Unusual sculpting of dipeptide particles by ultrasound induces gelation

Article abstract of DOI:10.1021/ja711342y

A readily synthesized dipeptide shows unprecedented gelation behavior when dispersed and submitted to ultrasound in nonsolvents. SEM and FFEM revealed spectacular shape changes from a sheet-like material into a highly interconnected fiber network and ribb

Full text of DOI:10.1021/ja711342y

Published on Web 02/21/2008
Unusual Sculpting of Dipeptide Particles by Ultrasound Induces Gelation
David Bardelang,*, Franck Camerel,* James C. Margeson, Donald M. Leek, Marc Schmutz,
†
,‡
§
†
|
†
†
⊥
‡
,†
Md. Badruz Zaman, Kui Yu, Dmitriy V. Soldatov, Raymond Ziessel, Christopher I. Ratcliffe,* and
†
John A. Ripmeester
Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario
K1A0R6 Canada, Laboratoire de Chimie Mol e´ culaire, ECPM, UMR 7509, CNRS - UniVersit e´ Louis Pasteur,
25 rue Becquerel, 67087 Strasbourg Cedex 02 France, Institute for Research in Construction, National Research
Council of Canada, 1200 Montreal Road, Ottawa, Ontario K1A0R6, Canada, Institut Charles Sadron, CNRS -
UPR 22, 6 rue Boussingault, 67083 Strasbourg France, and Department of Chemistry, UniVersity of Guelph,
5
0 Stone Road, East, Guelph, Ontario N1G2W1 Canada
Received December 21, 2007; E-mail: christopher.ratcliffe@nrc-cnrc.gc.ca
Recently, ultrasound has emerged to play unexpected roles in
1
physics and chemistry, for example causing mechano- and sonolu-
2
minescence, or to bias classical reaction pathways to reach
unexpected products.3 Another intriguing example of this peculiar
,4
influence of ultrasound was first described by Naota and co-workers
5
regarding organogelation by pallado-macrocycles. This result was
6
extended recently to new peptide-based palladium complexes. In
these two cases, the mechanism involved conformational changes
of the gelator triggered by a period of sonication which induces
supramolecular self-assembly leading to gelation of the liquid. Such
an unusual way to induce gelation is highly interesting, as it seems
counterintuitive that application of ultrasound will be compatible
with the successive self-assembly events leading to a gelation with
a low molecular weigth gelator through the formation of weak
intermolecular interactions.7 Indeed, ultrasound is usually used in
laboratories and in food industries to disrupt weak intermolecular
interactions, to solubilize molecules, or to disperse particles. Here
we describe the basis of a new gelation mechanism based on a
readily prepared dipeptide, in which ultrasound modifies the mor-
phology of the material from sheet-like particles into 3-dimen-
sional networks of fibers or ribbons, which are responsible for
gelation.
,8
Figure 1. (a) Molecular structure of dipeptide 1; (b) gels formed after
-
2
submitting 1 to sonication (40.0 kHz, 0.28 W.cm ) in various alkanes or
a mixture of polar solvents at 20 °C (c ) 26 mM) (from left to right:
n-hexane, n-hexadecane, cyclohexane, and water/methanol (20/80 vol %)
(tsonic ) 60 s, except for n-hexadecane tsonic ) 240 s).
Dipeptide 1 was prepared in a single step procedure from
N-Fmoc-protected Leucine and C-benzoyl-protected Leucine by
peptidic coupling in acetonitrile (Figure 1 and ESI for synthesis).
Although dipeptide 1 was soluble in a wide variety of solvents at
seems to be a unique feature of the present dipeptide since molecules
lacking the benzoyl group or with a benzoyl replacing the Fmoc
group do not show any solvent gelation by ultrasound.
