Peptide-nanofiber-supported palladium nanoparticles as an efficient catalyst for the removal of N-terminus protecting groups

Article abstract of DOI:10.1002/cplu.201300348

Sonication-induced tryptophan- and tyrosine-based peptide bolaamphiphile nanofibers have been used to synthesize and stabilize Pd nanoparticles under physiological conditions. The peptide bolaamphiphile self-assembly process has been thoroughly studied by using several spectroscopic and microscopic techniques. The stiffness of the soft hydrogel matrix was measured by an oscillatory rheological experiment. FTIR and circular dichroism (CD) experiments revealed a hydrogen-bonded β-sheet conformation of peptide bolaamphiphile molecules in a gel-phase medium. The π-π stacking interactions also played a crucial role in the self-assembly process, which was confirmed by fluorescence spectroscopy. Electron (SEM and TEM) and atomic force microscopy (AFM) studies showed that the peptide bolaamphiphile molecules self-assemble into nanofibrillar structures. Pd nanoparticles were synthesized in the hydrogel matrix in which redox-active tryptophan and tyrosine residues reduce the metal ions to metal nanoparticles. The size of the Pd nanoparticles are in the range of 3-9 nm, and are stabilized by peptide nanofibers. The peptide-nanofiber- supported Pd nanoparticles have shown effective catalytic activity for the removal of N-terminus protecting groups of amino acids and peptides.

Full text of DOI:10.1002/cplu.201300348

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DOI: 10.1002/cplu.201300348
Peptide-Nanofiber-Supported Palladium Nanoparticles as
an Efficient Catalyst for the Removal of N-Terminus
Protecting Groups
Indrajit Maity, Manoj K. Manna, Dnyaneshwar B. Rasale, and Apurba K. Das*[a]
Sonication-induced tryptophan- and tyrosine-based peptide
bolaamphiphile nanofibers have been used to synthesize and
stabilize Pd nanoparticles under physiological conditions. The
peptide bolaamphiphile self-assembly process has been thor-
oughly studied by using several spectroscopic and microscopic
techniques. The stiffness of the soft hydrogel matrix was mea-
sured by an oscillatory rheological experiment. FTIR and circu-
lar dichroism (CD) experiments revealed a hydrogen-bonded
b-sheet conformation of peptide bolaamphiphile molecules in
a gel-phase medium. The p–p stacking interactions also played
a crucial role in the self-assembly process, which was con-
firmed by fluorescence spectroscopy. Electron (SEM and TEM)
and atomic force microscopy (AFM) studies showed that the
peptide bolaamphiphile molecules self-assemble into nanofi-
brillar structures. Pd nanoparticles were synthesized in the hy-
drogel matrix in which redox-active tryptophan and tyrosine
residues reduce the metal ions to metal nanoparticles. The size
of the Pd nanoparticles are in the range of 3–9 nm, and are
stabilized by peptide nanofibers. The peptide-nanofiber-sup-
ported Pd nanoparticles have shown effective catalytic activity
for the removal of N-terminus protecting groups of amino
acids and peptides.
Introduction
Self-assembly-driven peptide hydrogels[1–3] have a wide range
of biological applications.[4,5] Self-assembly plays a crucial role
in constructing various nanostructures that have potential ap-
plications in nanoscience and nanotechnology.[6–9] Several stim-
uli have been used to tune the self-assembly process.[10] Hydro-
gel and organogel matrices have been extensively used as
a nanoreactor for the synthesis and stabilization of metal nano-
particles.[11–19] Metal nanoparticles have higher catalytic activi-
ties[20,21] than their bulk materials because of their ultra-small
size and large surface-to-volume ratio. One major disadvantage
is that the metal nanoparticles readily aggregate and thus lose
their catalytic activity. Therefore, a stabilizing agent is required
for the synthesis of metal nanoparticles. Various toxic organic
solvents and reagents have been used to stabilize metal nano-
particles.[22–25] Therefore, a simple and eco-friendly method to
synthesize stable metal nanoparticles needs to be developed.
