A terminally protected dipeptide: from crystal structure and self-assembly, through co-assembly with carbon-based materials, to a ternary catalyst for reduction chemistry in water

Article abstract of DOI:10.1039/c5sm02189h

A terminally protected, hydrophobic dipeptide Boc-l-Cys(Me)-l-Leu-OMe (1) was synthesized and its 3D-structure was determined by single crystal X-ray diffraction analysis. This peptide is able to hierarchically self-assemble in a variety of superstructures, including hollow rods, ranging from the nano- to the macroscale, and organogels. In addition, 1 is able to drive fullerene (C₆₀) or multiwalled carbon nanotubes (MWCNTs) in an organogel by co-assembling with them. A hybrid 1-C₆₀-MWCNT organogel was prepared and converted (through a high vacuum-drying process) into a robust, high-volume, water insoluble, solid material where C₆₀ is well dispersed over the entire superstructure. This ternary material was successfully tested as a catalyst for: (i) the reduction reaction of water-soluble azo compounds mediated by NaBH₄ and UV-light with an overall performance remarkably better than that provided by C₆₀ alone, and (ii) the NaBH₄-mediated reduction of benzoic acid to benzyl alcohol. Our results suggest that the self-assembly properties of 1 might be related to the occurrence in its single crystal structure of a sixfold screw axis, a feature shared by most of the linear peptides known so far to give rise to nanotubes.

Full text of DOI:10.1039/c5sm02189h

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A terminally protected dipeptide: from crystal structure and self-assembly, DthOIr: o10u.1g03h9/Cc5oS-M02189H
assembly with carbon-based materials, to a ternary catalyst for reduction chemistry in
water
Daniela Mazzier,a Francesco Carraro,a Marco Crisma,b Marzio Rancan,c Claudio Toniolo,a,b
and Alessandro Morettoa,b*
aDepartment of Chemical Sciences, University of Padova, 35131 Padova, Italy
bInstitute of Biomolecular Chemistry, Padova Unit, CNR, 35131 Padova, Italy
cInstitute for Energetics and Interphases, CNR, 35131 Padova, Italy
e-mail: alessandro.moretto.1@unipd.it
Abstract: The terminally protected, hydrophobic dipeptide Boc-L-Cys(Me)-L-Leu-OMe (1)
was synthesized and its 3D-structure determined by single crystal X-ray diffraction analysis.
This peptide is able to hierarchically self-assemble in a variety of superstructures, including
hollow rods, ranging from the nano- to the macroscale, and organogels. In addition, 1 is able
to drive fullerene (C60) or multiwall carbon nanotubes (MWCNTs) in an organogel by co-
assembling with them. A hybrid 1-C60-MWCNts organogel was prepared and converted
(through a high vacuum-drying process) into a robust, high-volume, water insoluble, solid
material where C60 is well dispersed over the entire superstructure. This ternary material was
successfully tested as a catalyst for: (i) the reduction reaction of water-soluble azo compounds
mediated by NaBH4 and UV-light with an overall performance remarkably better than that
provided by C60 alone, and (ii) the NaBH4-mediated reduction of benzoic acid to benzyl
alcohol. Our results suggest that the self-assembly properties of 1 might be related to the
occurrence in its single crystal structure of a sixfold screw axis, a feature shared by most of
the linear peptides known so far to give rise to nanotubes.
