Peptide nanofibers decorated with Pd nanoparticles to enhance the catalytic activity for C-C coupling reactions in aerobic conditions

Article abstract of DOI:10.1039/c3ra44787a

We report the synergistic effects of peptide nanofibers decorated with Pd nanoparticles to enhance the catalytic activity for C-C coupling reactions in mild and aerobic conditions.

Full text of DOI:10.1039/c3ra44787a

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Peptide nanofibers decorated with Pd
nanoparticles to enhance the catalytic activity for
Cite this: RSC Adv., 2014, 4, 2984
C–C coupling reactions in aerobic conditions†
Received 30th August 2013
Accepted 17th October 2013
*
Indrajit Maity, Dnyaneshwar B. Rasale and Apurba K. Das
DOI: 10.1039/c3ra44787a
www.rsc.org/advances
We report the synergistic effects of peptide nanofibers decorated with cetyltrimethylammonium bromide (CTAB) have been used for
Pd nanoparticles to enhance the catalytic activity for C–C coupling effective Suzuki coupling reactions in aqueous media.14 Suzuki
reactions in mild and aerobic conditions.
coupling reactions catalysed by peptide nanobers decorated
with Pd nanoparticles with improved yields under mild reaction
conditions still remains unexplored.
Bioinspired peptide nanobers1 have a wide range of applica-
tions in the elds of tissue engineering2 and nanotechnology.3
Peptides and peptide bolaamphiphiles can adopt various types
of well dened nanostructures4 in their self-assembled hydrogel
state. Several challenges have been achieved in the formation of
well-dened nanomaterials for use as catalysts, however, the
interactions at the biotic/abiotic interface remain poorly
understood. Recently, we reported a peptide bolaamphiphile
nanober templated synthesis of Pt nanoparticles in a hydrogel
matrix and the efficient catalytic activity for the hydrogenation
reaction of p-nitroaniline to p-phenylenediamine.5 A gel-based
trihybrid system containing Au nanoparticles has been used as
a catalyst for the reduction of 4-nitrophenol to 4-aminophenol.6
Recently, Knecht et al. reported the fabrication of peptide cap-
ped Pd nanoparticles, which enhanced the catalytic activity of
C–C coupling reactions under non-traditional conditions of a
water-based solvent at room temperature.7 The catalytic activity
of metal nanoparticles can be enhanced by using a nano-
structured templated synthesis of metal nanoparticles and
providing a high surface area.8 Therefore, bioinspired peptide
or peptide bolaamphiphile nanostructures are the best choice
to use as a template for the synthesis and stabilization of metal
nanoparticles. Several groups have used palladium nano-
particles (PdNPs) as a catalyst for a wide range of organic
reactions9 including Suzuki coupling reactions,10 Stille coupling
reactions,11 Heck reactions12 and Sonogashira coupling reac-
tions.13 Generally, various ligands and quaternary ammonium
salts such as tetrabutylammonium bromide (TBAB) or
In this work, we demonstrate the self-assembly of a peptide
bolaamphiphile molecule for the fabrication of Pd nanoparticles
on peptide nanobers to enhance the catalytic ability for C–C
Department of Chemistry, Indian Institute of Technology Indore, Khandwa Road,
Indore, 452017, India. E-mail: apurba.das@iiti.ac.in
Scheme 1 (a) Molecular structure of peptide bolaamphiphile 1 used in
† Electronic supplementary information (ESI) available: FT-IR and CD spectra,
TEM images of Pd nanoparticles, synthesis and characterization of peptide
bolaamphiphile and Suzuki coupling products. See DOI: 10.1039/c3ra44787a
the self-assembly study, (b) a photograph of a self-supporting
hydrogel which forms nanofibrillar structures, and (c) the in situ
synthesized Pd nanoparticles decorated on peptide nanofibers.
