A favorable, narrow, δh Hansen-parameter domain for gelation of low-molecular-weight amino acid derivatives

Article abstract of DOI:10.1002/chem.201101423

In recent years, the design of new low-molecular-weight gelators (LMWGs) has attracted considerable attention because of the interesting supramolecular architectures as well as industrial applications. In this context, the role of the organic solvent in d

Full text of DOI:10.1002/chem.201101423

FULL PAPER
DOI: 10.1002/chem.201101423
A Favorable, Narrow, dh Hansen-Parameter Domain for Gelation of Low-
Molecular-Weight Amino Acid Derivatives
Pasquale Curcio, Florent Allix, Guillaume Pickaert, and Brigitte Jamart-Grꢀgoire*[a]
Abstract: In recent years, the design of
new low-molecular-weight gelators
(LMWGs) has attracted considerable
attention because of the interesting
supramolecular architectures as well as
industrial applications. In this context,
the role of the organic solvent in deter-
mining the organogelation behavior is
a central question. Herein we report
the results of a systematic study of the
organogelation behavior of amino acid
derivatives in a wide range of solvents
to establish a relationship between the
nature of the solvent and the formation
of the gel. We highlight that the major-
ity of the gelified solvents are aromatic,
except for carbon tetrachloride and tet-
rachloroethylene. In addition, different
parameters related to the nature of the
solvent were considered and their in-
fluence on the physical properties of
gelation was evaluated. The hydrogen-
bonding Hansen parameter (dh) allows
us to draw
domain for gelation in the range of
0.2–1.4 (calcmꢀ3)1/2. Furthermore,
a narrow favorable dh
a
general increase of the Hildebrand pa-
rameter (d) leads to the formation of
poor gels (small gelation numbers,
GNs) in aromatic solvents. Scanning
electron microscopy (SEM) revealed
that the gels prepared from (l)-phenyl-
alanine and (l)-leucine derivatives in
different solvents are composed of an
entangled 3D fibrillar network, the di-
ameter of which is only slightly influ-
enced by the nature of the solvent.
Keywords: amino acids · fibrillar
networks · gels · Hansen parame-
ters · solvent effects
Introduction
van der Waals interactions, etc), the gels obtained are desig-
nated as supramolecular or physical gels.[6,7f]
Gels represent an interesting colloidal state of matter that is
present in many important consumables: cosmetics,[1] plastic
surgery,[2] contact lenses,[3] nanoscale electronics,[4] biomedi-
cal materials[5] and so forth. Generally, the solidlike phase of
gels is the result of solvent immobilization in 3D fibrillar
networks, which are, in many cases, made from polymeric
matrices.[6]
An alternative class of gels derives from self-assembly of
organic low-molecular-weight gelator molecules (LMWG
molecules: molecular mass usually <2000 gmolꢀ1) at very
low concentration.[6] These small molecules are able to
gelify a wide range of liquids, either organic (organogels) or
aqueous solvents, and even pure water (hydrogels).[7] Fur-
thermore, the existence of so-called ambidextrous LMWGs,
which are derived from vitamin C and amino acids and are
able to immobilize both organic and aqueous solvents, has
also been reported.[8]
Over the past ten years, this kind of organogels has at-
tracted much attention among scientists, as witnessed by the
exponential number of papers published in various jour-
nals.[6] This trend is justified by the fact that the intriguing
properties of gels allow the use of these supramolecular ma-
terials in various areas of chemistry and biology, such as sen-
sors,[9] drug-delivery agents,[10] biomimetics,[11] and in the
field of catalysis.[12] They can also be used to improve me-
chanical properties of soft solids.[13] A preliminary analysis
shows that the structures of LMWGs can be of different na-
tures.
Well-known organogelators include cholesterol deriva-
tives,[14] porphyrins,[15] carbohydrates,[16] urea/bisurea com-
pounds,[17] and oligoamides.[18] Moreover, several examples
of small amino acid derivatives have been reported in the
literature.[19] These molecules have the interesting ability to
self-assemble through highly specific noncovalent interac-
tions into long fibrous structures, which in turn form an in-
tertwined 3D network that is able to immobilize the sol-
vent.[6] This characteristic has allowed significant progress in
the understanding of gel-formation mechanisms, even
though many aspects of these phenomena, such as the influ-
ence of chemical structure of the gelator or the precise role
of the solvent in the organogelation, still remain un-
known.[20] It appears that the nature of the solvent has a fun-
damental importance in the delicate equilibrium that leads
to the formation of gel; that is why some attention has been
recently focused on the study of solvent–gelator interac-
tions.[21] Different parameters that characterize solvent
If 3D self-assembly of organogelators takes place through
noncovalent interactions (hydrogen bonds, p–p stacking,
[a] Dr. P. Curcio, Dr. F. Allix, Dr. G. Pickaert,
Prof. Dr. B. Jamart-Grꢀgoire
Laboratoire de Chimie-Physique Macromolꢀculaire
UMR CNRS-INPL 7568, Nancy-Universitꢀ
ENSIC, 1 rue Grandville, BP 20451
54001 Nancy Cedex (France)
Fax : (+33)383-36-83-36
E-mail: Brigitte.Jamart@ensic.inpl-nancy.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101423.