1

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DOI: 10.1039/C5SM02189H
1. Introduction
Since J.M. Lehn’s pioneering contributions, the bottom-up self-assembly strategy has
emerged as one of the most applicable approaches to drive small molecule arrangements.1-4
Following a hierarchical process, under appropriate conditions the combination of a variety of
non-covalent interactions may allow properly designed molecules to form stable, ordered
aggregates that can evolve into nano-, micro- and macroscale materials characterized by
discrete 3D-architectures with specific shapes, dimensions and functions, exploitable in areas
ranging from chemistry to materials science and medicine.5,6 In this connection, peptides
represent powerful building blocks on the nanoscale because of their modularity, molecular
diversity, which can be expanded beyond the repertoire of genetically coded amino acids, and
secondary structure variety.7-17 More specifically, nanotubes and nanofibers predominate as
molecular architectures among the self-assemblies formed by short, unprotected peptides18-21
of which the phenylalanine homo-dipeptide H-Phe-Phe-OH is the most extensively
investigated example (for review, see ref. 7). Notably, such architectures may adsorb or entrap
other molecules, thus generating ordered hybrid materials.22 Conversely, only a few examples
of terminally-protected peptides, the crystal structures of which are characterized by a
nanotube architecture, were reported, but their self-assembly properties have not been further
explored.23-26
Here, we present our data on the hydrophobic, terminally protected dipeptide Boc-L-
Cys(Me)-L-Leu-OMe (1) [where Boc is tert-butoxycarbonyl, Cys(Me) is S-methyl cysteine,
and OMe is methoxy] (Figure 1A) that was crystallographically characterized and
subsequently was found to be able to hierarchically self-assemble producing nano-, micro-
and macroscale tubes and organogels that we eventually exploited to incorporate guest
molecules such as gold nanoparticles, C60 fullerene and multiwall carbon nanotubes
(MWCNTs). Moreover, from an organogel composed of 1, C60 and MWCNTs, by a simple
vacuum-drying process we generated an ordered, robust, carbon-based non-covalently
2

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assembled material, which displayed high performances when used as catalyst DinOI:r1e0.d10u3c9/tCio5SnM02189H
chemistry.
2. Materials and methods
2.1 X-Ray diffraction
(i) Single crystal X-ray diffraction data were collected with an Agilent Technologies Gemini
E four-circle kappa diffractometer equipped with a 92 mm EOS CCD detector, using graphite
monochromated Cu K radiation ( = 1.54178 Å). Data collection and reduction were
performed with the CrysAlisPro software (Agilent Technologies). The structures were solved
by ab initio procedures of the SIR 2002 program,27 and refined by full-matrix least-squares on
F2, using all data, by application of the SHELXL-97 program.28 Additional details may be
found in the Supporting Information. (ii) Powder X-ray diffraction measurements were
carried out by means of a Bruker D8 Advance diffractometer equipped with a Göbel mirror
and a Cu Kα source (40 kV, 40 mA). Powder pattern indexing was performed by use of the N-
TREOR29 software in the framework of the EXPO201330 package.
2.2 Self-assembly investigations
(i) Self-assembly from solvent evaporation. Typically, from 500 mg to 1 g of 1 were dissolved
in 10 to 20 ml of the appropriate solvent (MeOH, CH3CN or EtOAc). The resulting clear
solution was allowed to stand in a large crystallization vessels at controlled temperature
(25 °C). After evaporation of the solvent macroscopic rods were obtained. (ii) Self-assembly
from water/EtOAc interfacial crystallization. Typically, from 100 to 200 mg of 1 were
dissolved in 10 to 20 ml of EtOAc. The resulting clear solution was gently layered over mQ
water in a crystallization vessels at controlled temperature (25 °C). After evaporation of
EtOAc, micrometric rods were collected from the water solution. (iii) Self-assembly from
MeOH/water. Typically from 0.1 to 1 mL of a MeOH solution of 1 (2 mg/mL) were added to
1 mL of H2O and allowed to stand in a sealed vessel for 1 to 3 days.
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DOI: 10.1039/C5SM02189H
2.3 Organogel preparations
(i) Pristine 1-organogel was prepared as follows. Dipeptide 1 was added in small amounts to
toluene to the concentration of 85 mg/mL. Then, the resulting suspension was heated at 60 °C
under sonication, for 5 min. After standing at rt, the clear solution turned into an organogel.
(ii) The 1-C60 organogel was prepared as follows. Typically, from 5 to 8 mg of C60 were
added to 1 mL of toluene and the suspension was heated at 70 °C up to complete
solubilization. Then, 85 mg of 1 were added to the C60 solution in small amounts to the
concentration of 85 mg/mL. The resulting suspension was heated at 60 °C under sonication
for 10 min. After standing at rt, the clear, dark-red solution turned into an organogel. (iii) The
1-MWCNTs organogel was prepared as follows. MWCNTs, 20 mg, were suspended in 1 mL
of toluene and sonicated for 5 min. Then, 1 was added to the suspension in small amounts to
the concentration of 85 mg/mL. The resulting suspension was heated at 65 °C under
sonication for 20 min. The black mixture gradually turned into an organogel.
