Tripeptide based super-organogelators: Structure and function

Article abstract of DOI:10.1039/c8nj05578e

A novel series of tripeptide-based low molecular weight super-organogelators were synthesized and characterized. Four tripeptides with diverse steric crowding at the central amino acid residue were studied. From this series, only sterically less hindered

Full text of DOI:10.1039/c8nj05578e

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Podder, S. Roy Chowdhury and S. Nandi, New J. Chem., 2019, DOI: 10.1039/C8NJ05578E.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Tripeptide based super-organogelators: structure and function
Debasish Podder,a Srayoshi Roy Chowdhury,a Sujay Kumar Nandia and Debasish Haldar*a
Received 00th January 20xx,
Accepted 00th January 20xx
A novel series of tripeptide based low molecular weight super-organogelators were synthesized and characterized. Four
tripeptides with diverse steric crowding at the central amino acid residue were studied. From these series, only sterically
less hindered peptide 1 and peptide 3 formed organogel in different saturated hydrocarbons, crude oil, and aromatic
solvents. In diesel, the peptides formed gel at 1 wt% i.e. act as super-organogelators. Interestingly both the peptides gel
show high stability and remarkable self-healing property. As the peptides have electron rich phenyl system, the
corresponding gel also interact with cationic dyes and can remove selectively cationic dyes from the waste water. The
super-gelators have very high efficiency to solidify only the oil from a biphasic mixture of oil-water. Due to the high
solubility of the super-gelators in non-toxic organic solvent ethanol, the solution is easy to handle and just by spraying the
ethanol solution over oil-water mixture be able to perform gelling function at room temperature. The trapped water under
the organogel can be pumped out and the oil can be recovered from the organogel by vacuum distillation. So, the super-
gelators can be used as a low cost, non toxic, room temperature easy to use material for the oil-spill recovery.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Since last century, oil spill accidents have occurred around
the world during oil transportation by ships or from natural
calamities in oil fields and pipelines. Due to the oil spill, marine
Introduction
Supramolecular gels are semi-solid like materials that can
encapsulate a large amount of solvent.1-4 These gels are also
known as low molecular weight gels2 (LMWG) which are
basically composed of elastic entangled fiber network
obtained by self-assembly of building blocks through various
non-covalent interactions such as hydrogen bonding, π-π
stacking, dipole-dipole interactions, and Van der Waals
interactions.5-9 These gels are responsive to different external
stimuli like temperature, pH, solvent polarity, light, ultrasound,
and salts.10-28 These make supramolecular gels suitable for
various applications such as in the field of sensing,
pharmaceutics, catalysis, actuating, charge transport, reaction
media, dye absorption and oil spill recovery.29-35 Although
there is a large number of reports of low molecular weight
organogelators, still discovery of a new organogelator having
low biphasic minimum gelling concentration (BMGC) at
ambient environment is challenging.36-37
Dyes are essential for the textile industry but they have
been found to pollute water which causes harmful effects on
the environment. There are various methods such as carbon
black adsorption, chemicoagulation, sedimentation, advanced
oxidation procedure to remove dyes from waste water.38-41
However, these processes have some disadvantages. Thus,
removal of dyes from waste water is highly demanding.
oil pollution is the most alarming issue and is a serious threat
to the marine life and marine ecosystem.42 Phase-selective
organogelator which can selectively solidify the oil layer from
the oil-water biphasic mixture can be a useful solution to this
problem. Toward this goal, three research groups (Sureshan,
Zeng and Chaudhuri) have carried out impressive works,43-46
with many different phase-selective organogelators, that can
work either in powder form or in solution phase due to their
high solubility in non-toxic organic solvents.47-52 But the
amount of expensive gelator is the major stumbling block. So,
we need to develop a super-organogelator that will form gel
with low BMGC at ambient condition.
Previously we have reported the self-assembly behaviour
of the dipeptide Boc-Phe-Phe-OMe by X-ray crystallography
and various microscopic techniques.53 This compound formed
an opaque gel in xylene by heating-cooling technique but the
minimum gel concentration (MGC) was very high (40mg/ml).