2984 | RSC Adv., 2014, 4, 2984–2988
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
coupling reactions under mild and aerobic conditions (Scheme
1). The peptide nanobers were anticipated to anchor the
aromatic reactants and metal nanoparticles, which would result
in an enhanced catalytic activity for C–C coupling reactions.
A synthesized peptide bolaamphiphile 1 (HO-F-Y-Suc-Y-F-
OH) was able to self-assemble in an aqueous medium and
formed a self-supporting hydrogel. We utilized the hydrolysis of
succinic anhydride in an aqueous medium to form succinic acid
and lower the pH in a controlled manner. In our experiment,
29.53 mg (20 mmol Lꢀ1) of peptide bolaamphiphile 1 was
dispersed in 2 mL of doubly distilled water. The peptide
bolaamphiphile was completely dissolved by the addition of
300 mL of 1 M NaOH and the nal pH of the solution was found
to be 11.2. Eight equivalents (32 mg, 160 mmol Lꢀ1) of succinic
anhydride were added to the peptide bolaamphiphile solution.
The hydrolysis of succinic anhydride to succinic acid is slow and
the pH continued to drop over time (ESI Fig. 1†). The nal pH of
the solution was found to be 3.3 aer 30 minutes and a self-
supporting hydrogel was formed.
The self-assembly study of the peptide bolaamphiphile 1 was
carried out using various spectroscopic and microscopic tech-
niques. FT-IR spectroscopy was used to study the secondary
structure of the hydrogel 1. The FT-IR spectrum of the hydrogel
exhibited an amide I band at 1629 cmꢀ1, which signies a
b sheet structure (ESI Fig. 2a†) of self-assembled gelator mole-
cules.5,15 A circular dichroism (CD) spectrum was recorded to nanoparticles without any external reducing agent. The redox
understand the secondary structural information of the self- active tyrosine molecules5,20 reduced the Pd2+ to Pd0. In our
assembled peptide bolaamphiphile 1 in the gel phase.
Fig. 1 (a) FE-SEM image showing the entangled nanofibrillar struc-
tures, (b) TEM image showing the nanofibrillar network structure of the
hydrogel. The TEM images show that: (c) Pd nanoparticles were
decorated on the surface of the peptide nanofibers and (d) Pd nano-
particles were completely aggregated without peptide nanofibers.
The nanobrillar hydrogel 1 matrix was used to synthesis Pd
experiment, the peptide bolaamphiphile 1 (20 mmol Lꢀ1) and
The CD spectrum of peptide bolaamphiphile 1 (ESI Fig. 3a†) PdCl2 (2 mg, 5.6 mmol Lꢀ1) was dispersed in doubly distilled
exhibited a strong positive band at 198 nm for the p–p* tran- water and the self-supporting hydrogel was prepared as per the
sition and a weak shoulder around 217 nm for the n–p* tran- above-mentioned method. The hydrogel formed as a light
sition in –CONH–.16 The CD signature showed a mixture of brown color. The formation of the PdNPs was monitored by the
b-sheet (42.8%) and random coil (39.6%) structures of the change in color. The color changed to dark brown within
hydrogel. To acquire more insight into the structural informa- 12 hours, which indicated the formation of the Pd nanoparticles
tion at the supramolecular level, the hydrogel was characterized in the hydrogel matrix. The effect in the secondary structure of
by uorescence spectroscopy. The emission spectrum of the the peptide bolaamphiphile from the interaction between the
hydrogel (ESI Fig. 4†) showed two characteristic peaks centered metal nanoparticles and the peptide bolaamphiphile was
at 297 nm and 320 nm. These two peaks are attributed to the recorded by FT-IR and CD analysis. The amide I band appeared
extensive p–p stacking interactions between the aromatic as a weak intensity band at 1635 cmꢀ1 for the peptide nano-
groups present in the backbone of the peptide bolaamphiphile bers decorated with Pd nanoparticles whereas a more intense
1.17 Another pronounced peak appeared at 468 nm, which is band at 1629 cmꢀ1 appeared for the peptide hydrogel without
assigned to a higher order supramolecular structure of the Pd nanoparticles (ESI Fig. 2†). This indicates that the secondary
peptide bolaamphiphile hydrogel 1.18 These results reveal that structure was slightly affected by the decoration of Pd nano-
the self-assembly of peptide bolaamphiphile 1 is governed by particles on the peptide nanobers. The secondary structure of
hydrogen bonding and p–p stacking interactions between the the Pd nanoparticles bound to the peptide bolaamphiphile
aromatic residues. The time correlated single photon counting nanobers was also studied by CD measurements. The CD
(TCSPC) experiment showed that the average lifetime for the spectrum of the Pd-bound peptide bolaamphiphile nanobers
hydrogel (10 mmol Lꢀ1) was 0.69 ns (ESI Fig. 5†).