2.4 1-C60-MWCNTs composite/catalyst preparation
Typically, 5 mL of the 1-C60 organogel and 5 ml of the 1-MWCNTs organogel (prepared as
described above) were separately melted in their sol state. The two solutions were combined
and heated at 75 °C under sonication for 5 min. The dark mixture gradually turned into a
stable organogel. The organogel was directly dried in vacuo. The nanocomposite contains
from 2.5 to 5 % w/w of fullerene.
2.5 Reduction of azo compounds under UV irradiation
Reduction of methyl orange (MO) was carried out in a 50 mL beaker. NaOH solution (1M,
0.5 mL), distilled water (40 mL), NaBH4 (100 mg), and MO (200 mg) were added. Then, the
1-C60-MWCNTs catalyst (100 mg, C60 as 2.5% w/w) was added to form a suspension. The
reaction proceeded by stirring under UV irradiation (UV lamp, 365 nm, 50 Hz) at rt for 45
min. After the catalyst was removed by filtration, the reaction mixture was characterized
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using HPLC-MS analysis. The experiment was repeated 3 times. The reduction procedure of
DOI: 10.1039/C5SM02189H
methyl red (MR) and 4-aminoazobenzene (AZ) in water was similar to that described above.
2.6 Reduction of benzoic acid under UV irradiation
Reduction of Benzoic Acid was carried in similar condition as described for Azo compounds.
The reaction proceeded by stirring under UV irradiation (UV lamp, 365 nm, 50 Hz) at rt for 3
hours. After the catalyst was removed by filtration, the reaction mixture was characterized
using HPLC-MS analysis.
3. Result and discussion
3.1 X-Ray diffraction
The X-ray diffraction structure of 1 [determined on a single crystal grown from MeOH
(methanol)] is illustrated in Figure 1B.31 The conformation adopted by both -amino acid
residues is semi-extended, and the N-H groups of Cys and Leu point to opposite directions. In
the packing mode (Figure 1C), the (urethane) N1-H group is intermolecularly H-bonded to the
(y, -x+y, z+1/6) symmetry equivalent of the (urethane) O0 atom, and the (amide) N2-H group
is H-bonded to the (x-y, x, z-1/6) symmetry equivalent of the (amide) O1 atom (Supporting
Information, Table S3). As a result, H-bonded molecules wrap around the sixfold screw axis
along the c direction, each molecule being connected to the next by two H-bonds. The left-
handed, supramolecular sixfold helix thus generated is characterized by a very narrow lumen,
about 2.5 Å in diameter (Figure 1C, bottom). The Cys(Me) and Leu side chains, as well as the
N- and C-terminal Boc and OMe groups of each molecule, are located on the external surface.
Each helical row of molecules is surrounded by six counterparts (Figure 1D). Lateral
stabilization between rows is provided, in addition to van der Waals interactions, by C-H···O
interactions taking place between the (Boc) C02-H02B group and the (x-1, y-1, z) symmetry
equivalent of the (methyl ester) OT atom (Supporting Information, Table S3).
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DOI: 10.1039/C5SM02189H
C
A
B
D
Fig. 1 (A) Chemical formula of dipeptide 1. (B) X-Ray diffraction structure of 1 with atom
numbering. (C) Packing mode of 1 in the crystal. Intermolecular H-bonding generates helical
rows of molecules (see text for details). A single row is shown as viewed perpendicular
(upper part) and parallel (lower part) to the sixfold screw axis. Intermolecular H-bonds are
indicated by dashed lines. (D) Overall packing of 1 molecules as viewed nearly along the c
axis. Carbon, grey; nitrogen, blue; oxygen, red; sulphur, yellow.
3.2 Self-assembly and gold nanoparticles inclusion
Dipeptide 1 is able to adopt a well-organized, micro-arrangement when allowed to self-
assemble under appropriate conditions. Upon slow evaporation of concentrated solutions of 1
in either EtOAc (ethyl acetate), CH3CN or acetone, long, aligned rods (up to 10 cm in length)
are formed as shown in Figure 2A. Interestingly, such rods are characterized by an overall
parallelepiped shape and present an empty inner cavity (Figure 2B).
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DOI: 10.1039/C5SM02189H
B
A
1 cm
B
D
A
C
200 m
200 m
C
E
D
F
20 m
20 m
Fig. 2 (A) Camera picture of macroscopic rods from 1 obtained after evaporation from a
concentrated solution in CH3CN. (B) Camera picture showing details of a macroscopic rod
obtained from a CH3CN solution. (C) SEM image of micrometric rods from 1 obtained from a
water/EtOAc interfacial crystallization. (D) SEM image of micrometric rods from 1 obtained
from a water/MeOH solution. (E,F): SEM images showing details of the rods (obtained from
a water/MeOH solution).