For this, peptides or peptide amphiphiles can serve satisfactori-
ly for both the synthesis and stabilization of metal nanoparti-
cles in aqueous medium. Recently, a nanostructured peptide
hydrogel matrix was used as a template for in situ synthesis
and stabilization of Pt nanoparticles.[26] The catalytic activity of
peptide-nanofiber-supported Pt nanoparticles was also report-
ed. Recently, Pd nanoparticles have attracted considerable at-
tention for their crucial applications in temperature reduction
of pollutant gases and most particularly in many organic reac-
tions as catalyst.[27–37]
Pd nanoparticles have been frequently used for CÀC cou-
pling reactions including Suzuki, Heck, and Sonogasira.[38–40]
Very recently, the synthesis of Pd nanoparticles supported on
mesoporous N-doped carbon and its catalytic ability for the
upgrading of biofuels has also been reported.[41]
In peptide chemistry, the number of N-protecting groups
are well known but some of them are base sensitive (9-fluore-
nylmethoxycarbonyl (Fmoc)) or acid sensitive (tert-butoxycar-
bonyl (Boc), naphthalene-2-methoxycarbonyl (Nmoc)), and
others (carbobenzyloxy (Cbz)) are stable towards both acid
and base. The protection and deprotection of N-terminus
amino acids and peptides are frequently used in synthesis. The
Cbz group was deprotected by using Pd metal on activated
charcoal in the presence of hydrogen gas.[42,43] The Fmoc
group can be deprotected from the N-terminus site by using
a base.[44] The Boc group can be removed from N-terminus
sites of amino acids and peptides by using a strong acid[45] in-
cluding trifluoroacetic acid (TFA) and HF, but these acids are
corrosive in nature. However, there is no report of a general
method to deprotect the N terminus of protected amino acids
and peptides. Therefore, there is a need for the development
of a general method to deprotect a wide range of N-protecting
groups from the N-terminus site of amino acids and peptides
that is quite easy and mild in nature. Our objective is to devel-
op such a method to deprotect the N-terminus protecting
groups under mild conditions.
[a] I. Maity, M. K. Manna, D. B. Rasale, Dr. A. K. Das
Department of Chemistry
Indian Institute of Technology Indore
Khandwa Road, Indore (India)
Fax: (+91)731-236-4182
E-mail: apurba.das@iiti.ac.in
Bolaamphiphiles are an interesting class of amphiphilic mol-
ecules, which can self-assemble into well-defined nanostruc-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300348.
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tures under aqueous conditions. Recently, several groups have
reported the self-assembly of bolaamphiphiles.[46,47] We have
designed a peptide bolaamphiphile with two dipeptide sites,
which are covalently connected with a succinic acid moiety hy-
drophobic spacer. Two dipeptides along with two carboxylic
acid groups can provide hydrogen-bonding interactions in the
self-assembly process. The hydrophobic aromatic side chains
can also play an important role in the self-assembly process by
providing p–p stacking interactions. These molecules are ideal
for the design of self-assembling soft materials through exten-
sive hydrogen bonding and p–p stacking interactions. The
peptide bolaamphiphile-based hydrogel matrix can be used as
a nanoreactor to synthesize metal nanoparticles.[48]
Suc-W-Y-OH; 16.8 mg, 10 mmolLÀ1) was dispersed in sodium
phosphate buffer solution (2 mL; pH 8, 10 mmolLÀ1) and soni-
cated for 5 min. A self-supporting hydrogel was formed after
only 10 min. The amino acid sequence including tryptophan
and tyrosine was chosen for its delicate balance of hydropho-
bic/hydrophilic character, which helps the hydrogelation pro-
cess.
The mechanical property of the hydrogel was characterized
by using an oscillating rheometer. Rheological analysis showed
that the storage modulus (G’) is greater than the loss modulus
(G’’) for peptide hydrogel 1. In the frequency sweep mode, the
values of the storage modulus (G’) exceed that of the loss
modulus (G’’) by a factor of 5–10 times that of hydrogel 1,
which indicates the formation of a strong and rigid hydrogel
(Figure 1).[49]
Herein, we report the sonication-induced formation of a tyro-
sine- and tryptophan-based peptide bolaamphiphile hydrogel.