showed the presence of a shoulder between 225 nm and 240 nm
The FE-SEM image (Fig. 1(a)) shows that the peptide and a wavelength shi from 198 to 200.5 nm at the positive side
bolaamphiphile molecules self-assembled into entangled and of spectrum (ESI Fig. 3b†). This indicates a compact peptide
helical nanobrillar structures.5,19 Each ber diameter was 20 ꢁ structure with the Pd nanoparticles.7 The decrease in ellipticity
5 nm and several micrometers in length. The TEM image of a at the wavelength of 200.5 nm reveals that the Pd-bound peptide
diluted solution of the peptide hydrogel shows three-dimen- hydrogel composite appeared to be of a less ordered structure
sional nanobrillar networks (Fig. 1(b)). The average diameter than the peptide bolaamphiphile hydrogel itself. The CD spec-
of the nanobers was 22 nm and each ber was several trum of the Pd-bound peptide bolaamphiphile hydrogel showed
micrometers in length. The nanobrillar network structures are a mixture of b-sheet (36.8%) and random coil (37.3%) struc-
responsible for the formation of the self-supporting hydrogel.
tures. The nanoparticles were characterized by transmission
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 2984–2988 | 2985

View Article Online
RSC Advances
Communication
electron microscopy. The TEM images show that the Pd nano- even in the cases of the electron-rich (R2 ¼ 4-OH, 4-NH2)
particles were synthesized and stabilized by the peptide nano- substrates (Table 1). Aer each reaction, the methanol was
bers (ESI Fig. 6†). The Pd nanoparticles were decorated and evaporated and 1 M Na2CO3 was added to the reaction mixture
monodispersed on the surface of the peptide nanobers to remove the peptide nanobers and the Pd nanoparticles. The
(Fig. 1(c)).21 The size of the nanoparticles ranged from 4 to 7 nm. reaction mixture was extracted with chloroform (3 ꢃ 20 mL).
To understand the role of the peptide nanobers in stabilizing The organic layer was evaporated to isolate the coupling prod-
and dispersing the Pd nanoparticles, we prepared Pd nano- ucts. The products were puried by column chromatography.
particles without nanobers for TEM characterization. NaBH4 The isolated yield for all these coupling reactions was around
(1 mmol) was added to a 2 mL solution of PdCl2 (5.6 mmol Lꢀ1
to reduce Pd2+ to Pd. Pd nanoparticles were collected by several recovered and reused for further reactions (ESI Table 2†).
)
90%. Aer the rst batch of reactions, the Pd nanoparticles were
washings and centrifugation. The TEM image shows that the Pd
The coupling products were well characterized by mass
nanoparticles were completely aggregated in this case spectrometry, and 1H and 13C NMR spectroscopy. We antici-
(Fig. 1(d)). This clearly suggests that the peptide nanobers pated that the coupling reactions happened on the surface of
have a crucial role in the stabilization and dispersion of Pd the peptide nanobers. The TEM image showed that the
nanoparticles.
