Multistimuli-responsive supramolecular organogels formed by low-molecular-weight peptides bearing side-chain azobenzene moieties

Article abstract of DOI:10.1002/chem.201300796

This work demonstrates that the incorporation of azobenzene residues into the side chain of low-molecular-weight peptides can modulate their self-assembly process in organic solvents leading to the formation of stimuli responsive physical organogels. The major driving forces for the gelation process are hydrogen bonding and π-π interactions, which can be triggered either by thermal or ultrasound external stimuli, affording materials having virtually the same properties. In addition, a predictive model for gelation of polar protic solvent was developed by using Kamlet-Taft solvent parameters and experimental data. The obtained viscoelastic materials exhibited interconnected multistimuli responsive behaviors including thermal-, photo-, chemo- and mechanical responses. All of them displayed thermoreversability with gel-to-sol transition temperatures established between 33-80 °C and gelation times from minutes to several hours. Structure-property relationship studies of a designed peptide library have demonstrated that the presence and position of the azobenzene residue can be operated as a versatile regulator to reduce the critical gelation concentration and enhance both the thermal stability and mechanical strength of the gels, as demonstrated by comparative dynamic rheology. The presence of N-Boc protecting group in the peptides showed also a remarkable effect on the formation and properties of the gels. Despite numerous examples of peptide-based gelators known in the literature, this is the first time in which low-molecular-weight peptides bearing side chain azobenzene units are used for the synthesis of "intelligent" supramolecular organogels. Compared with other approaches, this strategy is advantageous in terms of structural flexibility since it is compatible with a free, unprotected amino terminus and allows placement of the chromophore at any position of the peptide sequence. Intelligent response: The incorporation of the azobenzene moiety into the side chain of low-molecular-weight peptides allows for the preparation of multistimuli-responsive supramolecular organogels. The presence and position of the azobenzene residue act as a versatile regulator to reduce the minimum gelation concentration and enhance both the thermal stability and mechanical strength of the materials (see figure). Copyright

Full text of DOI:10.1002/chem.201300796

FULL PAPER
DOI: 10.1002/chem.201300796
Multistimuli-Responsive Supramolecular Organogels Formed by Low-
Molecular-Weight Peptides Bearing Side-Chain Azobenzene Moieties
Paola Fatꢀs,[a] Jꢁrgen Bachl,[a, b] Stefan Oehm,[b] Ana I. Jimꢂnez,[a] Carlos Cativiela,[a] and
David Dꢃaz Dꢃaz*[a, b]
Abstract: This work demonstrates that
the incorporation of azobenzene resi-
dues into the side chain of low-molecu-
lar-weight peptides can modulate their
self-assembly process in organic sol-
vents leading to the formation of stim-
uli responsive physical organogels. The
major driving forces for the gelation
process are hydrogen bonding and p–p
interactions, which can be triggered
either by thermal or ultrasound exter-
nal stimuli, affording materials having
virtually the same properties. In addi-
tion, a predictive model for gelation of
polar protic solvent was developed by
using Kamlet–Taft solvent parameters
and experimental data. The obtained
viscoelastic materials exhibited inter-
connected multistimuli responsive be-
haviors including thermal-, photo-,
chemo- and mechanical responses. All
of them displayed thermoreversability
with gel-to-sol transition temperatures
established between 33–808C and ge-
lation times from minutes to several
hours. Structure–property relationship
studies of a designed peptide library
have demonstrated that the presence
and position of the azobenzene residue
can be operated as a versatile regulator
to reduce the critical gelation concen-
tration and enhance both the thermal
stability and mechanical strength of the
gels, as demonstrated by comparative
dynamic rheology. The presence of N-
Boc protecting group in the peptides
showed also a remarkable effect on the
formation and properties of the gels.
Despite numerous examples of pep-
tide-based gelators known in the litera-
ture, this is the first time in which low-
molecular-weight peptides bearing side
chain azobenzene units are used for
the synthesis of “intelligent” supramo-
lecular organogels. Compared with
other approaches, this strategy is ad-
vantageous in terms of structural flexi-
bility since it is compatible with a free,
unprotected amino terminus and allows
placement of the chromophore at any
position of the peptide sequence.
Keywords: azobenzene · gels · pep-
tides · self-assembly · sol–gel proc-
esses
Introduction
ics, foods, and environmental remediation.[3] As soft materi-
als, gels are continuous in structure and solid-like in rheo-
logical behavior.[4] In contrast to chemical gels,[5] which are
based on covalent bonds (usually cross-linked polymers
unable to redissolve), physical (also called supramolecular)
gels[3c,6] are made of either low-molecular-weight (LMW)
compounds or polymers (so called gelators) through exten-
sive noncovalent interactions such as hydrogen bonding, van
der Waals, charge-transfer, donor-acceptor, dipole–dipole,
p–p stacking, and coordination interactions. The solid-like
appearance of the gels is the result of the entrapment of the
liquid (major component) in the interstices of a solid 3D
matrix of large surface area (minor component), usually
through surface tension and capillary forces.[3f,7] In the case
of LMW gelators, the formation of the viscoelastic matrix is
a consequence of the entanglement of one dimensional (1D)
supramolecular fibers, which is typically induced by cooling
their hot isotropic solutions to room temperature.
The ability of natural systems to alter function in direct re-
sponse to environmental conditions has inspired many scien-
tists to fabricate “smart” materials that respond to tempera-
ture, light, pH, electro/magnetic field, mechanical stress,
and/or chemical stimuli. These responses are usually mani-
fested as remarkable changes from the molecular (e.g., con-
formational state, hierarchical order) to the macroscopic
level (e.g., shape, surface properties).[1]
Among many types of stimuli-responsive materials, self-
assembled viscoelastic gels[2] of both organic solvents (orga-
nogels) and water (hydrogels) have been recognized as
promising materials for bottom-up nanofabrication tools in
various fields such as biomedicine, catalysis, sensors, cosmet-
[a] P. Fatꢀs, J. Bachl, Dr. A. I. Jimꢁnez, Prof. C. Cativiela,
Prof. D. Dꢂaz Dꢂaz
Instituto de Sꢂntesis Quꢂmica y, Catꢀlisis Homogꢁnea (ISQCH)
CSIC-University of Zaragoza, 50009 Zaragoza (Spain)
Due to the weakness of the noncovalent interactions that
maintain the dynamic supramolecular structure, physical
gels are usually thermoreversible. Moreover, the sol-to-gel
(and/or gel-to-sol) phase transition can also be triggered by
other stimuli such as light irradiation if a suitable chromo-
phore is present. This has been proven to be useful for un-
derstanding gelation mechanisms and for advanced applica-
[b] J. Bachl, S. Oehm, Prof. D. Dꢂaz Dꢂaz
Institut fꢃr Organische Chemie, Universitꢄt Regensburg
93053 Regensburg (Germany)
E-mail: david.diaz@chemie.uni-regensburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300796.