The peptide nanofibers were used as a template to synthesize
Pd nanoparticles. The Pd nanoparticles were decorated on the
surface of peptide bolaamphiphile nanofibers, which provide
extra stability to the Pd nanoparticles (Scheme 1). We report
Figure 1. Frequency sweep of peptide bolaamphiphile hydrogel
1 (c=10 mmolLÀ1). Storage modulus G’ is higher than the loss modulus G’’.
G’>104 Pa at low frequency, and the storage modulus is higher than the
loss modulus by a factor of 5–10, which indicates excellent solidlike behavior
of the gel materials.
An FTIR study was carried out to obtain the secondary struc-
ture of peptide bolaamphiphile 1 in the gel state (Figure 2a).
In the gel state, two peaks appeared at 1714 and 1736 cmÀ1
,
which suggests that some of the carboxylic acid groups are in-
volved in hydrogen-bonding interactions (1714 cmÀ1), whereas
some of them are free (1736 cmÀ1). The characteristic amide I
band appeared at 1635 and 1615 cmÀ1 along with an amide II
band at 1516 cmÀ1, which indicate that the peptide bolaam-
phiphiles self-assemble into a supramolecular b-sheet structure
through hydrogen-bonding interactions.[50] The self-assembly
of the peptide bolaamphiphiles was also examined by circular
dichroism (CD) spectroscopy to realize the role of the chirality
of the individual molecules owing to the presence of the chiral
amino acid moieties (Figure 2b). The hydrogel was diluted to
a 100 mm concentration in phosphate buffer to investigate the
nature of the secondary structure of the self-assembled pep-
tide bolaamphiphile molecules. The CD spectrum showed
a characteristic positive peak at around 198 nm and a negative
peak at 211 nm, which is attributed to the n–p* transition of
the COÀNH groups of the peptide bolaamphiphile molecule.
This signature is analogous to the reported CD signature of
peptides adopting the b-sheet secondary structure.[51] This ob-
servation indicates that the gelator molecules self-assemble
into a b-sheet structure through extensive hydrogen-bonding
Scheme 1. A self-supporting hydrogel was formed by the self-assembly of
peptide bolaamphiphile 1 upon sonication. The hydrogel matrix was used
for in situ generation of Pd nanoparticles, which were stabilized and deco-
rated on the surface of peptide bolaamphiphile nanofibers.
a general method in which peptide-nanofiber-supported Pd
nanoparticles can efficiently deprotect various types of N-pro-
tecting groups from the N terminus of amino acids and pep-
tides in the presence of NaBH4 in aqueous medium at room
temperature.
Results and Discussion
Preparation of the hydrogel and study of the self-assembly
process
The synthesized peptide bolaamphiphile molecule 1 (HO-Y-W-
Suc-W-Y-OH, Y = tyrosine and W = tryptophan) is capable of
forming a self-supporting hydrogel in phosphate buffer (pH 8,
10 mmol) upon sonication. Peptide bolaamphiphile 1 (HO-Y-W-
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emission maxima occurs as a result of the extensive self-assem-
bly of gelator molecules through p–p stacking interactions
and the decrease in intensity results from the quenching of
the aromatic core present in gelator molecules during the self-
assembly process.[53] Another important emission maxima at
468 nm supports the formation of a higher ordered supra-
molecular nanostructure.[26]
The nanostructural morphology[54–59] of hydrogel 1 was char-
acterized by field-emission scanning electron microscopy
(FESEM) and atomic force microscopy (AFM; Figure 4a,b). The
Figure 2. (a) FTIR spectrum of peptide bolaamphiphile hydrogel
1 (c=10 mmolLÀ1), which indicates the formation of a supramolecular
b-sheet structure in the gel phase and (b) the CD spectrum of hydrogel
1 (c=100 mm) reveals the formation of a b-sheet structure through extensive
hydrogen-bonding interactions.
interactions. Another strong positive band appeared at
230 nm, which is attributed to the electron-transfer transition
involving excitation of the nonbonding electron of the oxygen
atom into the p*-orbital system of the aromatic ring of the ty-
rosine moiety.[52]
Figure 4. (a) A SEM image showing the entangled nanofibrous aggregates
and (b) an AFM image showing the nanofiber morphology of hydrogel 1.