nanoparticles were decorated on the peptide nanobers. The
Pd nanoparticles are used as a catalyst in an extensive range peptide nanobers have hydrogen bonding sites along with
of reactions in organic synthesis. They are used in Suzuki aromatic cores. We anticipate that the hydrophobic aryl halides
coupling reactions, where the ligands and temperature play are stacked on the ber surface through hydrophobic and p–p
crucial roles. Here, we have efficiently used Pd nanoparticles for stacking interactions of the aromatic residues of the nanobers,
Suzuki coupling reactions under mild conditions and at a and the boronic acid groups interact with the nanober surface
relatively low temperature in an aerobic environment (Scheme through hydrogen bonding interactions. This surface
2). The in situ synthesized well dispersed and decorated Pd phenomenon could be related to the enhanced catalytic activity
nanoparticles with sizes ranging from 4 to 7 nm were used of Pd nanoparticles for C–C coupling reactions (Fig. 2). The
efficiently for Suzuki coupling reactions. All the coupling reac- yields were signicantly less when Pd nanoparticles catalyzed
tions were carried out in water–methanol (1 : 1) solvent.
the C–C coupling reactions without being decorated on peptide
All the aryl halides and boronic acids used were completely nanobers.
soluble in the water–methanol (1 : 1) solvent. In the Suzuki
In summary, we have reported the development of a peptide
coupling reactions, a mixture of 0.08 mmol of the aryl halide bolaamphiphile hydrogel. The self-assembly of the peptide
and 0.12 mmol of the aryl boronic acid were dissolved in 2 mL of nanostructures could be controlled by the hydrolysis of succinic
the water–methanol (1 : 1) solvent. The Pd nanoparticles anhydride. The self-assembly process was driven by hydrogen
(0.0024 mmol, 500 mL of the hydrogel containing Pd nano- bonding and p–p stacking interactions, which led to the
particles decorated on the nanobers) were added to the reac- formation of nanobrillar structures. This succinic anhydride
tion mixture. Potassium carbonate (6 equiv., 0.48 mmol) was hydrolysis method to trigger the formation of a biocompatible
used as a base for all the coupling reactions. The C–C coupling nanobrous hydrogel matrix was used to synthesis Pd nano-
reaction was very fast and completed within 2 hours at 40 ꢂC for particles. The Pd nanoparticles were stabilized and decorated
the aryl iodides. The Suzuki coupling reaction is less feasible on the surface of peptide bolaamphiphile nanobers. The Pd
under mild conditions for the comparatively less reactive aryl nanoparticles decorated on the peptide nanobers enhanced
bromides. We observed that all the C–C coupling reactions took the catalytic activity for C–C coupling reactions. This indicates
4 hours to complete at 50 ꢂC for the aryl bromides. The products that self-assembled peptide bolaamphiphiles could be used to
were obtained in high yields for all the C–C coupling reactions, switch and control the activity of a material’s functionality. We
believe that this study will lead to the development of bio-
inspired functional smart materials.
Experimental section
Preparation of the hydrogel
29.53 mg (20 mmol Lꢀ1) of the peptide bolaamphiphile (HO-F-
Y-Suc-Y-F-OH) 1 was dispersed in 2 mL of doubly distilled water.
The peptide bolaamphiphile was completely dissolved by the
addition of 0.3 mL of 1 M NaOH and the nal pH of the solution
was found to be 11.2. Eight equivalents (32 mg, 160 mmol Lꢀ1
)
of succinic anhydride was added to the peptide bolaamphiphile
solution. The hydrolysis of succinic anhydride to succinic acid
is slow and the pH continued to drop over time. The nal pH of
the solution was found to be 3.3 aer 30 minutes. A self-sup-
porting hydrogel was formed upon vortexing the resulting
solution for 10 seconds.
Scheme 2 Suzuki-coupling reaction using Pd nanoparticles deco-
rated on peptide nanofibers.
2986 | RSC Adv., 2014, 4, 2984–2988
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
Table 1 Table for Suzuki coupling reactions between aryl halides and phenyl boronic acids
Entry
Boronic acid
Aryl halide
Product
Time (h)
Yielda (%)
1
2
1.5
4
92
90
3
4
5
6
7
8
4
87
90
88
90
87
91
3
4
4
3
1.5
9
4
90
90
89
87
10
11
12
3
1.5
4
13
4
90
a
Isolated yield.