Smaller and packed rods of 1 (at the microscale level) were obtained by interfacial
crystallization between water and EtOAc, as revealed by SEM images (Figure 2C). Moreover,
by adding a diluted solution of 1 in MeOH to a large excess of water, formation of
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micrometric, isolated rods was observed (Figure 2D). According to a SEM analysis (Figures
DOI: 10.1039/C5SM02189H
2E and 2F), even at this microscale level, the rods are characterized by an empty inner cavity
while maintaining the overall parallelepiped shape found at the macroscale level.
Peptide 1 was further studied by circular dichroism (CD) in CH3CN solution and as solid-state
rods obtained after evaporation from the same solvent (Figures 3A and 3B). Notably, 1 shows
a strong aggregation propensity in CH3CN solution at concentrations above 1 mg/ml (Figure
3A). This conclusion is justified by the change of the Cotton effect near 195 nm from weakly
positive at 0.3 mg/ml concentration to strongly negative at concentrations in the range 2.0 –
10 mg/ml, accompanied by a marked increase in ellipticity of the positive Cotton effect at
about 218 nm. The CD spectra of the most concentrated solutions nicely resemble the
spectrum obtained in the solid state (Figure 3B), i.e. that of the rod structure, although the
latter would be entirely red-shifted, albeit slightly.
The powder X-ray diffraction (PXRD) technique was exploited in order to check whether
formation of rods characterized by an empty cavity at the macro- and microscale level
(prepared from CH3CN and MeOH/water, respectively) could be ascribed to a supramolecular
organization different from that determined at the single crystal level. The PXRD patterns of
the rods prepared by the two methods (CH3CN and MeOH/water) are identical to each other
all over the 2 range investigated, and virtually overlapping to the simulated pattern of the
single crystal X-ray diffraction sample (Figure 3C). In addition, the values of the unit cell
parameters extracted from the two experimental PXRD patterns are in excellent agreement
with those of the single crystal (Table 1). These results clearly indicate that, although different
preparation methods lead to different morphologies, the 3D-supramolecular organization
adopted by 1 in the crystal state is shared by the micro- and macro-hollow rods. Similar
conclusions were extracted from a Raman study (Figure 3D). Indeed, the rods obtained from
the EtOAc/water and CH3CN preparation methods exhibit identical Raman signatures.
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DOI: 10.1039/C5SM02189H
Fig. 3 (A) CD spectra of 1 at different concentrations in CH3CN solution. (B) Solid-state CD
spectra of rods obtained from CH3CN. (C) Comparison of simulated PXRD measurements
(from single-crystal X-ray diffraction) with experimental results on rods obtained from the
CH3CN and MeOH/water preparation methods. (D) Comparison of Raman spectra of the rods
obtained from the EtOAc/water and CH3CN preparation methods.
Table 1. Unit cell parameters of the single crystal structure of 1 and those extracted from
powder pattern indexing of rods obtained from different preparation methods.
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3
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a (Å)
b (Å)
c (Å)
α
β
γ
DOI: 10.1v03o9l/uCm5SMe0(2Å189)H
Single crystal 11.4458(2) 11.4458(2) 27.7551(5)
90
90
90
90
90
90
120
3148.95(10)
MeOH/water
CH3CN
11.468(4)
11.426(4)
11.468(4)
11.426(4)
27.810(16)
27.847(28)
120
120
3167(2)
3148(3)
To gain in-depth information on the nature of the rod architectures, we investigated the
mother liquor remained after the MeOH/water preparation method. The transmission electron
microscopy (TEM) analysis of this mixture carried out on stained (uranyl acetate) samples
showed the presence of nano- and micrometric rods (Figure 4A).
A
C
B
500 nm
5 m
2 m
Fig. 4 (A) Stained TEM image after filtration of the macroscopic rods from a MeOH/water
solution of 1. (B) Stained TEM image showing details of the self-assembled rods. (C) Stained
TEM image showing details of the multiwall nature of a nanoscopic rod.