Scanning electron microscopy (SEM) was used to investigate
the mechanism of gel formation in hexane. The as-synthesized
material is composed of nano- to micrometer size sheet-like particles
(Figure 2a). Dipeptide 1 immersed in hexane for several days prior
to solvent evaporation and imaging remains essentially identical
in appearance to the as-prepared material (see Figure S2, Supporting
Information). Likewise, when the dipeptide is dispersed in hexane
followed by a heating-cooling cycle, well-defined sheets are clearly
visible from the precipitate (Figure 2b). On the other hand, dramatic
morphological changes are observed from samples subjected to
ultrasound. After only 10 s of sonication (Figure 2c), the micro-
graphs clearly indicate a restructuring of the sheet via the appearance
of tiny holes and wrinkling of the surface. Longer exposure to
ultrasound (20 s, Figure 2d) confirms the emergence of holes. After
40 s exposure to ultrasound, SEM shows that the pore volume is
dramatically expanded and the peptide particles have been reshaped
from the interior (Figure 2e) revealing the appearance of an
entangled network of fibers (Figure 2e, inset). After 60 s of
sonication, the whole of the material was found to form a well
defined extended network of thinner fibers (Figure 2f) in accord
with the visual observation of a gel state. In a water/methanol
2
0 °C, it was soluble in water and cyclic and linear alkanes (Table
S1, Supporting Information) only at elevated temperatures. The
cooling of hot (∼80 °C) homogeneous alkane solutions of
compound 1 did not result in gel formation and the compound
precipitated out of the solutions (systematic precipitation for hexane
in the 25-100 mM concentration range). However, when disper-
sions of dipeptide 1 in alkanes (c ) 26 mM) were submitted to
sonication at 20 °C for a period of 1-4 min(s), a complete and
homogeneous liquid gelation was observed (Figure 1). Gelation was
only observed when ultrasound was used as an external stimulus.
The temperature of the sonication bath was found to be crucial
since high temperatures prevent gel formation (T > 45 °C). The
critical gelation concentration was found to be around 10 mM in
hexane. The gels are white, opaque, and stable for months at room
temperature. Interestingly, mixtures of polar solvents could also
be gelled by compound 1 as for example in water/methanol. This
†
Steacie Institute for Molecular Sciences.
‡
Universit e´ Louis Pasteur.
§
Institute for Research in Construction.
Institut Charles Sadron.
|
⊥
University of Guelph.
10.1021/ja711342y CCC: $40.75 © 2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 3313-3315
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C O M M U N I C A T I O N S
Figure 3. Freeze fracture electron micrograph obtained with a nonane gel
prepared with dipeptide 1 at 26 mM concentration after a sonication time
of 90 s.
Figure 4. (left) Solid-state CP-MAS 13C NMR spectra of (a) the precipitate
(
spin rate ) 7 kHz), (b) the xerogel (7 kHz). (Right) Proposed antiparallel
â-sheet model for compound 1 in the as-synthesized material and in the
gel.
Such a conformation is favorable for the organization of the
dipeptide inside â-sheet structures via intermolecular hydrogen
bonding, a structure often encountered in natural and synthetic oligo-
peptides (Figure 4).9
Figure 2. SEM micrographs obtained: (a) on the as-synthesized dipeptide;
(
(
b) on the precipitate isolated after a heating and cooling cycle in hexane;
c) after 10 s sonication in hexane; (d) after 20 s sonication in hexane; (e)
Solid-state cross-polarization magic angle spinning (CP-MAS)
after 40 s sonication in hexane; (f) after 60 s sonication in hexane; (g) after
0 min sonication in hexane; (h) after 60 s sonication in a water/methanol
13
C NMR spectra of as-synthesized material, a xerogel (desolvated
1
gel) obtained after slow evaporation of the solvent, and a gel with
nonane exhibit essentially the same patterns (Figure 4, left-hand-
side and Figure S5, Supporting Information). All three materials
show carbonyl and CR peaks at ∼173 and 53 ppm respectively,
characteristic of sheet rather than helical structures for which the
mixture (20/80 vol %). All images were recorded from xerogels and dried
solutions/suspensions prepared using 5 mg of dipeptide 1 dispersed into
3
50 µL of solvent.
mixture (20/80 vol %), the formation of well-defined fibers with
an average diameter of 60 nm is also clearly observed after 60 s of
sonication (Figure 2h).