In 2008 Li and co-workers reported the formation of self-
assembled organogel by NH2-Phe-Phe-COOH.54 Ulijn and co-
workers have reported the gelation behaviour of Fmoc
protected Phenylalanine and its derivatives.55-59 Recently, We
have introduced different achiral spacers between two
phenylalanine residues and study their stimuli-responsive
behaviour.60-61 In this regards here we have designed and
synthesized a series of tripeptides keeping phenylalanine
residue at two terminal positions and introducing different α-
amino acids with increasing steric hindrance at the α-carbon as
spacers between two phenylalanine residues (Fig.1). The
a.Department of Chemical Sciences, Indian Institute of Science Education and
Research Kolkata, Mohanpur -741246, West Bengal, India E-mail:
deba_h76@yahoo.com; deba_h76@iiserkol.ac.in
† Electronic Supplementary Information (ESI) available: [Peptide synthesis, NMR
spectroscopy, rheology curves, Figure ESI S1-S35, peptide single crystals (CCDC
1855060)]. See DOI: 10.1039/x0xx00000x
peptides contain phenylalanine, alanine,
-amino isobutyric
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acid, phenylglycine and 1-Aminocyclohexanecarboxylic acid done the Solid state FT-IR of all the peptides 1-4Vtieow kArnticolewOntlhinee
(Fig. 1). Peptides and are more sterically hindered than
backbone conformation and self-assembDlyOpI:a1t0t.e10r3n9./FCr8oNmJ05F5T7-8IRE
and 3. Among these peptides, only tripeptides and formed spectroscopy, for peptides and , there are intense bands at
transparent organogel in various aromatic solvents and 3302 cm-1 and 3306 cm-1 respectively which are responsible for
saturated hydrocarbons. The transparent gels have significant the hydrogen bonded NH. But in case of peptides and 4,
also exhibit there are bands at 3427 cm-1 and 3395 cm-1 respectively along
2
4
1
1
3
1
3
2
self-healing property. These peptides
phase selective gelation of oil from a biphasic mixture of oil with 3283 cm-1 and 3323 cm-1 bands that indicates all the NH
and water and can be used for oil spill recovery. As the the are not hydrogen bonded (ESI Figure 1). Hence, backbone
1 and 3
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super-gelators are high solubility in non-toxic organic solvent structures and hydrogen bonding patterns of peptides
ethanol, the solution is easy to handle and just by spraying the are similar whereas peptides and are structurally close.
ethanol solution over oil-water mixture be able to form Peptide was characterized by X-ray Crystallography Peptide
organogel at room temperature. The trapped water under the crystallizes with one molecule in the asymmetric unit from
organogel can be pumped out and the oil can be recovered methanol-water solution by slow evaporation. The peptide
1 and 3
2
4
2
.
2
from the organogel by vacuum distillation.
adopts a kink like structure (Fig. 2). There is no intramolecular
hydrogen bond. The important torsion angles are listed in ESI
Table 1. In higher order packing peptide
2 self-assembles
through intermolecular hydrogen bonding (N7-H7….O2, 2.02Å,
2.87Å, 169°, 1+x,y,z) to form a column-like structure (ESI Figure
2).
Fig. 2. The ORTEP diagram of peptide 2.
Initially, the gelation abilities of all the peptides were checked
in various organic solvents by dissolving 10 mg of compound in
1 ml of different organic solvents. Peptide
1 and Peptide 3
were found to form transparent gels (Fig 3 and ESI Figure 3) in
different aromatic solvents like xylene, o-xylene, p-xylene, m-
xylene, toluene, benzene (Electron donating solvents) as well
as chlorobenzene, 1,2-dichlorobenzene (Electron withdrawing
solvents) and different saturated hydrocarbons like petrol,
diesel, kerosene, mustard oil, body oil, olive oil, crude oil by
heating-cooling technique only (ESI Figure 4 and Fig.4). But
Fig. 1. The schematic representation of tripeptides 1-4
.