A more detailed investigation of these samples revealed two interesting features. First, we
observed the presence of “incompletely folded” rods, which confirmed the tubular nature of
such type of architecture (Figure 4B). Secondly (Figure 4C), we noticed a multilayer structure
of the external part of the rod. In a few cases, our TEM analysis of the same, but non-stained,
sample (Figure 5A) revealed the typical structure of a multiwall nanotube characterized by
two parallel, dark lines, associated to a higher material density, occurring at the boundaries of
the rods. Interestingly, these images are similar to those that can be obtained from the TEM
analysis performed on MWCNts samples.32 To prove the existence of such a type of hollow
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superstructure, we made use of citrate-passivated (water soluble) gold nanoparticles (10-nm
DOI: 10.1039/C5SM02189H
diameter)33 which were added to the MeOH/water solution. After the self-assembly process,
the results of TEM analysis (non-stained samples, Figure 5B), energy-dispersive X-ray
spectroscopy (EDX) (Figure 5C), and optical microscopy (polarized light, Figures 5D and 5E)
clearly showed that the gold nanoparticles are incorporated into the rod structures.
Fig. 5 (A) Non-stained TEM image of a nanorod of 1. (B) Non-stained TEM image showing
the encapsulation of water-soluble gold nanoparticles into a rod. (C) SEM image and EDX
pattern of the 1-nanotube / gold nanoparticles hybrid rods. (D) Optical microscopy image of
1-rods prepared from MeOH/water under polarized light. (E) Optical microscopy image of
co-assembled 1-rods/gold nanoparticles under polarized light.
Interestingly, we found that 1 is able to interact with citrate-passivated gold nanoparticles,
probably via its thioether sulfur atom, resulting in self-aggregation of the nanoparticles. This
hypothesis is supported by comparison of the TEM images (Figure 6A and 6B) and the UV-
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vis absorption spectra (Figure 6C) of the citrate-passivated gold nanoparticles and the 1-
DOI: 10.1039/C5SM02189H
passivated gold nanoparticles. The strong aggregation occurring for 1-passivated gold
nanoparticles results in a remarkable red-shift of its plasmonic band as compared with that of
the citrate-passivated gold nanoparticles. Moreover, the CD spectra of the 1-passivated gold
nanoparticles (Figure 6D) showed a chiral signature in correspondence to the plasmonic
region (400-600 nm), that may arise from the interactions of 1 (which is a chiral molecule)
with the gold core. A similar chiral signature of the plasmonic band was previoulsy reported
for a 10-nm gold nanoparticle-peptide conjugate.34
Fig. 6 (A) TEM image of citrate-passivated gold nanoparticles. (B) TEM image of 1-
passivated gold nanoparticles. (C) UV-vis absorption spectra of citrate-passivated gold
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nanoparticles (black line) and 1-passivated gold nanoparticles (red line). (D) CD DspOeI: c10t.r1a03o9/fC51S-M02189H
passivated gold nanoparticles.
In addition, treatment at 400 °C in oxygen atmosphere of the co-assembled 1-rods/gold
nanoparticles described above led to the formation of gold nanorods (about 100-200 nm in
length) as detected by TEM (Figures 7A and 7B). The UV-vis absorption spectrum of this
material (Figure 6C) is characterized by a relatively sharp band at about 525 nm, accompanied
by a much broader absoprtion centered near 720 nm, features typical of gold nanorods.35
0,20
A
B
C
0,15
0,10
0,05
0,00
400
500
600
700
800
900
500 nm
100 nm
Wavelength (nm)
Fig. 7 (A) and (B) TEM images of gold nanorods, obtained by treatment at 400 °C in oxygen
atmosphere of the the co-assembled 1-rods/gold nanoparticles, recorded prior and after a
sonication process, respectively. (C) UV-vis absorption spectra of gold nanorods in aqueous
suspension.
3.3 Organogel formation
To further expand our study of the self-assembly propensity of 1 in organic solvents, we
prepared a set of concentrated solutions of 1 in toluene. We observed formation of organogel
(after a gentle heating followed by a cooling process) when solutions of 1 reach the
concentration of 85 mg/ml. According to a TEM analysis, the organogel is characterized by a
dense network composed by long (up to 10 m) and twisted fibers (Figures 8A and 8B).