Synthesis of Pd nanoparticles
Pd nanoparticles were synthesized in the hydrogel matrix
without the addition of any external reducing agent. The
peptide bolaamphiphile 1 (20 mmol Lꢀ1) was dispersed in 2 mL
of doubly distilled water and the peptide bolaamphiphile was
completely dissolved by the addition of 0.3 mL of 1 M NaOH.
Then, PdCl2 (2 mg, 5.6 mmol Lꢀ1) was dispersed in the solution
and the solution was sonicated for a few minutes to completely
dissolve the dispersion. Eight equivalents (32 mg,
160 mmol Lꢀ1) of succinic anhydride was added to decrease the
pH in a controlled manner through its hydrolysis to succinic
acid. The hydrogel formed aer 30 minutes when the pH was
found to be 3.3. The formation of the PdNPs was monitored by
the change in color. The color changed to dark brown within
12 hours, which indicated the formation of Pd nanoparticles in
the hydrogel matrix.
Catalytic studies
Fig.
2 The possible reaction mechanism for the C–C coupling
(Suzuki) reaction on the surface of Pd nanoparticles decorated on In a Suzuki coupling reaction, a mixture of 0.08 mmol of the aryl
peptide nanofibers.
halide and 0.12 mmol of the aryl boronic acid were dissolved in
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 2984–2988 | 2987

View Article Online
RSC Advances
Communication
2 mL of water–methanol (1 : 1) solvent. The Pd nanoparticles
(0.0024 mmol, 500 mL of the hydrogel containing Pd nano-
particles decorated on the nanobers) were added to the reac-
tion mixture. Potassium carbonate (6 equiv., 0.48 mmol) was
used as a base for all the coupling reactions. The reactions were
N. Munichandraiah, ACS Appl. Mater. Interfaces, 2010, 2,
2965.
9 (a) X. Huang, Y. Li, Y. Chen, E. Zhou, Y. Xu, H. Zhou, X. Duan
and Y. Huang, Angew. Chem., Int. Ed., 2013, 52, 2520; (b)
O. Verho, A. Nagendiran, E. V. Johnston, C.-w. Tai and
ꢂ
¨
carried out at 40–50 C depending on the substrates used.
J.-E. Backvall, ChemCatChem, 2013, 5, 612.
10 (a) D. Saha, K. Chattopadhyay and B. C. Ranu, Tetrahedron
Lett., 2009, 50, 1003; (b) A. Ohtaka, T. Teratani, R. Fujii,
K. Ikeshita, T. Kawashima, K. Tatsumi, O. Shimomura and
R. Nomura, J. Org. Chem., 2011, 76, 4052; (c) A. Bej,
D. Srimani and A. Sarkar, Green Chem., 2012, 14, 661.
11 J. C. Garcia-Martinez, R. Lezutekong and R. M. Crooks, J. Am.
Chem. Soc., 2005, 127, 5097.
Acknowledgements
AKD thanks DST for funding this work. IM and DBR are
indebted to CSIR for their fellowships. We are thankful to SAIF,
IIT Bombay for the assistance in using the TEM facility.
12 (a) S. Bhattacharya, A. Srivastava and S. Sengupta,
Tetrahedron Lett., 2005, 46, 3557; (b) B. C. Ranu and
K. Chattopadhyay, Org. Lett., 2007, 9, 2409.
13 (a) S. Bhattacharya and S. Sengupta, Tetrahedron Lett., 2004,
45, 8733; (b) D. Saha, R. Dey and B. C. Ranu, Eur. J. Org.
Chem., 2010, 6067.
14 (a) B. Yuan, Y. Pan, Y. Li, B. Yin and H. Jiang, Angew. Chem.,
Int. Ed., 2010, 49, 4054; (b) M. Keller, V. Colliere, O. Reiser,
A.-M. Caminade, J.-P. Majoral and A. Ouali, Angew. Chem.,
Int. Ed., 2013, 52, 3626.