Chem. Eur. J. 2011, 17, 13603 – 13612
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13603

nature, such as solubility (Hildebrand, d), polarity
(ET(30)),[22] and the dielectric constant (e), were taken into
account and their influences on the physical properties of
gelation were highlighted. Very recently, Smith and co-work-
ers reported a study that promotes better understanding of
the role of the solvent in the gelation process.[23] They con-
sidered the solvent effect in terms of Kamlet–Taft parame-
ters.[24] They were able to establish, for their own class of
gels, based on a self-assembly process through hydrogen
bonding of the gelator molecules, that the a parameter of
the solvent (hydrogen-bond donor ability) had primary im-
portance in the control of the formation of hydrogen-bond
networks.
During the last few years, our research group has studied
a family of new l-amino-acid-type gelators with carboxyben-
zyl (Z) as the amine-protecting group and a naphthalimide
moiety as the fluorescent chromophore.[25a] Different spec-
troscopic techniques, such as NMR, FT-IR, and fluorescence
spectroscopy as well as circular dichroism (CD) have been
used to elucidate the detailed structures of supramolecular
gels obtained with (l)-phenylalanine (1a, Scheme 1a) and
(l)-leucine (1b, Scheme 1a) derivatives in aromatic solvents,
Results and Discussion
Synthesis: It is difficult to predict the gelation ability of a
molecule, and almost all small gelators were discovered by
accident. In most cases, new compounds described in the lit-
erature[6,7] are the result of fine structural changes of a gela-
tor lead molecules. Therefore, we decided to synthesize new
potential organogelators inspired by those previously discov-
ered by our research group.[25a] Thus, we have synthesized
compounds derived from (l)-alanine, (l)-valine, and (l)-iso-
leucine (1c–e respectively, Scheme 1a), which can be seen
as the result of small modifications of the aliphatic residue
of the (l)-leucine-based gelator 1b.
Subsequently, we hypothesized that replacing phenylala-
nine with a tryptophan residue (1 f, Scheme 1a) would in-
crease the aromatic part of lead-molecule 1a and would in-
crease the gelation ability through additional p–p stacking
interactions.[26]
Compounds 2a and 2b (Scheme 1b), enantiomers of 1a
and 1b, respectively, were also synthesized to check the in-
fluence of the a-CH chirality on the gelation properties in
achiral solvents.
Finally, to extend the results (see the section on solvent-
parameters analysis), we synthesized compounds 3a and 3b
(Scheme 1c) with a tert-butyl carbamate group (Boc) and 4a
(Scheme 1d) with a 9-fluorenylmethyloxycarbonyl group
(fmoc) instead of the carboxybenzyl (Z) amine protecting
group.
Preliminary organogelation behavior: The new synthesized,
potential gelators, listed in Scheme 1a and b, were screened
for gelation ability in 33 selected solvents: 10 aromatic sol-
vents, 2 linear and cyclic alkanes, 5 chlorinated solvents, 2
alcohols, 3 esters, 3 ethers, and 3 ketones. The class called
“others” included 4 solvents, such as dimethyl sulfoxide and
water, which do not match any other classes. The gelation
behaviors are classified as: insoluble (I) if the gelator is
completely insoluble in the solvent; precipitate (P) if the ge-
lator is soluble in hot solution, but precipitates in cold solu-
tion; soluble (S) if the gelator is completely soluble in the
solvent, and gel (G) if the gelator is able to gelify the sol-
vent. The results obtained are listed in Table 1.
Scheme 1. Structures of the amino acid derivatives, for which the synthe-
sis is described herein.
On first inspection of the data, it appears that only phe-
nylalanine 1a and leucine 1b derivatives are able to gelify
some of the tested solvents. (l)-Alanine 1c, (l)-valine 1d,
(l)-isoleucine 1e, and (l)-tryptophan 1 f derivatives are in-
soluble or precipitate under the same conditions (Table 1).
As expected, the d-enantiomers 2a and 2b, exhibit the
same behavior as the corresponding l-gelators 1a and 1b in
achiral solvents (Table 1). We performed circular-dichroism
experiments, which showed an inversion of chirality from
one enantiomer to the other (see the Supporting Informa-
tion).
such as toluene.[25b,c] These compounds are highly versatile
as they contain a lateral chain, a chiral center, and protect-
ing groups and allow the synthesis of a large number of ana-
logs. Actually, Hildebrand (d) and Hansen (dd, dp, and dh)[22]
parameters have been used to analyze gelation strength.
Herein, based on this new series of compounds we propose
the use of Hildebrand and Hansen parameters, a valuable
alternative to the Kamlet–Taft parameters, to predict gela-
tion behavior.
The second important observation concerns the nature of
the solvent. We notice that most of the gelified solvents are
aromatic; this might suggest a primary importance of the p–
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Chem. Eur. J. 2011, 17, 13603 – 13612

Amino Acid Gelation
FULL PAPER
Table 1. Gelation behavior for 1a–f in different solvents[a]
.