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tions like drug delivery and switchable devices with memory
function.[8] The simplest molecular design principle in this
case is to incorporate one or more light-responsive units in
the structure of the respective gelating moiety. Azobenzene
is the photochromic group[9] most widely used to generate
photoresponsive gels,[10] probably due to its facile incorpora-
tion into organic structures and its well-studied trans-to-cis
photoisomerization. In addition to the photoresponsive be-
havior, aromatic p–p stacking and other noncovalent inter-
actions have been found to cooperatively stabilize some or-
ganogel structures.[11] However, it should be considered that
the simple incorporation of switchable azo units into LMW
gelators does not warrant a rapid, if any, macroscopic re-
sponse of the gel material upon light irradiation because
photo-induced isomerization is very sensitive to the gel
structure.[12] Indeed, very slow kinetics have generally been
documented for supramolecular organized systems in which
the azobenzene moieties are tightly packed.[13] Nevertheless,
exceptions to this rule have been also reported in the case
of gels made from azobenzene dendrons and aliphatic
amide units.[14] In addition, one should consider that incor-
poration of relatively large stimuli responsive moieties in
the structure of a gelator may cause an important disruption
of the gelation ability and/or modification of the gel proper-
ties as a consequence of the delicate equilibrium between
crystallization and solubility that usually defines the meta-
stable gel state.[15]
Peptides are among the most popular gelating moieties,
with numerous examples of LMW peptide-based gelators
being reported in recent years.[16] However, only a few
works[14a,17] have focused on compounds of this type that
contain azobenzene groups for the preparation of photores-
ponsive gel materials. In most of such reports, the azoben-
zene unit is incorporated into the peptide backbone block-
ing the terminal amino moiety. Additionally, in some case-
s,[17a,e] hydrocarbon chains are also attached to the molecule
to promote self-assembly through hydrophobic interactions,
so that the compounds cannot be considered strictly as pep-
tides but rather as peptide-conjugates or peptide-hybrid ma-
terials. A different approach to generate photoresponsive
LMW peptide gelators while completely keeping the pep-
tide nature of the system can be the use of amino acids that
bear an azobenzene group in the side chain. This strategy
seems advantageous in terms of structural flexibility since it
is compatible with a free, unprotected amino terminus and
allows placement of the chromophore at any position of the
peptide sequence. We report herein the ability of small pep-
tides bearing a side chain azobenzene moiety to form stable
supramolecular gels that are responsive to several stimuli,
among which light. In particular, we have synthesized and
studied the gelation properties and responsiveness of tetra-
peptides containing p-(phenylazo)-l-phenylalanine, that is,
the l-phenylalanine (Phe) derivative formally resulting from
the attachment a phenylazo group to the para position of
the aromatic ring[18] (azoPhe in the abbreviated form;
Figure 1). After the pioneering studies of Goodman[19] on
the chirooptical and conformational properties of high-mo-
lecular-weight homo- and copolymers based on azoPhe, a
number of works[20] have dealt with this azobenzene-con-
taining amino acid but none of them are related to the
design of peptide gelators.
Results and Discussion
Peptide design and synthesis: In the course of an investiga-
tion on the structural behavior of peptides composed of
azoPhe and Asp
benzyl ester) residues, we observed that the intermediate
tetrapeptide Boc-azoPhe-[Asp(OBzl)]3-OMe (A4 in
ACHTUNGTRENN(UNG OBzl) (l-aspartic acid with a side chain
AHCTUNGTRENNUNG
Figure 1) exhibited a high propensity to form a gelatinous
material. We decided to investigate the gelation properties
of this compound and extended the study to the analogous
sequences B4, C4, and D4 (Figure 1), differing in the posi-
tion occupied by the azobenzene-containing residue
(azoPhe) within the peptide backbone. Given the different
solubility expected for tetrapeptides bearing an N-Boc-pro-
tected- (Boc=tert-butoxycarbonyl) versus a free amino
group, the study was also applied to the corresponding un-
protected derivatives (name preceded by H- in Figure 1).
Additionally, to evaluate the extent to which the gelation
ability of these compounds depends on the presence of the
azobenzene moiety, we considered the replacement of the
azoPhe residue by Phe in the peptide sequences mentioned.
Clearly, the compounds incorporating Phe (named as F- in
Figure 1) will not be photosensitive but their gelation ca-
pacity could be similar or even superior to that of the corre-
sponding azoPhe-containing counterparts depending on
whether the additional p-(phenylazo) group is beneficial or
detrimental to the gelation process. The absence of both
Phe and azoPhe was evaluated by considering Z4, whereas
the tripeptide A3 was included in the study to check wheth-
er A4 is the shortest sequence exhibiting gelation capacity.
Finally, we wondered about the effect produced when the
side chain azobenzene group acts as a bridge to connect two
peptide chains. This possibility was explored for the A4 and
D4 sequences, linked in this way to give rise to bis-A4 and
bis-D4, respectively (Figure 1).
The azobenzene-based amino acids azoPhe and bisazoPhe
were synthesized in the N-Boc-protected form appropriate
for subsequent peptide synthesis, Boc-azoPhe-OH (1) and
Boc-bisazoPhe-OH (2), by using commercial N-Boc-(p-
nitro)-l-phenylalanine (3) as
a
common precursor
(Scheme 1). In both cases, the synthetic strategy involved
condensation of a nitroso and an amino moiety, similarly to
that described by Goodman for the preparation of unpro-
tected azoPhe.[19a] Catalytic hydrogenation of N-Boc-(p-
nitro)-l-phenylalanine (3) yielded the p-amino derivative 4,
which was further reacted with nitrosobenzene in acetic acid
to afford the desired Boc-azoPhe-OH (1; Scheme 1). To
generate the bisazoPhe derivative, compound 4 was also
used as the amino component, whereas the nitroso moiety
(compound 5) was obtained from 3 upon reduction with
zinc and subsequent treatment with iron trichloride. Con-
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Multistimuli-Responsive Supramolecular Organogels
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Figure 1. Library of LMW peptides bearing side chain azoPhe synthesized for this work (right). A code according to key moieties was adopted to simpli-
fy the representation of the peptides (left). The nomenclature used for naming the peptides was based on the position of the azoPhe group with respect
to the N-terminal group (i.e., A, B, C, D), the length of the peptide (i.e., tripeptides=3; tetrapeptide=4). Definition of other abbreviations: F=Phe;
H=absence of N-Boc protecting group; Z=control peptide without azoPhe or Phe moieties.
densation of the two fragments in acetic acid yielded the bis-
AHCTUNGTREG(NNNU amino acid) bearing a Boc group in both amino termini (2;
Scheme 1). This methodology is more straightforward than
the one previously reported by Sewald,[21] which required
additional protection and deprotection steps. It should be
noted that the a carbon atom(s) in amino acids 1 and 2 ex-
hibit an l stereochemistry (equivalent to S), as in the precur-
sor 3.[22]
All peptides in Figure 1 were prepared following standard
procedures of peptide synthesis in solution,[23] using Boc as a
temporary protection for the amino function and N-(3-dime-
thylaminopropyl)-N’-ethylcarbodiimide
hydrochloride
(EDC)/1-hydroxybenzotriazole (HOBt) as coupling agents.