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DOI: 10.1039/C5SM02189H
A
B
5 m
1 m
C
D
E
5 m
1 cm
20 m
Fig. 8 (A and B) Stained TEM images showing the organogel microstructure (fibers) of 1 and
details of the twisted nature of the fibers. (C) Optical microscopy image of the 1/MWCNTs
organogel obtained after evaporation of toluene. Inset: Vials showing a 85 mg/ml solution of
1 in toluene at 70 °C (left) and the corresponding organogel formed after sonication in the
presence of MWCNTs and cooling to room temperature (right; the vial is upside down). (D)
and (E): SEM and non-stained TEM images, respectively, of the 1/MWCNTs composite
obtained after evaporation of toluene.
3.4 Multicomponent organogels
Addition of MWCNTs to a solution of 1 (85 mg/ml) in toluene, followed by sonication,
allowed us to obtain 1/MWCNTs organogel systems (Figure 8C, inset). Once deposited on a
glass surface, this organogel rapidly crystallized in microscopic black needles, as detected by
optical microscopy and SEM (Figures 8C and 8D, respectively). Moreover, small and large
nanostructures were unraveled also by TEM (Figure 8E). A comparison of the TEM analyses
of the organogel formed by 1 alone (stained sample, Figures 8A and 8B) and of the
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1/MWCNTs organogel (non-stained sample, Figure 8E), indicates a much higher matter
DOI: 10.1039/C5SM02189H
density for the latter. This finding suggests that in the 1/MWCNTs organogel the MWCNTs
might have been incorporated, and probably ordered, within the rods. We found that addition
of a 15% v/v MeOH (a good solvent for 1, but not for MWCNTs) was successful in
disassembling the 1/MWCNTs organogel system, resulting in a black MWCNTs precipitate
and a colorless solution (the same effect was also observed after addition of ethanol,
isopropanol, CH3CN and N,N-dimethylformamide). Therefore, we decided to progressively
disrupt the rods directly on the TEM-grid by adding increasing amounts of MeOH or CH3CN
to the 1/MWCNTs organogel drop prior to TEM detection. The resulting TEM images are
reported in Figure 9. In particular, by comparing the TEM images reported in Figure 8E
(needles obtained by evaporation) and Figure 9A (needles after addition of 0.5% MeOH) it
seems clear that addition of MeOH affects the overall morphology. Subsequent additions of
larger amounts of MeOH resulted in the collapse of the needle morphology (Figures 9B and
9C). This finding was also observed upon addition of CH3CN (Figures 9D and 9E). In
particular, Figures 9A-9E show formation of packed (or single) strands (between a 30-100 nm
width, and up to a 5-m length) that originate from the needle structures mentioned earlier.
Our most relevant observation is that strands are detected on a non-stained sample, which
suggests a high matter density occurring in these superstructures. We hypothesize that these
strands are related to the 1-MWCNTs co-assembled systems where the MWCNTs surfaces
are covered by 1.
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DOI: 10.1039/C5SM02189H
A
D
B
E
C
F
500 nm
1 m
1 m
1 m
500 nm
100 nm
Fig. 9 Non-stained TEM images showing the effect of the addition of MeOH (A-C) or
CH3CN (D,E) to the 1/MWCNTs organogel microstructure on the TEM-grid. Specifically:
additions of 0.5% (A), 2% (B) and 5% (C) MeOH (v/v); additions of 1% (D) and 3% (E)
CH3CN (v/v). (F) Non-stained TEM image after sonication of MWCNTs in a solution of 1 at
10 mg/ml in toluene.
As a control experiment, we prepared a solution of 1 at the concentration of 10 mg/ml in
toluene and added MWCNTs. Under these conditions, after a sonication process, no
microstructures were formed. However, it was possible to detect (by TEM) partially
unbundled MWCNTs (Figure 9F). The latter observation is an important evidence for the co-
assembly propensity of 1 with MWCNTs. Thus, 1 seems to be able to partially disperse
MWCNTs in toluene solution. Overall, we can likely assume that formation of 1/MWCNTs
co-assembled needles goes through a series of steps. Specifically, MWCNTs are first covered
by 1, then MWCNTs become disperse in the solution media, and eventually the covered 1-
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MWCNTs complexes align (with the support of the large excess of 1) giving rise to the
DOI: 10.1039/C5SM02189H
supramolecular hybrid architectures.