Longer exposure of the dipeptide dispersion to ultrasound (10
min) resulted in the formation of straight ribbon-like objects (Figure
10
resonances are expected at ∼179 and 58 ppm, respectively. These
results indicate a similar molecular organization in the as-syn-
thesized material and in the gel, a result confirmed by FT-IR
-1
spectroscopy. A unique NH band centered at 3302 cm is observed
and is typical of NH functions engaged in hydrogen bonding. Free
2g). The formation of ribbon-like structures for longer sonication
times was also confirmed by freeze fracture electron microscopy
experiments performed on a nonane gel (c ) 26 mM, 90 s
ultrasound). Well-defined ribbons, with a width ranging from 20
to 100 nm, with an average of ∼50 nm, and a length up to several
micrometers are clearly visible (Figure 3). This is in excellent
agreement with observations by SEM of samples sonicated for
longer than 1 min. The ribbon structures likely represent the final
architecture after the ultrasound has reshaped the peptide particles
with the formation of fiber networks as an intermediate structure.
-
1
NH functions are found at 3430 cm in chloroform. In CHCl
3
-1
solution, three stretching νCdO bands at 1732, 1716, and 1679 cm
are attributed to unbound ester, carbamate, and amide carbonyl
functions, respectively. In the solid, gels, and xerogels, all the amide
-
1
functions are engaged in hydrogen bonding (νCdO 1648 cm ),
whereas the carbonyl band of the carbamate function is split into
-
1
two equal bands at 1716 and 1693 cm . Regarding the results
obtained in CHCl , the first νCdO stretching vibration is character-
3
1
istic of free carbamate and the second to hydrogen-bonded
carbamate. These observations are in line with the proposed model
where half of the carbamate functions are hydrogen bonded. Thus,
the overall set of data suggests the formation of â-sheet-like
structures via intermolecular hydrogen bonding in which an anti
conformation is favored. The gelation seems to rely on a morpho-
logical change of the dipeptide particles induced by a sonocrys-
tallization process rather than changes in the aggregation mode or
in the conformation of the molecule inside the material.
To gather information at the molecular level, liquid state H NMR
experiments were performed. A 2D-NOESY spectrum of a CDCl
3
solution of dipeptide 1 at 295 K revealed the existence of several
conformations, whereas a unique conformation was deduced from
1
1
1
the NMR spectra ( H, H- H COSY, and 2D-ROESY) of a d14
-
hexane solution (c ) 15 mM) at 60 °C. ROE cross-peaks are in
line with an anti conformation presenting the two amide functions
(and also isobutyl chains) in opposite directions (Figure S3,
Supporting Information).
3314 J. AM. CHEM. SOC.
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C O M M U N I C A T I O N S
Therefore, this unique dipeptide/ultrasound process is able to
selectively gel oil in water without heat and may well open new
avenues for real-life applications such as the decontamination of
oil spills.12
In conclusion, ultrasound has been used advantageously to
reshape sheet-like dipeptide particles into elongated molecular
assemblies which are at the origin of solvent gelation. A sono-
crystallization process appears to be at the main origin of the
gelation. Sonication time, temperature, and solvents were found to
play a crucial role in determining the morphology of the particles,
13
features which are particularly appealing for templating materials.
This is a clear and rare example of how dipeptides can display
remarkable behavior when subjected to ultrasound.
Figure 5. Selective organic liquid gelation over water (1/1 vol %). From
left to right: paraffin oil, olive oil, and kerosene ([1] ) 26 mM, 700 µL)
after 2 min sonication. The aqueous phase (top) is trapped by the gel.
Acknowledgment. The National Research Council Canada is
acknowledged for financial support. We are also grateful to Dr.
Stephen Lang and Malgosia Daroszewska for technical assistance.
To gain better insights into the evolution of the morphology of
the material, X-ray diffraction and differential scanning calorimetry
Supporting Information Available: Preparation and full charac-
terization of dipeptide 1, a table of solvents tested for gelation, additional
photographs, SEM, a ROESY NMR spectrum, solid state NMR spectra,
differential scanning calorimetry, and powder X-ray diffraction mea-
surements. This material is available free of charge via the Internet at
http://pubs.acs.org.
(DSC) experiments have been performed on the as-synthesized
material and on xerogels (Figure S6, Supporting Information).
From X-ray experiments, it can be seen clearly that the material
is more crystalline in the xerogel than in the as-synthesized material.
In fact, sharp reflections are observed in the 5-35° 2θ range for
the xerogels whereas only a very broad feature centered at 2θ )
20° was observed in the starting material. The crystallization of
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