Class of
solvent
Solvents
(l)-Phe
(1a)
(l)-Leu
(1b)
(d)-Phe
(2a)
(l)-Ala
(1c)
(l)-Val
(1d)
(l)-Ile
(1e)
(l)-Trp
ACHTUNGTRENNUNG(1 f)
G
G
R
G
N
U
benzene
toluene
p-xylene
G (0.16)
G (0.17)
G (0.18)
G (0.18)
G (0.12)
G (0.30)
G (0.6)
G (0.18)
G (0.66)
G (0.30)
G (0.25)
G (0.42)
G (0.11)
G (0.5)
G (>2)
G (0.18)
G (0.66)
G (0.30)
G (0.25)
G (0.42)
G (0.11)
G (0.5)
G (2)
G (0.18)
I
P
I
I
I
I
I
P
I
I
P
P
P
I
I
I
P
I
P
S
I
I
I
P
I
I
ethylbenzene
p-diethylbenzene
o-diethylbenzene
chlorobenzene
tetraline
1-methylnaphthalene
nitrobenzene
n-nctane
aromatic
I
I
I
P
P
P
S
I
P
P
P
S
I
S
P
P
S
I
G (1.3)
S
I
S
S
I
S
S
I
alkanes
cyclohexane
I
I
I
I
I
I
I
carbon tetrachloride
tetrachloroethylene
1,2-dichloroethane
dichloromethane
chlorform
I
G (0.2)
G (0.05)
I
I
I
I
I
I
I
I
I
I
I
G (0.06)
G (0.06)
chlorinated
P
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
S
I
P
S
S
S
S
S
S
S
I
P
S
S
S
S
S
S
S
I
P
S
S
S
S
S
S
S
I
P
S
S
S
S
S
S
S
I
I
methanol
ethanol
S
S
S
I
I
I
alcohols
esters
methyl acetate
ethyl acetate
ethyl propionate
diethyl ether
diisopropyl ether
tetrahydrofuran
acetone
4-methyl-pentan-2-one
cyclopentanone
dimethyl sulfoxide
acetonitrile
ethers
I
I
I
I
I
I
I
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
I
S
S
S
S
S
S
S
I
S
S
I
S
S
P
S
I
ketones
others
dimethylformamide
water
[a] G(x): gel (critical gelation concentration (CGC) in wt%); S: soluble; I: insoluble; P: precipitate.
p stacking interactions between the gelator and the solvent.
The only exceptions to this trend are carbon tetrachloride
and tetrachloroethylene. Compound 1b is able to gelify the
first one, whereas gelator 1a form gels in both. Finally, gela-
tors 1a and 1b are soluble in all other solvents tested,
except in diethyl ether, diisopropyl ether, and water, in
which they precipitate.
We tried to apply this theory to our gels, but, unfortunate-
ly, we realized that the a parameter alone cannot explain
the behavior of our compounds 1a and 1b towards the
tested solvents. Indeed, as shown in Figure 1, many solvents
with an a parameter of zero are not gelified by gelators 1a
and 1b, which are soluble or insoluble in the considered sol-
vents.
To understand the solvent effects on gelation, we decided
to consider the Hildebrand and Hansen parameters[22] for
the different solvents.
Contrary to the conclusions given by Smith and co-work-
ers,[21b,23] we conclude that the consideration of the a param-
eter, which only represents the hydrogen-bond donor ability
of a solvent, is not sufficient to understand the gelation phe-
Solvent parameters analysis: Before turning to this study, we
analyzed the gelation behavior of our molecules as a func-
tion of most common solvent parameters, such as dielectric
constant (e), dipole moment (m), boiling point (bp), and
density (d). However, no relationship could be established
between these parameters and the gelation behavior (see
Table 1 in the Supporting Information). Then, following the
recent work reported by Smith and co-workers,[23] we have
tried to explain the organogelation behavior of 1a and 1b in
various solvents by using the Kamlet–Taft parameters (see
the Supporting Information). In their approach, the a pa-
rameter (hydrogen-bond donor ability, Table 2) appears to
have primary importance and should ideally be zero for ef-
fective gelation.
Figure 1. a-Depending behavior of gelators 1a and 1b in the studied sol-
vents.
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13605

B. Jamart-Grꢀgoire et al.
Table 2. Hydrogen-bond donor ability (a)[a] and Hildebrand (d)[b] and Hansen (dd, dp, dh)[b] parameters of the
solvents.
Figure 2 shows the behavior
of gelators 1a and 1b as a func-
tion of the value of d. Only
three classes of behavior (in-
soluble, soluble, and gel) have
been indicated; the precipitate
class is included in the insoluble
one.
Class of
solvent
Solvents
a
d
dd
[(calcmꢀ3
dp
[(calcmꢀ3
dh
[(calcmꢀ3
) ]
1/2
1/2
1/2
1/2
A
)
]
R
)
]
G
)
]
U
benzene
toluene
p-xylene
0.00
0.00
0.00
N.A.
N.A.
N.A.
0.00
9.1
8.9
8.8
8.7
8.8
N.A.
9.6
9.8
9
8.8
0
0.7
1
1
N.A.[c]
8.7
N.A.
0.3
0
N.A.
2.1
1
0.4
4.2
0
N.A.
0.7
0.3
N.A.
1
1.4
2.3
2
ethylbenzene
p-diethylbenzene
o-diethylbenzene
chlorobenzene
tetraline
1-methylnaphthalene
nitrobenzene
n-octane
8.8
N.A.
9.3
9.6
10
9.8
7.6
8.2
8.7
9.3
9.3
8.9
8.7
7.4
7.7
7.6
7.7
N.A.
7.1
N.A.
8.2
aromatic
alkanes
In a first approach, we can
define a favorable d domain for
gelation between 8.5 and
N.A.
N.A. 10
10.5 (calcmꢀ3)1/2
(Figure 2).
