CHEMPLUSCHEM
FULL PAPERS
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
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
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 413 – 420 413