The C-terminal methyl esters were prepared by treatment of
the corresponding N-Boc-protected amino acid with methyl
iodide in the presence of potassium carbonate. Boc depro-
tection was carried out by treatment with a 4:6 (v/v) tri-
fluoroacetic acid (TFA)/dichloromethane mixture. When bi-
sazoPhe was involved, the two amino or carboxylic acid
groups in the bis(amino acid) (or the bisazoPhe-containing
peptide chain) were made to react simultaneously. All N-
Boc-protected intermediate derivatives and final peptides
Scheme 1. Synthesis of the azobenzene-based amino acids Boc-azoPhe-
OH (1) and Boc-bisazoPhe-OH (2). Abbreviations: Boc, tert-butoxycar-
bonyl.
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Scheme 2. Synthesis of azoPhe-based peptides A4, H-A4 and bis-A4. Abbreviations: Boc, tert-butoxycarbonyl; Bzl, benzyl; EDC, N-(3-dimethylamino-
propyl)-N’-ethylcarbodiimide hydrochloride; HOBt, 1-hydroxybenzotriazole; TFA, trifluoroacetic acid.
were purified by column chromatography and fully charac-
terized by NMR spectroscopy and high-resolution mass
spectrometry. Isolation of the final peptides exhibiting a free
amino terminus (those denoted with H- in Figure 1) was ac-
complished by removal of the Boc protecting group in the
corresponding precursor under standard conditions and fur-
ther neutralization of the trifluoroacetate formed. As a
model example, the preparation of A4, H-A4, and bis-A4 is
shown in Scheme 2.
B4, D4, F-C4, bis-A4, H-bis-D4) and ethyl acetate (H-B4,
H-C4, bis-A4, H-bis-A4, bis-D4), in which inhomogeneous
or homogeneous opaque gels were obtained.[22] Conversely,
precipitate formation was observed in some solvents like
methanol (A4, H-A4, H-B4, H-C4) and diethyl ether (D4,
F-A4, F-B4) after cooling down the corresponding isotropic
solutions to room temperature. However, no apparent corre-
lation was found between the parameters of the above sol-
vents and the structure of the different peptides that formed
either gels or precipitates. Besides occasional gels observed
in nonprotic solvents (acetonitrile, ethyl acetate, diethyl
ether), gelation was effective in aromatic solvents (toluene,
benzene, xylene) with very few exceptions (see the Support-
ing Information, Table S1).
Excluding the solvents that did not provide any gel and
the reference peptides A3, Z4, and H-Z4, 60% of the
matrix elements afforded stable gels. In the case of alcohols,
a remarkable enhancement of the gelation ability of the
peptides was observed when increasing the molecular
weight (Mw) of the solvent, probably due to additional hy-
drophobic interactions stabilizing the aggregates. Thus,
methanol was gelled by 15% of the peptides, ethanol by
50% and higher Mw alcohols (isopropanol, 2-butanol, 1-hex-
anol) by 75–95%. Moreover, the azoPhe-containing pepti-
des showed greater gelation ability than their Phe counter-
parts both in aromatic and alcoholic solvents (e.g., A4 vs.
F-A4; H-A4 vs. H-F-A4; B4 vs. F-B4; C4 vs. F-C4; H-C4 vs.
H-F-C4; the Supporting Information, Table S1).
Gelation ability and solubility considerations: The gelation
ability of the peptide library (Figure 1) was systematically
studied for 20 solvents of different nature (polar protic,
polar aprotic, or nonpolar) by using the classical thermal
(heating–cooling) treatment. This provided a 2D rectangular
matrix of 460 elements (the Supporting Information,
Table S1). The state of the material was examined by the
“stable-to-inversion” method,[22] and the gel condition of the
samples that did not show gravitational flow upon turning
the vial upside down was further confirmed by dynamic
rheological measurements (see below).
All peptides studied in this work were found to be insolu-
ble in water, n-hexane, and glycerol upon heating and/or in-
tense ultrasound treatment. In contrast, the use of dimethyl
sulfoxide, acetonitrile, acetone, dichloromethane, ethyl ace-
tate, tetrahydrofuran, chloroform or 1,4-dioxane resulted in
complete solubilization of most of the peptides upon heat-
ing–cooling or ultrasound treatment. Among these solvents,
only a few exceptions were observed for acetonitrile (A4,
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On the other hand, tripeptide A3 showed gelation only in
two aromatic solvents, which asserts the tetrapeptides as the
shortest sequence leading to efficient azobenzene-based
peptide organogelator of this type. Notably enough, control
tetrapeptides Z4 and H-Z4 (containing no azoPhe or Phe
residue) provided gels only in 1-hexanol or toluene, respec-
tively. These results highlight the important role of p–p
stacking interactions of aromatic residues in the side chain
in promoting peptide supramolecular assembly in organic
solvents.[24] It is worth noting that both Z4 and H-Z4 bear a
benzyl group attached to the side chain of each Asp residue
(Figure 1). The fact that replacement of a single AspACTHUNTGRNEUNG(OBzl)
residue in Z4 or H-Z4, at any position of the peptide se-
quence, by Phe brings about a dramatic increase in gelating
capacity means that, at least for the kind of peptides under
study, the benzyl moiety in Phe is much more effective in
providing p–p stacking interactions than the one present in
AspACHTUNGTRENNUNG(OBzl). This distinct effect could be related to the closer
proximity of the aromatic group to the peptide backbone in
the case of Phe, which in turn results in a more restricted
flexibility.
Appearance of the gels: All organogels formed at the MGC
by the peptides containing an azobenzene group showed a
characteristic yellow-orange color (Figure 2).
The formation of aggregates smaller than l=400–700 nm
(visible wavelength range) was routinely observed in nonar-
omatic solvents, as indicated by the complete opacity of the
materials. In general, gels derived from bisazoPhe-based
peptides were more yellowish and pale than those made of
the analogous azoPhe derivatives, in which the ratio of
amino acid residues per azobenzene moiety is higher. In the
case of alcohols, the degree of opacity was independent of
the solvent structure (methanol, ethanol, isopropanol, 2-bu-
tanol, 1-hexanol). However, translucent or transparent or-
angeish-colored gels were always obtained in aromatic sol-
vents. The same opacity behavior was also observed for gels
prepared from peptides lacking the azobenzene units (F-
series).[22] Such marked optical differences point out the im-
portance of the interactions established between gelator and
solvent molecules for the growth of supramolecular aggre-
gates.