0.00
0.00
0.00
0.00
0.00
0.00
0.3
0.44
0.93
0.83
0.00
0.00
0.00
0.00
0.00
0.00
0.08
10.9
7.6
8.2
8.7
9.9
10.2
9.9
9.3
14.5
13
9.2
8.9
8.4
7.7
6.9
9.5
9.8
N.A.
10.4
13
0
However, this domain also in-
cludes solvents in which gela-
tors 1a and 1b are soluble.
Then, we decided to analyze
separately the three compo-
nents of d, the results are
shown in Figure 3.
The gelation behavior of 1a
and 1b as a function of dd (Fig-
ure 3a) and dp interactions (Fig-
ure 3b) gives two gelation do-
mains, which once more include
solvents that dissolve gelators
1a and 1b. We then focused on
dh, which evaluates the ability
of the solvent to donate or to
accept hydrogen bonds. The be-
havior of the two gelators 1a
and 1b defines another narrow,
favorable, dh domain for gela-
cyclohexane
carbon tetrachloride
tetrachloroethylene
0
0
0.1
0.3
1.4
2
3
2.8
10.9
9.5
3.7
3.5
N.A.
2.5
N.A.
3.9
3.4
N.A.
N.A.
5
3
5.5
20.7
3.2
3.6
3.1
1.5
6
4.3
3.5
2.6
N.A.
1.4
N.A.
2.8
5.1
N.A.
N.A.
8
chlorinated 1,2-dichloroethane
dichloromethane
chlorform
methanol
ethanol
methyl acetate
ethyl acetate
ethyl propionate
diethyl ether
diisopropyl ether
tetrahydrofuran
acetone
alcohols
esters
ethers
ketones
7.6
4-methyl-pentan-2-one N.A.
N.A.
N.A.
9
7.5
8.5
cyclopentanone
dimethyl sulfoxide
acetonitrile
dimethylformamide
water
0.00
0.00
0.19
0.00
1.17
12
12.1
23.4
8.8
6.7
7.8
others
7.6
[a] The data were extracted from Ref. [24]. [b] The data were extracted from Ref. [22]. [c] N.A.: not available.
nomena. The use of a parameter able to represent both abil-
ities of a solvent—to accept or donate hydrogen bonds—
would be more judicious.
The gelation phenomenon is a complex solubility–insolu-
bility balance between the gelator and the solvent. By
taking into account that the complete miscibility of a two-
components system is expected if the Hildebrand parame-
ters and the degree of hydrogen bonding (dh) are similar,[22]
we decided to report the behavior of our gelators as a func-
tion of these solvent parameters.
Despite the lack of data concerning our molecules, solu-
Figure 2. d-Depending behavior of gelators 1a and 1b in the studied sol-
bility parameters are well referenced in literature for a wide
range of solvents (Table 2). First we considered the Hilde-
brand parameter, which can be formulated as a function of
the Hansen parameters (dd, dp, dh):
vents.
tion between 0.2 and 1.4 (calcmꢀ3)1/2 at which only “gel” be-
haviors are observed (Figure 3c).
In our series based on amino acid derivatives, the hydro-
gen-bond interactions seem to play a key role in 3D self-as-
sembly of the organogelator molecules.[25c] In this context,
the dh parameter is assumed to be of primary importance
compared with the two other Hansen parameters (dd and
dp), which represent weaker interactions. In the defined,
narrow, favorable, dh domain the solvent–gelator interac-
d2 ¼ dd þ dp þ dh
2
2
2
where dd accounts for the dispersive interactions, dp for the
permanent dipole–dipole interactions, and dh for the hydro-
gen-bonding interactions.[22] Finally, d represents the total
solubility parameter.
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Amino Acid Gelation
FULL PAPER
p–p stacking interactions. This condition is not satisfied for
1b, which has a less aromatic character than 1a due to the
alkyl side chain. The organogelation behavior in carbon tet-
rachloride, in which 1b forms a gel and 1a is completely in-
soluble, although this solvent is inside the favorable dh
domain (dh =0.3 (calcmꢀ3)1/2, Figure 3c), is currently unex-
plained.
Moreover, above, we describe a favorable d domain for
gelation of 1a and 1b that also includes solvents in which
the gelators are soluble (Figure 2). After the dh analysis, we
concluded that these solvents have too many gelator–solvent
hydrogen-bonding interactions (Table 2); this prevents the
self-assembly of gelator molecules.
To complete this study, we decided to consider the behav-
ior of the new molecules 3a–b and 4a in different classes of
solvent. The results are shown in Table 3.
We can note that 3a–b and 4a have very similar gelation
abilities to 1a and 1b in the same solvents. They are able to
gelify most of the tested aromatic solvents and the chlorinat-
ed solvents, for example, tetrachloroethylene in the case of
3a and 3b. Derivatives 3a–b and 4a are soluble (S) or in-
soluble (I) in all other solvents tested (Table 3). The surpris-
ing exception to this trend concerns the behavior of gelator
4a in tetrachloroethylene. Indeed, whereas 3a and 3b form
a gel in this solvent at low concentration (CCG=0.12 wt%/
12.8 mm and 0.25 wt%/9.56 mm, respectively), gelator 4a ap-
pears to be completely insoluble.