Fluorescence spectroscopy results suggest that the p-stack-
ing interaction also plays a key role in the self-assembly pro-
cess towards hydrogel formation (Figure 3). The emission peak
of the aromatic phenolic groups of tyrosine residues and
indole groups of tryptophan residues at 380 nm after 10 min
shifts to 388 nm after 1 day for hydrogel 1, which suggests
that the aromatic groups are overlapped. The redshift of the
FESEM image of the hydrogel shows that compound 1 assem-
bled into dense entangled fibrous networks with fiber diame-
ters of (25Æ5) nm and lengths of several millimeters. The AFM
image also shows that the peptide bolaamphiphile molecules
self-assembled into nanofibrillar structures. The average diame-
ter of the fibers was found to be 30 nm and the average
height of an individual fiber was 4 nm (Figure S1 in the Sup-
porting Information).
To obtain further structural information, powder wide-angle
X-ray scattering (WAXS) was used to characterize peptide bo-
laamphiphile 1 and its dried hydrogel (Figure 5). We correlated
the different features of self-assembled 1 from the series of
characteristic reflection patterns obtained from the wide-angle
powder X-ray diffraction. Two reflection peaks at 2q=13.14
and 20.058 were observed for powdered compound 1, whereas
the dried hydrogel 1 showed several characteristic reflection
peaks. The characteristic peak at 2q=7.828, corresponding to
a d spacing of 11.29 ꢁ, is a signature of a b-sheet structure,
which is adopted by self-assembled peptide molecules. This re-
flection pattern indicates the distance between two successive
b sheets, that is, the intersheet distance. The characteristic
peak at 2q=18.158, corresponding to a d spacing of 4.88 ꢁ, re-
Figure 3. The fluorescence spectra of the peptide bolaamphiphile hydrogel
1 (c=10 mmolLÀ1), the results of which suggest extensive p–p stacking in-
teractions between the aromatic residues of peptide bolaamphiphile
1 during the self-assembly process.
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Figure 5. Wide-angle powder XRD pattern of (i) peptide bolaamphiphile
1 powder and (ii) its dried hydrogel 1. The powder X-ray diffraction pattern
exhibits the characteristic pattern of a b-sheet structure of peptide bolaam-
phiphile 1, which is adopted through p–p stacking interactions between the
aromatic moieties through the self-assembly process.
Figure 6. UV/Vis spectra used to monitor the synthesis of Pd nanoparticles
in gel nanofibers. The black line indicates PdCl2 in phosphate buffer whereas
the blue line indicates the generation of Pd nanoparticles.
veals the spacing between two peptide molecules within the
same b-sheet structure.[60] Another reflection peak at 2q=
27.358 (d spacing of 3.26 ꢁ) is expected from the p–p stacking
interaction[61] between two aromatic groups, which also plays
a vital role during the self-assembly process along with hydro-
gen-bonding interactions.
Generation and characterization of Pd nanoparticles
The redox-active tyrosine- and tryptophan-based peptide bo-
laamphiphile 1 was used to synthesize Pd nanoparticles in its
hydrogel matrix. Pd nanoparticles were synthesized in the hy-
drogel matrix without addition of any external reducing agent.
Redox-active tyrosine and tryptophan molecules in peptide bo-
laamphiphile molecules reduced Pd2+ to Pd0.[12,62] The synthe-
sized Pd nanoparticles were stabilized by the peptide bolaam-
phiphile nanofibers. In our experiment, peptide bolaamphi-
philes were sonicated to dissolve them in phosphate buffer
and then PdCl2 was mixed and sonicated for 5 min. The color
of the solution changed to brown within 2 hours, which is an
indication of the formation of Pd nanoparticles. The nanoparti-
cles were characterized by UV/Vis spectroscopy and transmis-
sion electron microscopy.
Figure 7. A TEM image of the Pd nanoparticles after 3 h of reaction time.
The Pd nanoparticles were decorated and stabilized on the surface of the
peptide nanofibers. The scale bar is 50 nm.
to 5 nm after 3 hours of reaction time (Figure 7) and the Pd
nanoparticles were decorated on the peptide nanofibers.