Figure 2. Representative digital photographs of upside-down vials con-
taining organogels obtained from different peptides in toluene or isopro-
panol. Gels were prepared at their MGC as indicated in Table S1 (in the
Supporting Information). In isopropanol: A4, 2% w/v; B4, 4% w/v; C4,
6% w/v; D4, 10% w/v; bis-D4, 2% w/v; F-A4, 4% w/v; F-D4, 5% w/v.
In toluene: A4, 2% w/v; B4, 1.5% w/v; C4, 2% w/v; D4, 4% w/v;
bis-D4, 1% w/v; H-A4, 7% w/v; F-D4, 9.5% w/v; H-F-D4, 2.5% w/v.
See the Supporting Information for a color version of this figure.
p*-parameter relates to the stabilization of charges and di-
poles associated to peripheral groups (solvation) and inter-
actions between fibers at the nanoscale. In our case, it was
possible to build an appropriate fitting Gaussian model
based on theoretical and experimental data distribution that
accounts for the gelation ability of the peptide library in
polar protic solvents (Figure 3).[22]
Thus, the protic solvents with a higher tendency to form
stable gels were those presenting simultaneously relatively
high and balanced hydrogen-bond-donor and -acceptor abil-
ities (e.g., aꢀ0.75, bꢀ0.85), as well as intermediate polariz-
ability (e.g., p*ꢀ0.5). In this scenario, solvents like MeOH,
water, or glycerol with much higher a-values and much
lower b-values should hinder initial steps of the gelation
process affording either precipitation or clear solutions. On
the other hand, both the high gelation efficiency of the pep-
tide library and similar Kamlet–Taft parameters in the case
of aromatic solvents caused a size population data too small
to build a statistically significant model for the gelation abil-
ity. Regardless the solvent, no significant differences could
be observed for the distribution of gel properties such as the
gel-to-sol transition temperature (Tgel) or the minimum ge-
lation concentration (MGC, i.e., lowest concentration of the
Correlations with solvent properties: To evaluate possible
correlations between gelation ability, gel properties, and sol-
vent nature, we paid attention to the study of the Kamlet–
Taft solvent parameters.[25] Kamlet–Taft solvatochromic pa-
rameters measure separately the acidity or hydrogen-bond-
donor ability (a-parameter), hydrogen-bond-acceptor (b-pa-
rameter), and dipolarity/polarizability (p*-parameter) prop-
erties of the solvent as contributing to overall solvent polari-
ty. Previous studies with physical gels have demonstrated
the usefulness of these parameters to categorize the interac-
tions between solvent and gelator molecules.[26] The results
of these studies indicate that a-parameter plays a key role
during the formation of hydrogen-bonded gel networks, the
b-parameter affects the thermal stability of the gel and
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aging time, thermal history, and degree of hysteresis, among
others,[29] they were correlated for model examples with the
first endothermic transition observed by modulated differen-
tial scanning calorimetry (DSC).[22] As typical feature of su-
pramolecular gels, Tgel increased considerably with increas-
ing initial gelator concentration (DTgel ꢀ35–408C, Figure 4),
Figure 3. General Gaussian-based predictive model built from the corre-
lation between Kamlet–Taft solvent parameters and gelation ability of
the peptide library in polar protic solvents (2-butanol, isopropanol, 1-hex-
anol, ethanol, methanol, water, and glycerin). Note: Gaussian distribu-
tions reflect the normal probability of the peptides to form gels in protic
solvents. Thus, highest probability is achieved when all Kamlet–Taft pa-
rameters of a given solvent are simultaneously at the x coordinate of the
corresponding maximum.
Figure 4. Phase boundaries defined by the evolution of Tgel as a function
of gelator concentration for bis-D4 in isopropanol (b.p.: 82.58C) and tol-
uene (b.p.: 110.68C). The plateau value in isopropanol was virtually
reached after increasing the concentration 6.5-fold with respect to MGC
(i.e., 2% w/v).
gelator at which gelation occurs) in function of a, b, or
p*-parameters. However, we could elucidate some general
trends by comparing directly these properties with the struc-
ture of the peptide-based gelators (see below).
Interestingly, we also found that gel formation could be
also achieved by ultrasound treatment during 5–30 min at
room temperature of the initial (gelator+solvent) mixture
instead of the typical heating–cooling process without major
alteration of the MGC. In general, gels prepared at the
same concentration by both methods did not show major
differences in terms of Tgel values and morphology. Howev-
er, the gelation times were considerably lower and the opac-
ity somewhat higher for gels prepared by ultrasound treat-
ment, suggesting the eventual formation of larger aggre-
gates. Rheological experiments also confirmed slightly
higher strength within the linear regime for gels prepared by
thermal process (see below). From a mechanistic point of
view, ultrasound has been proven to activate noncovalent in-
teractions (e.g., hydrogen bonding, p–p stacking, van der
Waals) through cavitation of the solvent, which can eventu-
ally dissolve the gelator molecules and trigger the gelation
phenomenon.[27]
indicating that self-assembly is driven by strong intermolecu-
lar interactions affording closely packed 3D networks. Re-
markably, the gelator concentration could be increased in
many cases until 50% w/v affording homogeneous gels,
which retained stability well above the boiling point of the
solvent. The impossibility to prepare more concentrated iso-
tropic solutions in those cases prevented the detection of
the typical plateau region before collapse of the gel. As ex-
pected, gelation time also decreased with increasing gelator
concentration.[22]
In terms of thermal responsiveness, all organogels were
found to be fully thermoreversible and stable to several
cycles of heating and cooling, without affecting the gelation
ability. Moreover, they remained homogeneous, without sig-
nificant change in the mechanical strength (e.g., caused by
microcrystallization of the gelator), reconstitution ability,
color, or appearance for several months when stored in
sealed glass vials at room temperature.
Minimum gelation concentration and gel-to-sol phase transi-
tion temperature: MGC values were established in most
cases between 1–10% w/v (that is, 1% w/v=1 g of solute
per 100 mL of solution). These values involve the immobili-
zation between hundreds and thousands of solvent mole-
cules per gelator molecule.[22] The gel-to-sol transition tem-
peratures (Tgel) of the gels were obtained by the “inverse
flow method” (IFM).[28] However, as Tgel values determined
by IFM strongly depend on factors such as the cooling rate,
Structure–property relationship studies: Figure 5 outlines an
overview of the Tgel and MGC values corresponding to the
gels prepared with all peptides in two of the most statistical-
ly relevant solvents, namely isopropanol (polar protic) and
toluene (nonpolar). The results show that the presence of
the side chain azoPhe unit in A4–D4 leads to the formation
of organogels in isopropanol that exhibit higher thermal sta-
bility (DTgel ꢀ10–208C) in comparison with the analogous
gelators containing Phe (Figure 5A). However, this effect
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slightly higher Tgel values for the gels prepared with the
azoPhe series in isopropanol (Figure 5A, C). This effect was
nearly irrelevant in toluene, except for the gels made with
H-A4 that were intriguingly more thermostable (DTgel
ꢀ148C) than those made with A4. A much higher gelation
tendency was also observed for the N-Boc protected in the
F- series, although no clear correlation was found between
solvent nature and gelation ability for the latter.