As described above for 1a and 1b (Figure 3c), we were
able to establish a relationship between the dh parameter
and the gelation ability of 3a–b and 4a (Figure 4). There-
fore, also for these analogs, the dh values of the gelified sol-
vents are located in a very narrow domain. In accordance
with recent reports,[23] we demonstrated that the ability of a
solvent to support self-assembly and that gelation of our
class of molecules is dependent of the ability to form hydro-
gen bonds. However, in our case, both hydrogen-bonding ac-
ceptor and donor abilities have to be taken into account.
Figure 3. Hansen-parameter-depending behaviors of gelators 1a and 1b
in the studied solvents: a) dd, b) dp, and c) dh.
tions have the right force to allow molecules 1a or
1b to interact through hydrogen bonds and form
Table 3. Gelation behavior for 3a–b and 4a in different solvents[a]
.
the fibrous network. Moreover, the fact that this fa-
vorable domain includes quite low values of dh
means that the hydrogen-bonding interactions be-
tween solvent and gelator have to be sufficiently
weak to not compromise the self-assembly of gela-
tors.
Nevertheless, two exceptions can be noticed: In
1-methylnaphthalene, which is slightly outside the
dh domain (Figure 3c), 1b is soluble whereas 1a
forms a gel, but at higher concentrations than in
other aromatic solvents (critical gelation concentra-
tion (CGC)=1.3 wt%, Table 1). It can be assumed
that the high dh value of this solvent (dh =2.3) de-
stabilizes the formation of the 3D network based
on hydrogen bonds. However, the gelation ability
of 1a could be explained by the presence of aro-
matic groups on the lateral chain; this could induce
[b]
Class of
solvent
Solvents
(l)-Phe
(3a)
(l)-Leu
(3b)
(l)-Leu
dh
[(calcmꢀ3
) ]
1/2
G
E
(4a)
G
benzene
toluene
p-xylene
chlorobenzene
tetraline
G (0.57) G (1.63) G (0.97)
G (0.46) G (1.43) G (0.77)
G (0.35) G (0.63) G (0.46)
G (0.81) G (1.79) G (1)
G (0.35) G (1.04) G (0.57)
1
1
N.A.
1
1.4
2.3
2
0.1
0.3
1.4
2.8
10.9
3.5
aromatic
alkanes
1-methylnaphthalene
nitrobenzene
cyclohexane
carbon tetrachloride
S
S
I
S
S
I
S
S
I
P
P
I
chlorinated tetrachloroethylene
chlorform
G (0.12) G (0.25)
I
S
P
S
I
S
S
S
I
S
S
S
I
S
I
alcohols
esters
ethers
ketones
others
methanol
ethyl acetate
diethyl ether
acetone
2.5
3.4
20.7
S
I
S
I
water
[a] G(x): gel (CGC in wt%); S: soluble, I: insoluble, P: precipitate, N.A.: not avail-
a self-assembly of gelator molecules by additional able. [b] H-bonding Hansen parameter; the data were extracted from Ref. [22].
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B. Jamart-Grꢀgoire et al.
Figure 4. dh-Depending behavior of gelators 3a–b and 4a in the studied
solvents.
As a result, we demonstrated that the capacity of a sol-
vent to form a gel by our series of compounds can be pre-
dicted by the dh parameter representative of the ability to
establish hydrogen bonds. However, we think that the com-
plete description of the gelation phenomenon, even of more
complex systems, necessitate other parameters, such as one
describing p–p stacking interactions, to be taken into ac-
count.[6,7] Indeed, as observed herein and in another recent
work of our group,[25c] these latter interactions are driving
forces in the organogelation phenomenon.
Figure 5. a) Effect of the d on GN of gelators 1a and 1b. b) Effect of the
dh on GN of gelators 1a and 1b.
Gelation-strength analysis: The CGC, defined as the mini-
mum concentration at which a gel is formed, is one of the
most common criterions to evaluate the strength of a gel as
well as the sol–gel transition temperature (Tgel).[6] The
values of CGC expressed in wt% depend on the density of
the solvent, whereas the values of CGC expressed in mm
depend on the molecular weight of the gelator. To eloquent-
ly describe the strength of the gel, we translated the CGC
into gelation number (GN), which gives the number of sol-
vent molecules gelified per molecule of gelator.[21d]
Following the analysis of the previous section, we first
tried to correlate the GN with d and then with dh (Figure 5).
Generally, an increase in the d parameter represents a
higher solvation of polar gelators; this prevents the H-bond-
ing interactions responsible for gelation and consequently
leads to a GN decrease. A similar behavior was been recent-
ly reported by Zhu and Dordick for thehalose-based gelator
structures in different solvents.[21d] In the same way, Hirst
Figure 6. GN for gelators 1a and 1b in the studied solvents. The error
bars represent 5% of the total value.
2
2
2
and Smith reported that an increase of da (da =dp +dh )
leads to a gel-strength decrease (i.e., Tgel decrease).[21b]
Even though we can notice a similar trend for gelators 1a
and 1b in aromatic solvents as far as d is concerned (see
arrow, Figure 5a), the data does not seem to correspond en-
tirely to this assumption. Furthermore, we were not able to
find any correlation between GN and dh; the GN of gelators
1a and 1b in the chosen solvents seem to be completely
random (Figure 5b). Furthermore, gelators 1a and 1b show
the highest GN in tetrachloroethylene, which does not have
the lowest d value compared with other gelified solvents
(Figure 6).