Results and discussion
We have designed and synthesized four tripeptides Boc-Phe-
XXX-Phe-OMe (XXX = Alanine/ -Aminoisobutyric acid/ Phenyl
glycine/ 1-Aminocyclohexanecarboxylic acid) (Fig.1). Steric
peptide
2
and peptide
4
failed to form gels in any organic
and 3, the spacer i.e
solvent as well as in oil. For peptides
1
the central amino acid between the two phenylalanine
residues containing one hydrogen atom along with one methyl
crowding at the α-carbon of the central amino acid spacer
gradually increases from peptide
1 to peptide 4. We wish to
group in case of peptide
peptide
1 and one phenyl group in case of
study the effect of the spacer in the tripeptide self-assembly.
The reported tripeptides have been synthesized by
conventional solution phase methodology. The final
compounds have been purified and characterized by H-NMR,
13C-NMR, FT-IR and Mass spectrometry analysis. First we have
3
.
1
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gel is sufficiently strong to tolerate the weight aVbieowuAtrt1ic5le0Onglimne
(Fig. 5).
Further, we have synthesized other three tripeptides by
replacements of phenylalanine with Leucine, Valine and
Alanine and studied their gelation behavior in diesel.44,46,48 We
have used leucine, valine and alanine as a terminal amino acid
DOI: 10.1039/C8NJ05578E
Fig.3. Transparent gels of peptide 1 in different aromatic solvents.
for peptide
three peptides only leucine containing
but at higher MGC and BMGC compare to peptide
mg/ml and BMGC is 17.3 mg/ml for peptide . The peptides
and failed to form gel in diesel. So, from all of these peptides,
peptide has lower MGC and BMGC in diesel.
5
, peptide
6
and peptide
7
respectively. From these
forms gel in diesel
. MGC is 8
Gelation was confirmed by the inverted vial method (Fig 3).
The minimum gel concentrations for peptide and are very
less compared to the only Boc-Phe-Phe-OMe. Here the MGC
varies in the range 2.5 mg-6 mg/ml for peptide and 2.2-4.8
mg/ml for peptide . For both the peptides, the gels from
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1
3
1
5
6
1
7
3
1
xylene having lowest MGC and the gels from chlorobenzene
having highest MGC. Minimum gelation concentrations of all
the reported gels are summarized in ESI Table 2. Surprisingly
the MGC for the gels in various oils were much lower
compared to those in aromatic solvents. In different oils, the
The mechanical strength of the gel was measured by
rheology experiments. Storage modulus (G’) and loss modulus
(G”) are the main two factors in rheology where G’ represents
the elastic response and G” represents the viscous behaviour
of the gel. We have prepared the gel of peptides
1 and 3 in
MGC for Peptide
1 and 3 varies from 1.2-2.1 mg/ml and 1.4-2.1
xylene and performed oscillatory frequency sweep
experiment. In a typical frequency sweep experiment, the
variation of G’ and G” is monitored as a function of angular
mg/ml respectively and are summarized in ESI Table 2. So, the
peptides
petrol.
1 and 3 can act as super-gelators for diesel and
frequency at a constant strain 0.01%. For both peptides
1 and
As gelation in crude oil is highly demanding, we have also
checked the gelation of peptide and peptide in crude oil.
Peptide and peptide can successfully form gel in crude oil
by normal heating-cooling technique (ESI Figure 5.)
3
gels G’ is greater than G” and did not cross each other over
1
3
the entire the range of angular frequency, which indicates that
the gels are elastic in nature (Fig. 6). We have also performed
the rheology experiment of the gel from diesel (ESI Figure. 6)
1
3
Fig.4. Transparent gels of peptide 3 in different oils.
Water
Diesel
Fig. 5. The peptide
3
gel in diesel is sufficiently strong to tolerate the weight of
150 gm water. CoCl2 was added to water for clarity.
Fig. 6. Rheology of organogel in Xylene of (a) peptide
storage modulus of the gel (1 wt%) was found to be larger than the loss
modulus indicates an elastic rather than viscous material.