Moreover, the bisazoPhe-based gels usually displayed Tgel
values superior to those of the corresponding azoPhe-based
gels (DTgel ꢀ15–308C) with little influence of the solvent or
presence/absence of N-Boc protection. In general, the posi-
tion of the azoPhe or Phe residue within the peptide se-
quence did not have a major impact on the Tgel of the gel
materials. Conversely, MGC values were typically lower in
toluene than in isopropanol, regardless of the gelator family
in the case of N-Boc protected peptides, except for F-D4 for
which the opposite was unexpectedly observed (Figure 5B).
Much lower MGC were also achieved in the case of bisazo-
Phe-containing peptides. In general, the location of azoPhe
at the A or B position in N-Boc protected peptides provided
lower MGC values, especially in isopropanol. This effect
was even more marked in the F- series. For instance, F-B4
formed gels in isopropanol and toluene at 1.0 and 0.5% w/v
respectively, whereas F-C4 required 13.5 and 7.0% w/v re-
spectively. Interestingly, the absence of N-Boc protection in
the gelator structure of the azoPhe-based peptides resulted
in a lower MGC value, especially in isopropanol,[30] with the
exception of H-A4 (Figure 5D). In contrast, complete inhib-
ition of the gelation was observed for some peptides of the
H-F- series in isopropanol (H-F-C4, H-F-D4). These results
support the importance of remote groups on fine-tuning the
properties of physical gels.[31]
Driving-force studies
Temperature-dependent FT-IR experiments: We carried
out temperature-controlled FT-IR experiments to classify
the major driving forces for the gelation phenomenon. As
expected, the intensity of the bands involving the amide
groups were found to be much higher in the gel samples
compared with the solid gelator indicating more efficient in-
teractions of these groups in the gel phase.[22] To induce the
gel-to-sol transition, the samples were submitted to a con-
trolled increase of temperature. Figure 6 shows a very re-
ꢁ
markable shift observed in the region of the NH stretching
vibrational bands upon heating the gel made from the
model azopeptide C4. The spectra of the isotropic solution
ꢁ
(T=708C) displayed the characteristic NH stretching band
at 3391 cmꢁ1, which is broadened and 109 cmꢁ1 blueshifted
in comparison to the hydrogen bonded gel state (T=308C).
The evolution of the spectra showed a drastic change over
608C, which corresponds to the Tgel of the material. A less
dramatic lower-energy shift (Dlmax =39 cmꢁ1) attributed to
Figure 5. Bar graphs showing the influence of peptide structure on Tgel
and MGC for freshly prepared gels in two representative solvents (iso-
propanol, toluene).
ꢁ
the amide I (80% C=O stretching) resonance band was
was not evident in toluene for the above series or when con-
sidering bisazoPhe-based gelators (Figure 5A). The pres-
ence of an N-Boc protecting group generally resulted in
also observed in the gel compared to the solution phase
(lmax =1643 cmꢁ1). Overlapped noise in the region 1515–
1550 cmꢁ1 did not allow a clear distinction between the
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D. Dꢂaz Dꢂaz et al.
Figure 7. Representative temperature-induced chemical shift of Boc-CO-
NH- proton of B4 in [D8]toluene (1.5% w/v). The inflection point corre-
sponds to the Tgel
.
ꢁ
significant variations were observed for NH amide protons
Figure 6. Typical temperature-controlled FT-IR spectra showing the blue-
ꢁ
ꢁ
and the closest NH-CH-CH2-R protons (Dd/DTꢀ3.5ꢆ
shift and intensity change of the NH stretching vibration band during
10ꢁ3 ppmKꢁ1). In these cases, a downfield shift (Dd/DT
ꢀ7.1ꢆ10ꢁ4 ppmKꢁ1) was first observed in the range 25–
458C, followed by a more evident upfield shift (Dd/DT
ꢀ2.8ꢆ10ꢁ3 ppmKꢁ1) in the range 45–608C. The inflection
point of the curve at 318 K (44.858C) matched with the Tgel
of the material calculated by IFM (45(ꢂ1)8C). This behav-
ior has been already attributed by others[34] to the existence
of a different type of disassembly process during the initial
heating period, and it is overall in agreement with the split
of hydrogen bonding as the major driving force of the gelati-
on process.
the gel-to-sol transition upon heating. Experiment performed with pep-
tide C4 in toluene at the MGC (2% w/v). The Tgel of the material was
59(ꢂ2)8C as determined by using IFM. The arrows indicate the direc-
tions of motion. See the Supporting Information for a color version of
this figure.
ꢁ
bands associated to the amide II (60% NH bending) reso-
nance. Similar observations were made for other azopeptide
gelators.[22]
These results point out the key role of hydrogen bonded
amide groups during the formation of the 3D supramolecu-
lar gel network, besides the great stabilization effect of side
chain aromatic residues through p–p stacking interactions
(this is in agreement with the poor gelation ability of control
peptides such as Z4, see above).[31e,32]
Morphological characterization: To gain visual insights into
the microscopic morphologies of the organogels, we con-
ducted transmission electron microscopy (TEM) observa-
tions of the corresponding xerogels (Figure 8). The photo-
graphs show characteristic fibrillar networks in all cases re-
gardless of the position of the Phe, azoPhe, or bisazoPhe
residue in the peptide gelator or the free or protected state
of the N terminus. Most of the fibers presented lengths on
the micrometer scale and relatively uniform diameters in
the range of 10–60 nm (smallest feature). Typically, the high
aspect ratio of the entangled networks is a consequence of a
strong anisotropic growth process, which indicates a well-or-
dered molecular packing to form the unit nanofiber. In gen-
eral, the more densely entangled fibrous structure of some
samples was in agreement with their higher thermal stability
and intermolecular cohesiveness as indicated by the Tgel
values (e.g., Figure 8A and B: Tgel (A4 in toluene)=378C;
Tgel (A4 in isopropanol)=578C; see the Supporting Infor-
mation, Table S1). In contrast to the materials prepared in
toluene, those in isopropanol presented somehow numerous
patches that made the observation of the fibers generally
more difficult. No evident morphological differences were
observed by TEM analysis of a given azoPhe-based gel and
its analogous Phe-derived material.[22] To ensure that the ob-
served fibrillar structures were not artifacts, we also record-
ed atomic force microscopy (AFM) height images of the xe-
rogels,[22] which were in good agreement with the TEM pho-
Temperature-dependent 1H NMR spectroscopy experiments:
To gain an insight into the protons involved in the stabiliza-
tion of the self-assembled network, we recorded 1H NMR
spectra of different model azoPhe-based gels in [D8]toluene
within a temperature range comprising both gel and solution
states. Previous studies have demonstrated that the signals
of gelator molecules forming part of the gel network cannot
be observed due to long correlation times. Hence, any ob-
served 1H NMR signal of a gel is a consequence of small
amounts of gelator molecules, either aggregated or disaggre-
gated, dissolved in the immobilized solvent. Thus, the in-
crease of gelator signals upon heating is due to the enhance-
ment of molecular mobility and dissociation of the gel net-
work.[31d,33] As a model example, Figure 7 shows the chemi-
cal shift variation of the amide proton (Boc-CO-NH-) of B4
upon heating the corresponding gel. In general, a gradual
improvement of the signals resolution and slight chemical
shift displacements were observed as the temperature in-
creased. This observation was made at different levels for all
protons.[22] Both aromatic and benzyl ester protons
ꢁ
(PhCH2O ) shifted upfield, practically in a linear manner,
upon heating from 258C (gel state) to 608C (solution state;
Dd/DTꢀ6.7ꢆ10ꢁ4–1.0ꢆ10ꢁ3 ppmKꢁ1). However, the most
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Multistimuli-Responsive Supramolecular Organogels
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ure 9I–O). At least two levels
of surface roughness were ob-
served within this globular
population
(Figure 9N–O).