These latter results indicate that the variables involved in
the gelation process are much more complicated than those
described to date. Nevertheless, if we carefully analyze the
evolution of GN for gelators 1a and 1b in aromatic solvents
(Figure 6), we can still draw some general conclusions. The
best result for gelator 1b has been found with p-diethylben-
zene as an aromatic solvent (GN=3058ꢁ153), whereas ge-
lator 1a presents the best value in benzene (GN=3951ꢁ
198). Moreover for 1b, GN is very low in benzene (GN=
884ꢁ44), probably due to the lack of an aliphatic moiety in
the solvent structure. However, for gelator 1a, we note that
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Amino Acid Gelation
FULL PAPER
independent of the nature of the aliphatic moiety of the sol-
vent, the GN value is always smaller than the one found for
benzene (Figure 6). In this case, the presence of alkyl chains
on the structure of the solvent seems to be a disturbing pa-
rameter for the gelation phenomenon. For chlorobenzene,
the disturbing effect seems to be due to high polarity of this
solvent; this contributes to the weakness of the hydrogen-
bonding network of gelator molecules.[21b,d,e]
These last observations are in accordance with the results
reported by Banerjee et al. concerning the gelation property
of a family of tripeptides gelators.[19c] They found that chang-
ing the protecting group from tert-butyloxycarbonyl (Boc) to
benzyloxycarbonyl (Z) improved the gel properties in vari-
ous aromatic solvents. By taking these last conclusions into
account, we could have expected that the best solvent for
gelator 1a would be 1-methylnaphthalene, because of the
higher aromatic character than benzene. Actually, as dis-
cussed above, only poor gels were obtained in this solvent;
the high value of dh for this aromatic solvent does not allow
a good self-assembly of the gelator molecules.
Gel morphology: Scanning electron microscopy (SEM) is a
good tool to analyze the influence of the nature of the sol-
vent on gel morphology.[6] We have compared the pictures
of xerogels (dried gels, Figure 7) obtained from organogels
of compounds 1a and 1b in solvents with different solubility
and polarity (Table 2). A xerogel is a shrunk gel for which
the structure has collapsed because of the capillary forces
upon solvent evaporation during the drying process.[27]
First of all, we notice that despite the shrinking of the
structure, all xerogels are composed of a similar and highly
entangled 3D fibrillar network (Figure 7). Indeed, we never
observed a total loss of the 3D network due to solvent po-
larity (solvent–gelator interactions)[21d] or induced by the ca-
pillary forces.[28] The only observable difference is given by
the gels obtained from gelator 1a in toluene (Pictures F1
and F2, Figure 7), which seem to have less fibrillar density
than other gels. This result would suggest that weaker capil-
lary forces have been applied on the gel structure during the
preparation of these xerogels. However, the same effect was
not obtained with gelator 1b in toluene (Picture L1,
Figure 7), which presents the same compactness as other xe-
rogels. Furthermore, the variation of the concentration of
the gelator in the gel does not seem to have a significant
effect on the size and shape of the fibers. As a result, no dif-
ferences can be observed when comparing 0.23 and 1 wt%
of gelator 1a in toluene (Figure 7, pictures F1 and F2, re-
spectively).
Figure 7. SEM pictures of dried gels (xerogel) in different solvents:
F1) gelator 1a in 0.23 wt% toluene; F2) gelator 1a in 1.0 wt% toluene;
F3) gelator 1a in 0.07 wt% tetrachloroethylene; F4) gelator 1a in
0.22 wt% p-xylene; F5) gelator 1a in 0.72 wt% chlorobenzene; F6) gela-
tor 1a in 1.55 wt% 1-methylnaphthalene; L1) gelator 1b in 0.36 wt% tol-
uene; L2) gelator 1b in 0.06 wt% tetrachloroethylene; L3) gelator 1b in
0.28 wt% p-xylene; and L4) gelator 1b in 3.4 wt% chlorobenzene.
Nevertheless, a closer look at the SEM pictures reveals a
small influence of the solvent on the gel morphology, espe-
cially on the fiber diameter. Figure 8 shows the average
fiber diameters of the gels obtained in different solvents.
The fiber sizes of gelators 1a and 1b are slightly different
in a given solvent. The most obvious cases are the gels pre-
pared by using p-xylene as solvent. For gelator 1a, the aver-
age diameter is approximately 75 nm, whereas for gelator
1b the average diameter is approximately 195 nm with a
Figure 8. Average fiber diameters for the gels prepared from gelators 1a
and 1b in different solvents. The horizontal black lines represent the
maximum and minimum fiber diameter observed for each sample.
wide distribution of fiber size (Figure 8). In tetrachloroethy-
lene, for which we found the highest values of GN
Chem. Eur. J. 2011, 17, 13603 – 13612
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13609

B. Jamart-Grꢀgoire et al.
Data for compound 1a: m.p.=188–1908C; [a]2D0 =ꢀ27.98 (c=
0.067 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO, RT): d=11.07
(s, 1H), 8.58–8.52 (m, 4H), 7.96–7.90 (m, 2H), 7.74 (d, 1H, J=9.0 Hz),
7.42–7.22 (m, 10H), 4.97 (d, 2H, J=3.8 Hz), 4.70–4.63 (m, 1H), 3.32–3.28
(m, 1H), 2.95–2.87 ppm (m, 1H); 13C NMR (75 MHz, [D6]DMSO, RT):
d=170.8, 161.7, 161.5, 155.8, 137.9, 137.0, 135.2, 131.54, 131.47, 129.3,
128.3, 128.1, 127.6, 127.44, 127.36, 127.2, 126.4, 121.8, 65.2, 64.8,
37.8 ppm; HRMS (ESI):
(Figure 6), the difference between the average diameters of
the gels prepared from the two gelators is less marked.