Peptide bolaamphiphile 1 consists of tyrosine and trypto-
phan residues along with two carboxylic acid groups. The Pd
nanoparticles are stabilized by carboxylic acid groups. The at-
tachment of Pd nanoparticles onto the surface of peptide bo-
laamphiphile nanofibers is due to the charges on ÀCOOÀ in
peptide bolaamphiphile 1.[7] Another possibility is that the CO
and NH of amide bonds, which do not participate in back-
bone hydrogen bonding, may play a crucial role in decora-
tion.[8] We characterized the Pd nanoparticles by HRTEM. The
HRTEM image shows clear lattice fringes with a d spacing of
0.228 nm, which is attributed to the (111) lattice planes[64] of
face-centered cubic (fcc) Pd (Figure 8). The HRTEM image con-
firms the formation of crystalline Pd nanoparticles.
The PdCl2 salt was dissolved in phosphate buffer and ana-
lyzed by UV/Vis spectroscopy. The UV/Vis spectra showed one
characteristic peak at around 332 nm and another weak inten-
sity peak at 430 nm for the PdCl2 salt. After the generation of
Pd nanoparticles, the solution was centrifuged three times and
washed with methanol. The black mass was dispersed in phos-
phate buffer and characterized by UV/Vis spectroscopy. The
UV/Vis spectrum showed that the peaks at 332 and 430 nm
completely disappeared and a new peak appeared at 274 nm.
The absorption band at 274 nm is attributed to Pd0 nanoparti-
cles (Figure 6).[27,63]
Catalytic activity of Pd nanoparticles
After the synthesis of Pd nanoparticles in the hydrogel
matrix, the size of the nanoparticles was investigated by trans-
mission electron microscopy. The TEM images of the in situ
synthesized nanoparticles in the hydrogel showed that the di-
ameter of the nanoparticles increased with reaction time. TEM
images revealed that the diameter of the Pd nanoparticles is 3
Pd nanoparticles have many potential applications. One of the
most important applications is to act as a nanocatalyst in
many chemical reactions.[65,66] The in situ synthesized Pd nano-
particles with a diameter of 3–9 nm were used as a potential
catalyst for the deprotection of various types of N-protected
amino acids and peptides (Scheme 2). Pd nanoparticles in the
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without peptide nanofibers. These Pd nanoparticles are not
stable in methanol/water solvent and the Pd nanoparticles ag-
gregate. The Pd nanoparticles lose their catalytic activity owing
to aggregation, whereas the peptide-bolaamphiphile-support-
ed Pd nanoparticles are stable and show efficient catalytic ac-
tivity in deprotection reactions. Several control experiments
were performed without the Pd nanoparticle catalyst. The
N-protected amino acids and peptides were treated with only
NaBH4 for 5 hours. The Fmoc group of Fmoc-Trp-OH (substrate
5) was deprotected with 48% yield. Boc-, Nmoc-, and Cbz-pro-
tected substrates remained silent (Table S2) under the same
conditions. So, peptide nanofiber-supported Pd nanoparticles
play a vital role in the deprotection reactions.
Figure 8. The HRTEM image of a Pd nanoparticle showing the lattice fringes.
Conclusion
The tyrosine- and tryptophan-containing bolaamphiphile pep-
tide forms a self-supporting hydrogel upon sonication under
physiological conditions. The self-assembly process was stud-
ied through CD, fluorescence spectroscopy, and rheological ex-
periments. Hydrogen bonding and p-stacking interactions are
the driving force for the formation of the self-assembled pep-
tide hydrogel. The SEM and AFM images reveal that the bo-
laamphiphile peptide molecules self-assembled into nanofibril-
lar structures. Furthermore, peptide nanofibers were used as
a template for in situ generation of Pd nanoparticles. Peptide-
nanofiber-supported Pd nanoparticles show efficient catalytic
activity towards the deprotection of the N terminus of amino
acids and peptides.
Scheme 2. The palladium-nanoparticle-catalyzed deprotection reaction of
N-terminus amino acids and peptides.
presence of NaBH4 were effectively used for deprotection reac-
tions.