The cross-section view of one
of these large structures re-
vealed a highly interconnected
porous (Ø=30–70 mm) and fi-
brillar network of the inner
part. Nevertheless, it should
emphasized that due to the
drying process such structures
may not necessarily always
represent those in the gel
phase, and, therefore, any po-
tential relationships should be
relativized within this context.
Indeed, more detailed research
is still needed to explain the
differences observed in mor-
phology.
Dynamic rheological proper-
ties: Oscillatory rheological
measurements unequivocally
confirmed the viscoelastic gel
state of the materials that did
not show gravitational flow
upon turning the vial upside
down.[22] Thus, their storage
modulus G’ (energy storage
modulus) and loss modulus G’’
(energy loss modulus) were
first measured at room temper-
ature as a function of angular
frequency w (dynamic frequen-
cy sweep experiment, DFS)
and shear strain (dynamic
Figure 8. Representative TEM and AFM images of xerogels obtained from the corresponding organogels pre-
pared at their MGC as described in Table S1 (in the Supporting Information). TEM: A) A4 in toluene (2%
w/v); B) A4 in isopropanol (2% w/v); C) C4 in toluene (2% w/v); D) C4 in isopropanol (6% w/v), E) D4 in
toluene (4% w/v); F) bis-A4 in toluene (0.6% w/v). AFM: G) A4 in toluene (2% w/v); H) B4 in toluene
(1.5% w/v). See the Supporting Information for a color version of images G and H.
tographs (Figure 8A vs. G). 3D surface topographic images
showed fiber bundles and established average heights in the
range of 250–350 nm for the largest individual feature (Fig-
ure 8H).
strain sweep experiment, DSS) to determine the linear vis-
coelastic regime associated to the material (Figure 10A).
Relatively constant tan d (G’’/G’) values during the frequen-
cy sweep indicate a good tolerance of the gels to external
forces. Within the linearity limits of deformation (solid-like
response), the storage modulus was always about one order
of magnitude higher than the loss modulus during the flow
Furthermore, scanning electron microscopy (SEM) of the
corresponding xerogels revealed a 3D appearance of the sur-
face structure of the samples resembling lava flows or wrin-
kles in drapery (Figure 9). It was common to observe nu-
merous fibers fused to form large longitudinal fibrillar archi-
tectures with diameters of about 10–40 mm. Such morpholo-
gies were routinely observed when the gels were prepared
in toluene regardless the presence of a Phe or an azoPhe
residue and its position in the peptide. However, rougher
surfaces were somehow observed in the case of gels made of
peptides bearing an azoPhe moiety.[22] Interestingly, when
protic solvents such as isopropanol were used for the prepa-
ration of the gels, the SEM images showed the presence of
well-defined spherical structures resembling a cluster of
marine sponges with diameters of about 35–70 mm (Fig-
experiment (e.g., G’ꢀ26.6
N
U
for the gel made of A4 in toluene). Further dynamic time
sweep (DTS) measurements at 0.1% strain and 1 Hz fre-
quency confirmed the stability (no phase transition) of the
gel materials as a function of the ageing time at room tem-
perature (Figure 10B). In general, the tan d values between
random measurements of the same material were reproduci-
ble and increased with the concentration of the gelator, sug-
gesting an enhancement of the mechanical damping proper-
ties.
Notably enough, the replacement of the Phe unit in the F-
series by azoPhe caused an increase of the internal resist-
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D. Dꢂaz Dꢂaz et al.
Figure 9. Representative SEM images of xerogels obtained by freeze-drying the corresponding organogels prepared at their MGC as described in
Table S1 (in the Supporting Information). A) A4 in toluene (2% w/v); B) B4 in toluene (1.5% w/v); C) C4 in toluene (2% w/v); D) D4 in toluene (4%
w/v); E) D4 in toluene (4% w/v); F) H-A4 in toluene (7% w/v); G) F-A4 in toluene (2.5% w/v); H) bis-A4 in toluene (0.6% w/v); I) A4 in isopropanol
(2% w/v); J) A4 in isopropanol (2% w/v); K) C4 in isopropanol (6% w/v); L) C4 in isopropanol (6% w/v); M) D4 in isopropanol (10% w/v); N) D4 in
isopropanol (10% w/v); O) D4 in isopropanol (10% w/v); P) bis-A4 in isopropanol (1% w/v).
ance of the gels, as evidenced by the lower tan d value for
the azobenzene-containing peptides.[22] Such reduction of
tan d was about 60–70% in gels prepared from azoPhe-
based gelators in toluene. In isopropanol, the reduction was
estimated to be about 40%. Tan d also increased slightly
upon removal of the N-Boc protection for gels prepared in
toluene. Interestingly, the opposite tendency was observed
for some isolated cases in isopropanol, probably due to ad-
ditional noncovalent interactions established between the
gelator and solvent molecules. For the case of azoPhe-pepti-
des bearing an N-Boc group, increased relative elasticity of
the gel network was routinely observed for the gels pre-
pared in toluene when compared with those in isopropanol.