Thus, the solvent somehow affects the microscopic struc-
ture of the gel similarly to what has been reported previous-
ly.[21,28] For example, Zhu and Dordick reported that an in-
crease of the solvent–gelator interactions favors the forma-
tion of fine nanofibres.[21d] Nevertheless, in the present work,
no relationship could be found between the nature of the
solvent and the size of the fibrillar network of the xerogels.
Further SEM experiments on organogels are currently in
progress to settle this question.
m/z: calcd for C29H23N3Na1O5: 516.1535 [M+Na]+; found: 516.1530.
Data for compound 1b: m.p.=164–1668C; [a]2D0 =ꢀ17.78 (c=
0.067 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO, RT): d=10.89
(s, 1H), 8.55–8.50 (m, 4H), 7.93–7.88 (m, 2H), 7.62 (d, 1H, J=8.9 Hz),
7.44–7.29 (m, 5H), 5.08 (d, 1H, J=1.9 Hz), 4.51–4.43 (m, 1H), 1.84–1.59
3
(m, 3H), 0.97–0.94 ppm (m, 6H); C NMR (75 MHz, [D6]DMSO, rt): d=
171.4, 161.6, 161.4, 165.9, 137.0, 135.1, 131.5, 131.4, 128.3, 127.8, 127.7,
127.4, 127.2, 121.7, 65.5, 51.6, 41.1, 24.2, 23.1, 21.5 ppm; HRMS (ESI): m/
z: calcd for C26H25N3Na1O5: 482.1692 [M+Na]+; found: 482.1686.
Conclusion
Data for compound 1c: m.p.=195–1978C; [a]2D0 =ꢀ25.88 (c=
0.057 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO, RT): d=10.81
(s, 1H), 8.53–8.46 (m, 4H), 7.91–7.86 (m, 2H), 7.66 (d, 1H, J=8.0 Hz),
7.38–7.28 (m, 5H), 5.12–5.02 (m, 2H), 4.50–4.45 (m, 1H), 1.45 ppm (d,
3H, J=7.6 Hz); 13C NMR (300 MHz, [D6]DMSO, RT): d=171.6, 161.6,
161.4, 155.6, 137.0, 135.0, 131.5, 131.4, 128.3, 127.8, 127.4, 127.1, 121.7,
65.5, 48.7, 18.6 ppm; HRMS (ESI): m/z: calcd for C23H19N3Na1O5:
440.1222 [M+Na]+; found: 440.1217.
Data for compound 1d: m.p.=229–2318C; [a]2D0 =ꢀ17.48 (c=
0.067 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO,): d=10.88 (s,
1H), 8.56–8.52 (m, 4H), 7.95–7.90 (m, 2H), 7.48–7.32 (m, 5H), 5.10 (s,
2H), 4.31–4.26 (m, 1H), 2.19–2.12 (m, 1H), 1.07–0.98 ppm (m, 6H);
13C NMR (300 MHz, [D6]DMSO): d=169.9, 161.5, 161.4, 156.1, 137.0,
135.1, 131.4, 128.3, 127.7, 127.6, 127.4, 127.2, 121.7, 65.4, 58.6, 30.8, 19.1,
17.9 ppm; HRMS (ESI): m/z: calcd for C25H23N3Na1O5: 468.1535
[M+Na]+; found: 468.1530.
Data for compound 1e: m.p.=239–2418C; [a]2D0 =ꢀ23.78 (c=
0.067 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO): d=10.89 (s,
1H), 8.55–8.50 (m, 4H), 7.94–7.88 (m, 2H), 7.49 (d, 1H, J=9.0 Hz),
7.42–7.30 (m, 5H), 5.10 (s, 2H), 4.34–4.28 (m, 1H), 1.94–1.85 (m, 1H),
1.63–1.56 (m, 1H), 1.32–1.17 (m, 1H), 1.06 (d, 3H, J=6.8 Hz), 0.89 ppm
(t, 3H, J=7.4 Hz); 13C NMR (300 MHz, [D6]DMSO): d=170.0, 161.5,
161.4, 156.0, 137.1, 135.1, 131,5, 131.44, 131.41, 128.3, 127.7, 127.6, 127.4,
127.2, 121.7, 65.4, 57.7, 36.9, 24.2, 15.2, 11.0 ppm; HRMS (ESI): m/z:
calcd for C26H25N3Na1O5: 482.1692 [M+Na]+; found: 482.1686.
Herein we report the synthesis of several low-molecular-
weight amino acid derivatives and their organogelation be-
havior in a large variety of solvents. We observed that small
variations in the structure of the gelators can induce a dras-
tic change in the gelation properties. Indeed, only leucine
and phenylalanine derivatives are able to gelify different ar-
omatic solvents, carbon tetrachloride, and tetrachloroethy-
lene. The highest gelation number was found for the latter
solvent for both gelators 1a and 1b (GN=4894ꢁ245 and
5437ꢁ272, respectively). SEM pictures revealed that the ob-
tained gels are composed of entangled 3D fibrillar networks,
and that the fiber diameters are slightly influenced by the
nature of the solvent. Importantly, we point out herein that
dh appears to have primary importance in controlling wheth-
er hydrogen-bond networks of gelators can be established.