1 and (b) peptide 3 . The
G

G

We have also measured the Tgel (gel to sol transition
temperature) for the reported gels (ESI Table 3). For both the
The morphologies of the gels were examined by optical
microscope. The optical microscopic images of peptide
peptides
1 and 3, Tgel of gels from the aromatic organic
1
solvents are lower than the gels in oils. For both the peptides,
with increasing peptide concentration Tgel increases. All the
organogels are thermo-reversible. The gels are stable up to 2
months without loss of transparency, colour, and shape. The
xerogels exhibit elongated fiber-like morphologies (ESI Figure
7). To better understand the morphologies of the gels, FE-
SEM is a wonderful technique. From FE-SEM, xerogels of
peptide
1
obtained from different solvents show the branch of
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nanorods or nanofibers of length up to few micrometers (Fig. these solutions were placed in vials containing gel of peptide 1
7). The nanofibers are interlinked to produce a 3D network in xylene. However, the gel matrix selectively entraps cationic
View Article Online
DOI: 10.1039/C8NJ05578E
that can entrap solvents. However, the xerogels of peptide
exhibit fibers of small length (ESI Figure 8).
3
dyes from the aqueous medium. The dye absorption study of
the gelator was very fast and monitored by Uv-Vis
spectroscopy as a function of time (Fig. 10). Within 24 h more
than 95% removal of the dyes was observed. As the peptide is
electron-rich due to presence of aromatic rings and the amide
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Fig. 7. FE-SEM images of the xerogels of peptide
1 from (a) 1,2-dichlorobenzene
(b) m-Xylene (c) Toluene (d) Xylene (e) p-Xylene and (f) o-Xylene.
Fig. 9 Alternate arrangement of Rhodamine 6g-doped and undoped gel cylinders
that self-heal to form a single cylinder with sufficient mechanical strength and
the diffusion of the dye from doped to the undoped portion of the gel.
To further investigate the morphologies of gels in different
organic solvents, AFM experiment was performed. The AFM
images of the xerogel obtained from different organic solvents
show nanorods like morphologies for the peptide 1 with
almost similar size and shape (Fig. 8).
bonds, it has an attractive interaction with the cationic dyes but not
with anionic or neutral dyes. The peptide 1 gel can be reused
several times for the cationic dye removal with almost same
efficiency.
Fig. 8. AFM images of the xerogels of peptide
Dichlorobenzene (b) Toluene and (c) p-xylene.
1 obtained from (a) 1,2-
More interestingly these gels exhibit amazing self-healing62-
63property. When a cylindrical gel was cut into several small
pieces and joined together, the gel pieces were found to
integrate into a single cylindrical block within 15 minutes.
When dye-doped (Rhodamine 6G) gel pieces were combined
with undoped gel pieces in an alternative manner, the flow of
the dye from doped to the undoped pieces confirmed the
diffusion of the dye to the undoped portion that indicates the
dynamic flow of the solvent through the joining point (Fig. 9).
The self-healing may be due to dynamic equilibrium between
the generation of the new fibers and the disconnection of the
old fibers and that governs the reformation of gel.
Fig. 10. Dye removal studies using peptide 1 gel in xylene (a) Methyl Violet (b)
Rhodamine 6G (c) Pyrocatechol Violet (d) Methyl Orange
We have examined the phase selective gelation of both the
peptides
investigated phase selective gelation by standard heating
cooling technique. We have added 2.5 mg of peptide in the
1 and 3 from oil-water mixture. Primarily we have
It is interesting to note that the gel of the peptide
1 has
1
enormous potential to absorb toxic organic dyes. There are
various types of dyes like cationic, anionic and neutral. For this
purpose, 1 mM solution of dyes rhodamine 6G (cationic),
methyl violet (cationic), methyl orange (anionic), and
pyrocatechol violet (neutral) was prepared. Then, 1 mL of
mixture of water and diesel (each 1 ml) and warmed the
solution for solubilizing the peptide. Then the mixture was
kept to cool at room temperature and instantly within 2
minutes diesel layer was solidified above the water. However,
heating-cooling in phase-selective gelation is an inappropriate
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1
by 500 MHz, 400MHz H NMR spectroscopy, 13C NMR spectroscopy
technique for practical application such as oil-spill recovery
due to the high flammability of most oils.35,64 To avoid such
incident, a room temperature alternative procedure has
View Article Online
(125 MHz, 100MHz), mass spectrometry andDIROIs:p10e.c1t0r3o9s/cCo8pNyJ.05578E
NMR Experiments
developed. BMGC of Peptide
mg/ml and 3.6 mg/ml respectively. In this protocol, 20 mg of
peptide was dissolved in 1 mL of ethanol. This aliquot was
1 and Peptide 3 for diesel are 2.5
All NMR experiments were performed on a Jeol 400 MHz or
Bruker 500 MHz spectrometer. Compound concentrations were
in the range 1–10 mM in CDCl3 and DMSO-d6.