Moreover, addition of C60 to a solution of 1 (85 mg/ml) in toluene, followed by sonication,
allowed us to obtain 1/C60 organogel systems (Figure 10A, and inset). Inspection of a diluted
1/C60 organogel sample deposited on the TEM grid revealed the presence of small particles
(from 5- to 15-nm in diameter) spread all over the organogel network (Figure 10B). These
particles are possibily due to formation of C60 aggregates. The 1/C60 organogel, once
deposited on a glass surface, rapidly crystallized in microscopic red/orange needles (Figure
10C inset) that were characterized by SEM (Figure 10C).
A
B
C
0.5 mm
5 m
500 nm
10 m
D
E
2 m
20 m
Fig. 10 (A) Non-stained TEM images showing the 1/C60 organogel microstructure (formed in
toluene). Inset: Vials showing a 85 mg/ml solution of 1 in toluene at 70 °C (left) and the
corresponding organogel formed after sonication in the presence of C60 and cooling to room
temperature (right; the vial is upside down).. (B) TEM details showing the presence of small
particles (in a diluted sample) incorporated into the microstructure network. (C) SEM image
of the 1/C60 organogel microscopic needles obtained after rapid crystallization. Inset: camera
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picture of microscopic red/orange needles. (D) SEM image of the 1/C60/MWCNTs composite
DOI: 10.1039/C5SM02189H
obtained from the corresponding organogel in toluene after the high vacuum-drying process.
Inset: camera picture of the 1/C60/MWCNTs composite. (E) SEM details of the
1/C60/MWCNTs composite showing the presence of ordered co-assembled rods.
The organogels formed by 1 alone, 1/C60, and 1/MWCNTs are characterized by a transition
temperature for the sol-gel process (Tsol-gel) of 55, 47, and 85 °C, respectively. These data
indicate that, in comparison to 1, the gel stability is only marginally affected by the presence
of fullerene, but it is strongly increased by MWCNTs.
3.5 Three-component catalyst
With the aim at preparing a robust material with catalytic properties, we decided to combine
the 1/C60 and 1/MWCNTs carbon-based organogels. In particular, the two gels were melted in
their sol (liquid) forms and combined together. After a sonication process followed by
standing at room temperature, a novel 1/C60/MWCNTs organogel was obtained, which
displayed a Tsol-gel of 79 °C. Removal of the solvent by a high vacuum-drying process at room
temperature afforded a mechanically robust self-assembled solid material, characterized by a
high porosity (300 mg of the material prepared by this process occupy as many as 6 cm3) and
insoluble in water (Figure 10D, and inset). The vacuum-drying process itself helps keeping
the gel size, because the fast evaporation procedure used generates a low temperature. The
consequent gel hardening at low temperature is beneficial to retain the size. Furthermore, the
fast drying process shortens the time allowed to capillary forces to act on the gel, thus
moderating the structure collapse caused by strain accumulation.36 The SEM details reported
in Figure 10E showed the presence of co-assembled rods which exhibit ordered and aligned
domains on their surfaces. Remarkably, the same processes performed on the 1 or 1/C60
organogels did not provide any organized material. The interest for nanocarbon materials as
catalysts is rapidly growing, as highlighted by recent review articles.37,38 In this connection,
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39
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Khashab and coworkers reported the successful reduction of a set of azo compDoOuIn: 1d0.s10t3o9/Cth5SeM02189H
corresponding aromatic amines by NaBH4 under UV irradiation in basic water using C60 as
the catalyst. It is known that NaBH4 is unable to reduce the azo group without the assistance
of a metal catalyst, such as Pd or Ag.40,41 The catalytic effect of C60 was ascribed to the
electron acceptor properties of its UV-promoted triplet state, which lead to activation of the
N=N bond. The limitations of such a system are related to the insolubility of C60 in water,
which results in its poor dispersion and formation of large aggregates. Indeed, 10 mg of C60
were required in order to catalyze the reduction of 10 mg of methyl orange after 5 hours under
illumination in the presence of a large excess of NaBH4.39 We envisioned that our
1/C60/MWCNTs hybrid material could be a more efficient catalyst than C60 alone in the
reduction of azo compounds. Actually, by using 100 mg of the 1/C60/MWCNTs self-
assembled solid material (which contains approximately 2.5 % w/w of C60) and 100 mg of
NaBH4, quantitative conversion of 200 mg of either metyl orange, methyl red, or 4-amino-
azobenzene to the corresponding amines was achieved within 45 minutes under UV
irradiation at 365 nm in water. The water-insoluble catalyst was easily recovered by filtration.