The catalytic activity was examined with different types of
N-terminus (Boc-, Cbz-, Fmoc-, and Nmoc) amino acids and
peptides (Table 1). All of these palladium-catalyzed deprotec-
tion reactions were carried out in a water/methanol (1:1) solu-
tion at room temperature. Peptide-nanofiber-supported Pd
nanoparticles completed the reaction within 2 hours and all
the reactions were monitored by mass spectrometry. Boc and
Nmoc groups were deprotected by acid whereas Fmoc group
was deprotected by base. This novel method can effectively
deprotect Boc, Nmoc and Fmoc groups from the N-protected
amino acids or peptides. N-terminus protected Cbz group is
stable towards acid or base. In this case, palladium-nanoparti-
cle-catalyzed deprotection reactions are very effective. For the
Nmoc-protected amino acid or peptide, the reactions give un-
protected amino acid or peptide and the corresponding naph-
thalene methanol. The isolated yield was about 80–90% for all
the reactions. After the first batch of reactions, the Pd nano-
particles were recovered by using centrifugation and washing
of the reaction mixture. The recovered Pd nanoparticles were
also used for deprotection reactions, which were completed
successfully within 2 hours.
Experimental Section
Preparation of the hydrogel
Peptide bolaamphiphile
1
(HO-Y-W-Suc-W-Y-OH; 16.8 mg,
10 mmolLÀ1) was dispersed in sodium phosphate buffer solution
(2 mL, pH 8, 10 mmolLÀ1) and sonicated for 5 min. A self-support-
ing hydrogel formed after 10 min.
Synthesis of the Pd nanoparticles
Pd nanoparticles were synthesized in a hydrogel matrix without
the addition of any external reducing agent. In our experiment,
peptide bolaamphiphile molecules with tyrosine and tryptophan
residues (10 mmolLÀ1) were sonicated to enable dissolution in
phosphate buffer (2 mL) and then PdCl2 (2.8 mmolLÀ1) was mixed
followed by sonication for 5 min. It was seen that the gel became
brown after 2 h, which is an indication of the formation of Pd
nanoparticles inside the hydrogel matrix.
Here, peptide-nanofiber-supported Pd nanoparticles act as
an efficient catalyst for deprotection reactions. Pd nanoparti-
cles were also synthesized without a peptide support to deter-
mine the catalytic effect of the deprotection reactions. The
Boc-protected tyrosine remained unaffected by the catalysis re-
action. No significant yield was observed for the palladium-cat-
alyzed deprotection reactions with Nmoc-Phe-OH and Cbz-pro-
tected lysine (Table S1). In a control experiment, the Pd nano-
particles were synthesized by reduction of Pd2+ with NaBH4
Rheology
Oscillating rheology was used to quantify the final mechanical
properties of the peptide bolaamphiphile hydrogels. The experi-
ment was performed on a Paar Physica Modular Compact Rheome-
ter (MCR 301, Austria). A 25 mm cone plate with 18 angle configu-
ration was used and the temperature was set constant at 258C.
Storage (G’) and loss (G’’) moduli were measured at 0.1% strain
with a true gap of 0.05 mm.
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Fluorescence spectroscopy
Table 1. Summary of the deprotection reactions.
The fluorescence emission spectra
of the gel (10 mmolLÀ1) were re-
corded on a Horiba Scientific Fluo-
romax-4 spectrophotometer with
a 1 cm path length quartz cell at
room temperature. The slit width
for the excitation and emission
was set at 5 nm and a 1 nm data
pitch. Excitation of gel sample
1 was performed at 280 nm and
the data range was between 290
and 550 nm.
Entry Substrate (N-terminus-protected amino acid/ Product (deprotected amino acid/ Time [min] Yield [%][c]
peptide)[a]
peptide)[b]
1
2
2
3
2a
3a
60
60
87
80
Field-emission gun scanning
electron microscopy (FEG-SEM)
study
3
4
4
5
4a
60
60
82
90
For the SEM study, the hydrogel
was dried on a glass slide and
coated with platinum. Then the
micrographs were recorded in
a
SEM microscope (Jeol JSM-
5a
7600F).
