Bunzen and coworkers [25], the complex
responsible for gelation was tried to be determined by using mass
Acknowledgements
374
2
+
spectrometry. The samples were prepared by using ligand:Cu
The National Graduate School of Organic Chemistry and Chem-
ical Biology (V.N.), University of Jyväskylä (V.N.), and The Academy
of Finland (project Nos. 119616, 218178, and 255648 (E.S., K.T. and
V.N.)) are acknowledged for financial support. Professor Erkki
375
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378
molar ratios of 3:1, 2:1, and 1:1 and 30% of MeOH as the co-sol-
vent. In order to investigate the effect of the counter anion both
sulfate and nitrate were exploited as the copper source. For the
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[
[
[
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Kolehmainen is thanked for valuable collaboration. The authors are
grateful to M.Sc. Esa Haapaniemi (NMR spectroscopy) and Lab.
Tech. Elina Hautakangas (elemental analysis) for technical assis-
tance and measurements. The Department of Biological and Envi-
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Please cite this article in press as: Noponen V et al. Stimuli-responsive bile acid-based metallogels forming in aqueous media. Steroids (2014), http://
dx.doi.org/10.1016/j.steroids.2014.10.001