Chemical selectivity was observed as indicated in Figure 11, which was identical to that
reported in the previous study.39 A comparison of our results with those reported for the same
reaction by Khashab and coworker39 strongly supports the view that in our catalyst C60 is
molecularly well dispersed on a large porous surface, thus allowing a more effective
interaction with the azo compounds. These findings prompted us to explore additional
potential applications for our 1/C60/MWCNTs co-assembled solid material as a catalyst in
NaBH4-mediated reduction reactions. NaBH4 is commonly used to reduce aldehydes and
ketones to give the related alcohols, but is not reactive enough to reduce carboxylic acids or
esters. We found that, under experimental conditions closely related to those exploited for the
reduction of azo compounds, our catalyst is able to promote the reduction of benzoic acid to
benzyl alcohol (4 hours, 96% conversion, see Supporting Information).
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DOI: 10.1039/C5SM02189H
MO
MR
AZ
benzoic acid
benzyl alcohol
Fig. 11 Chemical structures of methyl orange (MO), methyl red (MR), 4-amino-azobenzene
(AZ), and benzoic acid, and the corresponding products of reduction reactions.
It has to be noted that although methyl red carries a carboxylic group, its NaBH4-mediated
reduction reaction catalyzed by our 1/C60/MWCNTs co-assembled solid material did not led
to any detectable formation of ortho-aminobenzyl alcohol. In this case the effects of the
amino substituent at the ortho position prevent the reduction of the carboxylic function under
our experimental conditions.
4. Conclusion
To summarize, we have shown that a terminally protected, hydrophobic dipeptide, Boc-L-
Cys(Me)-L-Leu-OMe (1), is able to hierarchically self-assemble in well-defined structures. In
particular, evaporation from organic solvents (e.g., EtOAc, CH3CN, acetone) or interfacial
(EtOAc/water and MeOH/water) crystallizations led to the formation of hollow rods ranging
from the nano- to the macroscale. In addition, an organogel could be obtained using toluene,
the microstructure of which is characterized by twisted fibers. The organogel of 1 formed in
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the presence of C60 or MWCNTs shows a different morphology (needles). Evidence is
DOI: 10.1039/C5SM02189H
provided in support of the view that 1, besides helping in dispersing the otherwise insoluble
MWCNTs, is able to co-assemble with them. We succeeded in obtaining a novel ternary, co-
assembled, carbon-based material by taking advantage of the self-assembly tendency of 1.
Specifically, by using a high vacuum-drying process, the 1/C60/MWCNTs organogel was
converted into a robust, high-volume, water insoluble, solid material where C60 is well
dispersed over the entire superstructure. We exploited this new material as a catalyst in the
reduction reaction of azo compounds (mediated by NaBH4 and UV-light). Our material
proved to perform remarkably better than C60 alone in this reaction. Moreover, we extended
its usefulness to a novel application, namely the reduction of benzoic acid to benzyl alcohol.
Finally, it is worth recalling that most of the linear peptides reported to form nanotubes are
characterized by the occurrence of a sixfold screw axis as a symmetry element in their crystal
structures.[18,19,23-26] This property is also shared by our terminally protected dipeptide Boc-L-
Cys(Me)-L-Leu-OMe (1) that crystallizes in the hexagonal space group P65. However, at
variance with 1, its positional isomer Boc-L-Leu-L-Cys(Me)-OMe (2, see Supporting
Information), that crystallizes in the orthorhombic space group P212121, in our hands was
unable to generate any superstructure, thus suggesting a strong connection between packing
mode and self-assembly behavior. Interestingly, quite recently a strictly related observation
was published, namely that sequence exchange in an unprotected dipeptide (Val-Ala vs. Ala-
Val) strongly affects its molecular structure and the related self-assembly tendency.42
Supplementary Information (ESI) available: Synthetic details, X-ray crystal structural analysis
CCDC 1057628 (1) and 1057629 (2) and catalysis results.
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DOI: 10.1039/C5SM02189H
Acknowledgements
The authors thank Dr. F. Bertasi for Raman spectra technical support. Financial support from
the University of Padova (PRAT C91 J11003560001) is gratefully acknowledged.
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DOI: 10.1039/C5SM02189H
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An efficient catalyst for reduction chemistry in water was prepared by co-assembly of
hydrophobic dipeptide, multiwall carbon nanotubes, and C60.
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1