AFM study
5
6
6
7
6a
7a
80
80
88
85
The gel sample was diluted in Milli
Q water to a final concentration of
0.5 mmolLÀ1 and placed on a mi-
croscopic cover slip. Then it was
dried by slow evaporation. Images
were taken with an AIST-NT instru-
ment (model smartSPM 1000)
using the soft tapping mode.
7
8
8
9
8a
9a
90
78
90
Wide-angle X-ray diffraction
Data was collected on a Bruker D8
Advance X-ray diffractometer with
a
wavelength of 1.5406 ꢁ. The
X-rays were produced using
a sealed tube and the X-rays were
detected using a fast counting de-
tector based on silicon strip tech-
nology (Bruker LynxEye detector).
120
[a] The compound numbers (e.g., 2, 3, 4, etc.) refer to the specific substrate, which was used for the deprotec-
tion reaction. [b] The products (e.g., 2a, 3a, 4a, etc.) obtained from the corresponding substrates after pep-
tide-nanofiber-decorated palladium-catalyzed deprotection reactions. [b] Yield of isolated product.
HRTEM study
High-resolution transmission elec-
tron microscopic images were re-
corded using a PHILIPS electron microscope (model: CM 200) oper-
ated at an accelerating voltage of 200 kV. Dilute solutions of the
Pd nanoparticles embedded in the hydrogel were dried on carbon-
coated copper grids (300 mesh) by slow evaporation in air, then al-
lowed to dry separately under vacuum at room temperature. The
average size of the nanoparticles was determined from the TEM
images.
CD spectroscopy
The secondary structure of the peptide bolaamphiphile was ana-
lyzed with a Jasco J-815 circular dichroism spectrometer. The pep-
tide hydrogel (10 mmolLÀ1) was diluted in double-distilled H2O
and measured from 250 to 190 nm with
a 0.1 data pitch,
a 20 nmminÀ1 scanning speed, a 1 nm band width, and a 4 s data
integration time (D.I.T).
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Catalytic studies
The peptide bolaamphiphile hydrogel (2 mL, 10 mmol) containing
Pd nanoparticles (0.006 to 0.01 mmol) was used for catalytic stud-
ies. In a typical experiment, the N-terminus amino acid or peptide
substrate (0.1 mmol; Table 1, entries 1 to 8) was dissolved in water/
methanol (1:1) solution (1 mL). The peptide hydrogel (1 mL,
10 mmol) containing Pd nanoparticles (0.006 to 0.01 mmol) was
added to the substrate (compounds 2 to 9) and cooled in an ice
bath for 5 min. After cooling, NaBH4 (0.264 mmol) was added and
sealed in the reaction tube. The reaction mixture was stirred and
the reaction was completed within 2 h at room temperature.
NaBH4 plays an important role in the reductive cleavage of N-pro-
tected groups. NaBH4 provides HÀ for reductive cleavage of N-ter-
minus protecting groups. The success of the catalytic effect of Pd
nanoparticles on the deprotection reaction was confirmed by mass
spectral studies.
For a control experiment, Pd nanoparticles were synthesized by re-
duction of Pd2+ with NaBH4. PdCl2 (1 mg, 0.0056 mmol,
2.8 mmolLÀ1) was dissolved in methanol/water (1:1; 2 mL) and
cooled in an ice bath for 5 min. NaBH4 (0.529 mmol) was added to
the above reaction mixture to reduce Pd2+ to Pd. The Pd nanopar-
ticles were isolated by centrifugation followed by washing with
methanol/water (1:1) solution. The protected amino acid
(0.1 mmol) was dissolved in methanol/water (1:1; 1 mL) and Pd
nanoparticles (0.01 mmol) were added to it. After cooling for
5 min, NaBH4 (0.264 mmol) was added to the reaction mixture and
the reaction tube was sealed. The reaction mixture was kept stir-
ring for 2 h at room temperature to allow the deprotection reac-
tions to occur. After 2 h, the reaction was monitored by mass anal-
ysis.
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Acknowledgements
We acknowledge CSIR, New Delhi, India for financial support. I.M.
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Keywords: bolaamphiphiles · gels · nanoparticles · palladium ·
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