1
injected at the interface of the diesel–water mixture (5ml diesel
and 10ml water) (Fig. 11) at room temperature. Immediately
diesel layer formed gel keeping the water phase intact at room
temperature. We have introduced an injection syringe through
the self-healing gel layer to the water layer and removed the
water completely. Finally, by vacuum distillation process we
recovered the diesel and collect the super organogelator. The
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FT-IR Experiments
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FT-IR spectroscopy in solid-state was performed with a Perkin
Elmer Spectrum RX1 spectrophotometer using KBr disk
method.
Absorption spectroscopy
The absorption spectra of peptides were measure on a Perkin
Elmer UV/Vis spectrometer (Lambda 35) using quartz cell
having 1 cm path length.
material cost of the peptide
reported diesel gelators.
1 is INR 30/gm which is lower than
Mass Spectrometry
Mass spectrometry was carried out on a Waters Corporation Q-
Tof Micro YA263 high-resolution mass spectrometer by
electrospray ionization (positive-mode).
Polarised optical Microscope
A small amount of solution of the compound was placed on a
clean glass cover slip and then dried by slow evaporation, then
visualized at 40 magnification (Olympus optical microscope
equipped with polarizer and CCD camera).
Field Emission Scanning Electron Microscopy
Field emission-scanning electron microscopy (FE-SEM) was
performed to examine the morphologies of the synthesized
peptides. A drop of peptide solution was casted on a clean
microscopic glass slide and dried under vacuum. The samples
were gold-coated, and the images were captured in an FE-SEM
apparatus (Jeol Scanning Microscope-JSM-6700F).
Atomic force microscopy
The morphology of the reported compound was investigated by
atomic force microscopy (AFM). A drop of the sample solution
were placed on a clean microscope cover glass and then dried
by slow evaporation. The material was then allowed to dry
under vacuum at 30°C for two days. Images were taken with an
NTMDT instrument, model no. AP-0100 by semicontact-mode
Fig. 11. Removal of diesel from water. (a) Diesel layer above water. (b) Addition
of ethanol solution of peptide 1. (c) The phase selective gel. (d) Insertion of
injection syringe in to water layer through gel. (e) Suction of water in syringe. (f)
Collection of water and (g) Separated water and organogel.
Experimental
General
All L-amino acids (L-phenylalanine, 2-aminoisobutyric acid, L-
alanine, L-phenylglycine and 1-Aminocyclohexanecarboxylic acid)
Gelation
1.5 mg of compound was dissolved in 1 mL of various organic
solvents like different aromatic solvents like xylene, o-xylene, p-
xylene, m-xylene, toluene, benzene (Electron donating solvents) as
well as chlorobenzene, 1,2-dichlorobenzene (Electron withdrawing
solvents) and different saturated hydrocarbons like petrol, diesel,
kerosene, mustard oil, body oil, olive oil, crude oil by heating-
cooling technique only. Homogeneous gel was formed.
were
purchased
from
Sigma
chemicals.
HOBt
(1-
hydroxybenzotriazole) and DCC dicyclohexylcarbodiimide) were
purchased from SRL.
Peptide Synthesis
Rheology Experiments
All the peptides were synthesized by conventional solution phase
methods by using racemization free fragment condensation
strategy. The Boc group was used for N-terminal protection and the
C-terminus was protected as a methyl ester. Deprotections of
methyl ester were performed using saponification method.
Couplings were mediated by dicyclohexylcarbodiimide/1-
hydroxybenzotriazole (DCC/HOBt). All of the intermediates were
To examine the thixotropic behaviour and mechanical strength
of the gel, we have done rheological measurements on a MCR
102 rheometer (Anton Paar, Modular Compact Rheometer) by a
steel parallel plate geometry having 40 mm diameter at 20 °C.