Notes and references
1 (a) C. Tomasini and N. Castellucci, Chem. Soc. Rev., 2013, 42,
156; (b) K. J. Nagy, M. C. Giano, A. Jin, D. J. Pochan and
J. P. Schneider, J. Am. Chem. Soc., 2011, 133, 14975; (c)
D. A. Stone, L. Hsu and S. I. Stupp, So Matter, 2009, 5,
1990; (d) X. Zhang, X. Chu, L. Wang, H. Wang, G. Liang,
J. Zhang, J. Long and Z. Yang, Angew. Chem., Int. Ed., 2012,
51, 4388; (e) J. Raeburn, T. O. McDonald and D. J. Adams,
Chem. Commun., 2012, 48, 9355.
15 J. T. Pelton and L. R. McLean, Anal. Biochem., 2000, 277, 167.
16 (a) X. Yan, Y. Cui, Q. He, K. Wang and J. Li, Chem. Mater.,
2008, 20, 1522; (b) S. M. Kelly, T. J. Jess and N. C. Price,
Biochim. Biophys. Acta, Proteins Proteomics, 2005, 1751, 119.
17 I. Maity, D. B. Rasale and A. K. Das, RSC Adv., 2013, 3, 6395.
18 A. K. Das, A. R. Hirst and R. V. Ulijn, Faraday Discuss., 2009,
143, 293.
19 (a) R. Afrasiabi and H.-B. Kraatz, Chem.–Eur. J., 2013, 19,
1769; (b) K. P. Divya, S. Sreejith, C. H. Suresh and
A. Ajayaghosh, Chem. Commun., 2010, 46, 8392; (c)
D. B. Rasale, I. Maity, M. Konda and A. K. Das, Chem.
Commun., 2013, 49, 4815.
2 (a) J. L. Frandsen and H. Ghandehari, Chem. Soc. Rev., 2012,
41, 2696; (b) J. Nicolas, S. Mura, D. Brambilla, N. Mackiewicz
and P. Couvreur, Chem. Soc. Rev., 2013, 42, 1147.
3 A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
Chem., Int. Ed., 2008, 47, 8002.
4 (a) F. Versluis, H. R. Marsden and A. Kros, Chem. Soc. Rev.,
2010, 39, 3434; (b) N. Kameta, M. Masuda and T. Shimizu,
ACS Nano, 2012, 6, 5249; (c) M. Reches and E. Gazit, Curr.
Nanosci., 2006, 2, 105.
5 I. Maity, D. B. Rasale and A. K. Das, So Matter, 2012, 8, 5301.
6 J. Nanda, A. Biswas, B. Adhikari and A. Banerjee, Angew.
Chem., Int. Ed., 2013, 52, 5041.
20 S. Ray, A. K. Das and A. Banerjee, Chem. Commun., 2006,
2816.
21 (a) P. Koley and A. Pramanik, Adv. Funct. Mater., 2011, 21,
4126; (b) S. Guha and A. Banerjee, Adv. Funct. Mater., 2009,
19, 1949; (c) Z. Yang, H. Gu, J. Du, J. Gao, B. Zhang,
X. Zhang and B. Xu, Tetrahedron, 2007, 63, 7349.
7 R. Coppage, J. M. Slocik, H. Ramezani-Dakhel,
N. M. Bedford, H. Heinz, R. R. Naik and M. R. Knecht,
J. Am. Chem. Soc., 2013, 135, 11048.
8 (a) H. F. Lang, R. A. May, B. L. Iversen and B. D. Chandler,
J. Am. Chem. Soc., 2003, 125, 14832; (b) S. Patra,
B.
Viswanath,
K.
Barai,
N.
Ravishankar
and
2988 | RSC Adv., 2014, 4, 2984–2988
This journal is © The Royal Society of Chemistry 2014