This study led us to determine a narrow dh domain favorable
to gelation. We think that this approach could be applied to
other amino-acid-derivative gelation systems, as well as to
other classes of gelators based on the self-assembly through
hydrogen-bond interactions.
Data for compound 1 f: m.p.=184–1868C; [a]2D0 =ꢀ42.38 (c=
0.067 gmLꢀ1 in DMSO); 1H NMR (300 MHz, [D6]DMSO): d=11.11 (s,
1H), 10.86 (s, 1H), 8.59–8.53 (m, 4H), 7.96–7.91 (m, 2H), 7.78 (d, 1H,
J=7.7 Hz), 7.60 (d, 1H, J=9.0 Hz), 7.39–7.24 (m, 6H), 7.10–7.02 (m,
3H), 4.97 (s, 2H), 4.72–4.67 (m, 1H), 3.44–3.39 (m, 1H), 3.12–3.04 ppm
(m, 1H); 13C NMR (300 MHz, [D6]DMSO): d=171.1, 161.6, 161.5, 155.8,
136.9, 136.1, 135.1, 131.5, 131.4, 128.2, 127.6, 127.4, 127.2, 124.0, 121.7,
120.8, 118.5, 118.2, 111.3, 109.9, 65.2, 54.1, 28.2 ppm; HRMS (ESI): m/z:
calcd for C31H24N4Na1O5: 555.1644 [M+Na]+; found: 555.1639.
Experimental Section
Synthetic procedure: Compounds 1b–f as well as 2a and 2b were pre-
pared in three steps (Scheme 2) from l-amino-acid esters according to a
general procedure described previously.[25]
Compounds 3a and 3b were prepared according to the Scheme 2 by
using the amino acids protected with a Boc group as starting material.[25]
Data for compound 3a: m.p.=158–1618C; 1H NMR (300 MHz, CDCl3):
d=8.79 (s, 1H,), 8.57 (br, 2H,), 8.22 (d, 2H, J=8.2 Hz), 7.73 (br, 2H,),
7.37–7.22 (m, 5H), 5.22 (d, 2H, J=7.6 Hz), 4.76 (br, 1H), 3.40–3.12 (m,
2H), 1.42 ppm (s, 9H); 13C NMR (300 MHz, CDCl3): d=171.3, 162.6
(2C), 156.8, 137.2, 135.4, 132.7, 132.5, 130.3, 129.3, 128.8, 127.7 (2C),
123.0, 81.6, 55.0, 38.2, 28.9 ppm.
Data for compound 3b: m.p.=181–1848C; 1H NMR (300 MHz, CDCl3):
d=8.78 (s, 1H), 8.65 (br, 2H), 8.24 (d, 2H, J=9.0 Hz), 7.83–7.72 (m,
2H), 5.0 (d, 1H, J=8.5 Hz), 4.48 (br, 1H), 1.93–1.85 (m, 2H), 1.68–1.51
(m, 10H), 1.04–0.99 ppm (m, 6H); 13C NMR (300 MHz, CDCl3): d=
171.8, 162.1, 162.0, 156.4, 135.5, 134.7, 133.5, 131.9, 131.8, 128.1, 127.6,
127.0, 122.4, 118.9, 80.6, 51.4, 40.5, 28.5, 24.8, 23.2, 22.2 ppm.
Scheme 2. General procedure for the preparation of gelators 1b–f, 2a
and 2b: a) Benzyl chloroformate (Z-Cl), NaHCO3 (saturated solution),
RT, overnight; b) NH2NH2·H2O, MeOH, reflux, overnight; c) Naphthalic
anhydride, toluene, reflux, 10 h.
Finally, compound 4a was prepared in two steps from Boc-protected
amino-acid 3b according to the procedure described in the Scheme 3.
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Amino Acid Gelation
FULL PAPER
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Data for compound 4a: m.p.=186–1908C; 1H NMR (300 MHz, CDCl3):
d=8.63–8.56 (m, 3H), 8.24 (d, 2H, J=8.1 Hz), 7.76 (d, 4H, J=7.3 Hz),
7.67–7.61 (m, 2H), 7.41–7.29 (m, 4H), 5.28 (d, 1H, J=7.7 Hz), 4.58–4.53
(m, 2H), 4.45–4.40 (m, 1H), 4.31–4.26 (m, 1H), 1.90–1.68 (m, 3H), 1.04–
0.99 ppm (m, 6H); 13C NMR (300 MHz, CDCl3): d=171.5, 161.9, 156.9,
144.1, 143.7, 141.4, 134.8, 132.1, 131.8, 128.1, 127.7, 127.2, 127.0, 125.4,
125.3, 122.3, 120.0, 67.4, 52.0, 47.2, 40.8, 24.8, 23.1, 22.2 ppm.
Gelation test: A given amount of potential organogelator (ca. 5 mg) in
1 mL of organic solvent was placed in a flask fitted with a reflux condens-
er and heated until complete dissolution of the solid. Compounds that
did not dissolve under these conditions were classified as insoluble (I).
When cooled down to RT (ca. 30 min.), the solution was transferred into
a closed glass vial and cooled at 48C for 24 h. The gelation was simply
confirmed by turning the glass vial upside down. For the CGC measure-
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until no further formation of gel was observed or until a viscous liquid
was obtained by inverting the glass vial.
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Acknowledgements
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Published online: October 25, 2011
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