The rheometer was attached to a Peltier circulator thermo cube
in order to control the temperature accurately. The storage
modulus (G′) and loss modulus (G″) of the gel were then
recorded by using the setup.
1
characterized by H NMR spectroscopy, 13C NMR spectroscopy and
mass spectrometry. The final compounds were fully characterized
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Conflicts of interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/C8NJ05578E
X-ray crystallography
Diffraction quality white crystals of peptide
2 were obtained
from methanol-water solution by slow evaporation. Intensity
data were collected with MoK radiation by Bruker APEX-2
CCD diffractometer. Data were processed using Bruker SAINT
package. The structure solution and refinement were performed
by SHELX97. Refinement of non-hydrogen atoms was
performed using anisotropic thermal parameters. Crystal data of
Peptide 1: C28H37N3O6, Mw = 511.61, P1 21/C 1, a = 5.9854(3)
Å, b = 19.6802(8) Å, c = 23.5930(16) Å,  = 90°,  = 90°,  =
90°, V = 2779.1(3) Å3, Z = 4, dm = 1.22 Mg m-3, , R1 =
0.0537 and wR2 = 0.1548 CCDC 1855060 contains the
crystallographic data for the crystals respectively.
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Acknowledgements
We acknowledge IISER Kolkata, India, for financial assistance.
D. Podder and S. K. Nandi thanks CSIR, India for research
fellowship. S. R. Chowdhury acknowledges the IISER-K, India
for fellowship.
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Notes and references
1
2
P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699–2715.
N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34
821–836.
,
,
Self-Healing Study
We have prepared two set of cylindrical gel (dye doped and dye
3
4
5
6
7
8
9
G. Yu, X. Yan, C. Han and F. Huang, Chem. Soc. Rev., 2013, 42
6697—6722.
undoped) by dissolving 30 mg of peptide
1 in 6 ml of xylene in
test tube. We have used Rhodamine 6G dye for doping. Then
we cut both the gel cylinder (dye doped and dye undoped) into
small pieces using a blade and arranged them in an alternative
manner in close pack.
Dye removal study
To check the dye absorption ability of the organogel, initially
S. Bera and D. Haldar, J. Mater. Chem. A, 2016, 4, 6933–
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,
we have prepared a organogel by dissolving 5 mg of peptide
1
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in 1 ml of xylene. Then we have placed 1 ml of different dye
solution (1mM) in vials containing the gel. The dye absorption
ability of the gel was monitored by Uv-Vis spectroscopy. We
have taken out 0.01 ml of aqueous solution containing the dyes
and dilute it up to 1ml and measured the absorbance and follow
the same procedure with different time interval.
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Phase-selective gelation
Commun., 2007, 0, 4758-4760.
In a typical phase-selective gelation experiment conducted at room
temp, 20 mg of peptide 1 was dissolved in 1 mL of ethanol. This
aliquot was injected at the interface of the diesel–water mixture (5ml
diesel and 10ml water).
12 J. J. D. Jong, L. N. Lucas, R. M. Kellogg, J. H. V. Esch and B. L.
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In conclusion, we have designed and synthesized a new class
of super-gelators that can separate oil by phase-selective
gelation of oil from water-oil mixture at room temperature.
The super-gelators are high soluble in non-toxic organic
solvent ethanol, the ethanol solution is easy to handle and
just by spraying the ethanol solution over oil-water mixture be
able to perform gelling function at room temperature. The
trapped water under the gel can be pumped out without
disturbing the gel. Finally, the oil can be recovered from the
organogel by vacuum distillation. The organogel show high
stability and significant self-healing property. As the peptides
have electron rich phenyl groups, the corresponding gels also
exhibit interactions with cationic dyes and can remove those
cationic dyes selectively from the waste water. So, the super-
gelators can be used as a low cost, non toxic, room
temperature easy to use material for the oil-spill recovery and
water cleaning. The results are promising and environmentally
benign.
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Debasish Podder, Srayoshi Roy Chowdhury, Sujay Kumar Nandi and Debasish Haldar*
The peptide based super-gelators are highly soluble in non-toxic organic solvent ethanol, the
solution is easy to handle and just by spraying the ethanol solution over oil-water mixture be able
to form organogel at room temperature.