The influence of the position occupied by the Phe or azoPhe
residue within the peptide sequence on the mechanical
damping properties of the materials was found to be irrele-
vant except for the D gelators. Thus, a slight increase in tan
d was observed for both D4 and F-D4.[22]
Remarkably, we also found a smart thixotropic response
of all azoPhe-based gels to external large straining. The phe-
nomenon of thixotropy was properly described for the first
time by Peterfi in 1927[35] and, nowadays, it is considered a
key property for real-life applications of gel-based materi-
als.[36] Figure 10 shows a simple thixotropy three-steps rheo-
logical loop test[37] for the organogel made of D4 in toluene
(3.8% w/v). The experiment is based on successive cycles in-
volving the following steps: 1) application of a shear strain
as defined by DTS experiments (gel state, G’>G’’), 2) in-
crease of the shear strain until the gel fractures (viscous sol-
ution, G’<G’’), and 3) return at the same rate to the initial
strain% value (recovered gel state, G’>G’’).[22] In the
model example shown in Figure 11, the gel displayed more
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Multistimuli-Responsive Supramolecular Organogels
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Figure 10. Oscillatory rheological measurements of model gels: A) Representative DSS and DFS experiments of the gel made from A4 in isopropanol
(2% w/v). B) Representative DTS experiments of gels prepared from A4 in toluene or isopropanol (2% w/v). All gels were prepared at their MGC as
indicated in Table S1 (the Supporting Information); Tol=toluene. See the Supporting Information for a color version of this figure.
than 90% recovery in its G’ value in only 5 s after the
second cycle. Moreover, a similar behavior was observed in
other solvents like isopropanol (the recovery of the initial
G’ value was about 85%) and regardless the method of
preparation (i.e., heating–cooling, ultrasound). The dramatic
loss of viscosity and recovery of the gel state were also mac-
roscopically observed in a standard glass vial upon a vigo-
rous shaking–resting process.
material to the appropriate wavelength. An appropriate ex-
perimental set up ruled out any temperature effect on gel-
to-sol transition caused by the irradiation process.[22] As
shown in Figure 12 for H-C4 as a model example, UV irra-
diation at lmax =366 nm of the gel made in toluene resulted
in photoisomerization of the side chain azobenzene moiety
from the trans to the cis state, as indicated by the gradual
decrease of the p–p* transition band at 328 nm and concom-
itant increase of the n–p* transition centered at lmax
=
Responsiveness to light: The expected trans-to-cis photoiso-
merisation was observed in solution for all peptides contain-
ing azoPhe or bisazoPhe upon irradiation with UV light at
room temperature. Moreover, light-induced reversible gel-
to-sol transition was achieved for all azoPhe-based peptides
exhibiting a free amino terminus upon exposure of the gel
Figure 12. Reversible changes in the UV/Vis absorption spectra of the gel
made from H-C4 in toluene (2.5% w/v; 0.1 mm path-length). The transi-
tion from the trans-rich gel state to the cis-rich solution state was induced
by UV irradiation at 366 nm for 10 min. The sol-to-gel transition was fur-
ther achieved by exposure of the cuvette to ambient visible light. The
arrows indicate the directions of motion. Original gel state (t=0); solu-
tion state upon 10 min irradiation at 366 nm (t=10 min, 366 nm); pro-
gressive solution-to-gel phase transition under visible light (t=1, 2, 5,
10 min, room light). See the Supporting Information for a color version
of this figure.
Figure 11. Typical thixotropy loop test of the model gel made from D4 in
toluene (3.8% w/v). The strain for each step during shear was first in-
creased from 0.1% (linear viscoelastic regime; gel state) to 100% (rup-
ture of the gel; solution state) and subsequently returned to the original
value (recovery of the gel state). Asterisks indicate the increase of the
shear strain at the time scale. See the Supporting Information for a color
version of this figure.
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D. Dꢂaz Dꢂaz et al.
439 nm. Such trans-to-cis photoisomerization led to a gel-to-
sol transition of the material after 10 min of UV irradiation.
Moreover, cis-to-trans back isomerization of the azobenzene
unit also took place upon exposure of the cuvette to room
light. These reversible transitions could be repeated for sev-
eral cycles. These results indicate that the isomerization
toward the trans-form of the azobenzene residue is gradually
hindered by the growth of the supramolecular fibrillar net-
work. The final content of the trans-isomer after irradiation
was estimated by integrating the difference of the curves in
approximately 15–20% of the original content in the gel
state. Despite the fast trans-to-cis and cis-to-trans isomeriza-
tion processes, the complete gel-to-sol and sol-to-gel transi-
tions, respectively, required prolonged exposure times to
light of appropriate wavelength. Thus, complete macroscop-
ic gel-to-sol transition in toluene was observed within 30–
60 min of UV irradiation, whereas complete sol-to-gel was
achieved after exposing the material to visible light for 3–
8 h.[22] The absolute values are specific for each case as the
time required for isomerization is both solvent and concen-
tration-dependent (e.g., the response times of gels made in
isopropanol were found to be longer than those in tol-
uene).[22]
The isosbestic points of the solution and gel states are
centered at 290 and 378 nm, in which the absorbance values
differ by about 0.03ꢂ0.01 (Au) with respect to the inter-
mediate states achieved during irradiation. As these values
are within the standard deviation of absorbance for replicate
measurements, only two absorbing species (i.e., trans- and
cis-azobenzene-containing molecules) are likely to be in-
volved during the light-induced gel,sol transition. The ab-
sorption corresponding to the p--p* transition band exhibit-
ed a narrow 3 nm redshift in the gel state with respect to the
solution state (i.e., 1 min after exposure to room light),
which based on the exciton theory suggests that azobenzene
groups likely packed into 1D J-aggregates instead of H-ag-
gregates.[38,39]
Figure 13. Diagram showing the preparation of multistimuli responsive
organogels from the azoPhe-containing peptide H-D4 and the interrelat-
ed response between different materials. Tgel values: E=47(ꢂ1)8C; C=
50(ꢂ1)8C; G= ꢂ28C. Tan d values: E=0.47; C=0.41; G=0.42. Gelation
times: A!E=2–3 min; B!C=5–10 min; F!G=5–10 min; I!C=10–
12 h. Liquefaction times: C!I=45–60 min; C!D=20–30 min; C!J=
10–30 min. Selected FE-SEM images are shown at the bottom: K: gels
prepared by heating–cooling (G’=1.78ꢆ105 Pa; tan d=0.42); L: gel pre-
pared by ultrasound treatment (G’=1.21ꢆ105 Pa, tan d=0.49); M: solu-
tion obtained upon UV-irradiation of gel K. See the Supporting Informa-
tion for a color version of this figure.
Solutions F and B were found to be equivalent according to
both thermal and mechanical properties of the correspond-
ing gels (Figure 13C and G). Additionally, the existence of a
free amino terminus in the peptide gelator allowed for a
light-induced gel-to-sol transition (Figure 13I), which was
reversible (Figure 13C). High-resolution FE-SEM images
demonstrated the complete destruction of the fibrillar net-
work upon UV-irradiation (Figure 13K, L vs. M), as well as
the general preservation of the gel morphology for materials
prepared either by thermal or ultrasound treatment (K vs.
L). On the other hand, the transition from I to C was not as-
sisted by ultrasound within minutes under visible light, as
occurs for the A!E transformation. Hence, ultrasound
treatment of peptide solution does not accelerate the cis-to-
trans isomerization of the azobenzene group. Responsive-
ness to chemical stimuli was also observed (Figure 13H and
J). This behavior, exemplified in Figure 13 for H-D4, was
general for all the organogels prepared regardless the gela-
tor family and solvent. Thus, addition of some solvents in
which gelation did not take place led to the formation of
clear solutions. For instance, when the gels made of C4,
H-D4 or F-A4 or H-D4 were overlaid with tetrahydrofuran,
1,4-dioxane or dichloromethane (gel volume/additional sol-
Remarkably, the light-induced gel-to-sol transition was
only observed for azobenzene-based organogels lacking the
N-Boc protection. In general, this process was also faster in
aromatic solvents than in alcohols. In agreement with previ-
ous studies in other systems,[13] this may be due to some ster-
ically hindered spatial conformation of the aggregates that
prevents the trans-to-cis isomerization of the azoPhe moiety
when the N-Boc protection is present.
Multiresponsive map: Figure 13A summarizes the study car-
ried out with the model peptide H-D4, which is insoluble in
toluene at room temperature (2.5% w/v). An isotropic solu-
tion could be achieved upon heating (Figure 13B), which
led to the thermoreversible formation of a stable organogel
upon cooling (Figure 13C). Vigorous shaking of the vial af-
forded a viscous fluid state (Figure 13H), which returned to
the precedent gel state upon resting (thixotropy). Ultra-
sound could be also used to dissolve H-D4 and generate a
dense fibrillar gel material (Figure 13E), which was also liq-
uefied upon heating affording a clear solution (Figure 13F).
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ed to Prof. Claudio Toniolo (University of Padova) for pointing out the
potentialities of the combination of AspACHTNUTGRNEUNG(OBzl) and azoPhe switch pepti-
des, Prof. A. Gçpferich (University of Regensburg) for granting access to
his rheometer, and Ms. E.-M. Schçn for assistance with the rheological
measurements.
vent volume=1:1), they were dissolved within 20–30 min
without shaking. In contrast, the gels were stable for several
days in the presence of solvents like pentane. Moreover, the
gels were transformed into either solutions or dispersions
upon gentle shaking in the presence of 0.1m aqueous NaOH
within 10–30 min.[22] Solvent evaporation (Figure 13B–I) and
drying of the residue under vacuum allowed full recovery of
H-D4, which could be reused without losing its properties.
In addition, the structural anisotropies of the organogels ob-
served by electron microscopy switched on their birefrin-
gence under polarized light,[22] which could be also switched
off upon gel-to-sol transition. Such fine birefringence con-
trol is important for a number of optical device applica-
tions.[40]
[1] a) P. Bawa, V. Pillay, Y. E. Choonara, L. C. du Toit, Biomed. Mater.
2009, 4, 022001; b) M. A. C. Stuart, W. T. S. Huck, J. Genzer, M.
Mꢃller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V.
Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov, S. Minko,
Nat. Mater. 2010, 9, 101–113; c) M. W. Urban, Handbook of Stimuli-
Responsive Materials, Wiley-VCH, Weinheim, 2011.
[2] a) X. Y. Liu, Top Curr. Chem. 2005, 256, 1–37; b) R. G. Weiss, P.
Terech, Molecular Gels: Materials with Self-Assembled Fibrillar Net-
works, Springer, New York, 2006; c) M. George, R. G. Weiss, Acc.
Chem. Res. 2006, 39, 489–497; d) G. C. Maity, J. Phys. Sci. 2007, 11,
156–171; e) S. Banerjee, R. K. Das, U. Maitra, J. Mater. Chem. 2009,
19, 6649–6687.
[3] a) D. Derossi, Y. Kajiwara, Y. Osada, Polymer Gels: Fundamentals
and Biomedical Applications, Plenum, New York, 1991, and referen-
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Conclusion
The foregoing results have showed that the incorporation of
azobenzene residues into the side chain of LMW peptides
can modulate their self-assembly in organic solvents leading
to the formation of stimuli responsive supramolecular orga-
nogels. The synthetic strategy is advantageous in terms of
structural flexibility since it is compatible with a free, unpro-
tected amino terminus and allows placement of the chromo-
phore at any position of the peptide sequence. The major
driving forces for the observed gelation process are hydro-
gen bonding and p–p interactions, which can be triggered
either by thermal or ultrasound external stimuli. In addition,
a predictive model for gelation of polar protic solvent was
developed by using Kamlet–Taft solvent parameters and ex-
perimental data. Remarkably, the obtained viscoelastic ma-
terials exhibited interconnected multistimuli responsive be-
haviors including thermal-, photo-, chemo-, and mechanical
responses. Structure–property relationship studies have
demonstrated that the presence and position of the azoben-
zene residue can be operated as a versatile regulator to
reduce MGC and enhance both the thermal stability and
mechanical strength of the gels. To the best of our knowl-
edge, this is the first time in which LMW peptides bearing
side-chain azobenzene units are used for the synthesis of
“intelligent” supramolecular organogels. This investigation
opens new perspectives for the rational design of tunable
multiresponsive physical gels from small peptides. Efforts
towards potential applications of the described materials, as
well as structural and charge modifications of the peptides
in order to expand the scope to biocompatible hydrogels are
currently underway in our laboratories.
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Acknowledgements
The authors thank the Ministerio de Ciencia e Innovaciꢇn–FEDER
(project CTQ2010-17436, FPU fellowship to P.F.), Gobierno de Aragꢇn–
FSE (research group E40), CSIC (project PIE 200980I059), Alexander
von Humboldt Foundation (fellowship for Experienced Researchers to
D.D.D) and the University of Regensburg (Anschubfinanzierung von
Wissenschaftlichen Projekten-2011) for financial support. We are indebt-
Chem. Eur. J. 2013, 00, 0 – 0
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Received: February 28, 2013
Published online: && &&, 2013
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ÝÝ
These are not the final page numbers!

Multistimuli-Responsive Supramolecular Organogels
FULL PAPER
Gels
P. Fatꢀs, J. Bachl, S. Oehm,
A. I. Jimꢁnez, C. Cativiela,
D. Dꢂaz Dꢂaz* ................ &&&& — &&&&
Multistimuli-Responsive Supramolec-
ular Organogels Formed by Low-
Molecular-Weight Peptides Bearing
Side-Chain Azobenzene Moieties
Intelligent response: The incorporation
of the azobenzene moiety into the side
chain of low-molecular-weight peptides
allows for the preparation of multisti-
muli-responsive supramolecular orga-
nogels. The presence and position of
the azobenzene residue act as a versa-
tile regulator to reduce the minimum
gelation concentration and enhance
both the thermal stability and mechan-
ical strength of the materials (see
figure).
Chem. Eur. J. 2013, 00, 0 – 0
ꢅ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!