Supramolecular control of self-assembling terthiophene-peptide conjugates through the amino acid side chain

Article abstract of DOI:10.1039/c2cc34375d

The self-assembly of oligothiophene-peptide conjugates can be directed through the systematic variation of the peptide sequence into different nanostructures, including flat spicules, nanotubes, spiral sheets, and giant, flat sheets. Furthermore, the asse

Full text of DOI:10.1039/c2cc34375d

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COMMUNICATION
Supramolecular control of self-assembling terthiophene–peptide
conjugates through the amino acid side chainw
Jessica A. Lehrman,a Honggang Cui,b Wei-Wen Tsai,a Tyson J. Moyerb and
Samuel I. Stupp*abcd
Received 18th June 2012, Accepted 10th August 2012
DOI: 10.1039/c2cc34375d
The self-assembly of oligothiophene–peptide conjugates can be
directed through the systematic variation of the peptide sequence
into different nanostructures, including flat spicules, nanotubes,
spiral sheets, and giant, flat sheets. Furthermore, the assembly of
these molecules is not controlled by steric interactions between
the amino acid side chains.
Table 1 Structures of the terthiophene amphiphiles varying only in
the amino acid side chain closest to the terthiophene segment and
corresponding amino acid dimensions and properties. Values of the
amino acid properties were obtained from protein databanks, which
calculate hydrophobicity values from statistical data based on actual
proteins8,9
The self-assembly of aromatic rod-like molecules in water can
provide a strategy to construct well-defined, discrete nano-
structures that integrate photonic, electronic, and biological
functions.1,2 The ability to create and control new structures is
of interest because different supramolecular nanostructures
can exhibit novel properties as a result of different molecular
packing arrangements.3,4 Inspired by the well-defined assem-
blies achieved in water using peptide amphiphiles (PAs),4–6
there has been significant interest in using peptides to direct the
spatial orientation and packing of p-conjugated oligomers through
the peptide sequence.1,7 However, the design rules for the self-
assembly of these hybrid molecules are still not well understood.
In recent work, the sequence of tetrapeptide PAs containing
hydrophilic and hydrophobic residues was shown to greatly
impact nanostructure morphology.6 These PAs assembled into
large, flat nanobelt structures through a combination of
hydrophobic collapse of alkyl tails and hydrogen bonding.
The curvature-free, flat architecture observed was attributed
to van der Waals interactions among amino acid side chains
leading to dimerization of molecules and close packing of the
headgroups.
Side chain
volume (A3)
Hydrophobicity
(kcal molÀ1
Compound
R
)
3T-GE
3T-VE
0.63
80.5
0
2.2
3T-IE
107.2
107.2
2.7
2.9
3T-LE
balance of forces that govern their assembly (Table 1). Our
molecular design included terthiophene to integrate opto-
electronic properties into our assemblies and to direct the
assembly in water through hydrophobic collapse and p–p
stacking interactions. A dipeptide was included for water
solubility and for potential integration with biological systems
in the future. The sequence of the dipeptide, consisting of
alternating hydrophobic and hydrophilic residues, should
promote intermolecular hydrogen bonding and van der Waals
interactions between the amino acid side chains.6
To understand the influence of specific amino acids in the
self-assembly of oligothiophene–peptide conjugates, a series of
four molecules was designed and synthesized to probe the
a Department of Chemistry, Northwestern University, USA
b Department of Materials Science and Engineering,
Northwestern University, Evanston, IL 60208, USA.
E-mail: s-stupp@northwestern.edu; Fax: +1 (847) 491-3010;
Tel: +1 (847) 491-3002
In order to specifically examine the roles of steric hindrance
and hydrophobic van der Waals interactions on the supra-
molecular structure, the hydrophobic amino acids were selected
based on their van der Waals volume and hydrophobicity
values (Table 1).8,9 Amino acids with hydrophobic, alkyl side
chains—glycine, leucine, valine, and isoleucine—were chosen
to minimize additional interactions among the side chains.
Since leucine and isoleucine have the same van der Waals volume,
and isoleucine and valine share similar b-branched structures,
c Institute for BioNanotechnology in Medicine, Northwestern
University, USA
d Department of Medicine, Northwestern University, Chicago,
IL 60611, USA
w Electronic supplementary information (ESI) available: Materials
and methods, synthetic details and molecular characterization, small
angle X-ray scattering, circular dichroism, and transmission FTIR
data. See DOI: 10.1039/c2cc34375d
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9711–9713 9711

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these amino acids were selected to systematically examine the
steric influence of the side chains on the molecular self-
assembly of these molecules. In addition, these amino acids
all vary in their hydrophobic character. Glycine, which lacks a
side chain, serves as both a reference point, and a control for
the hydrophobicity of amino acids. The molecules were
synthesized via a solution-phase coupling of a-terthiophene
carboxylic acid to the N-terminus of the dipeptide (Scheme S1,
ESIw). All of the molecules were assembled in water (purified
to 18.2 MO cm) at a concentration of 0.2% (w/v), heated at
80 1C for 24 h to give a molecularly dissolved, transparent,
yellow solution, and then slowly cooled to room temperature
on the benchtop overnight.
various nanostructures. UV-Vis spectra for each of the assem-
blies (Fig. 2) show a hypsochromic shift in the absorbance
maximum (lmax), indicating that the terthiophene chromo-
phores are participating in face-to-face p–p stacking (H-aggre-
gation).11 Under assembling conditions, 3T-GE displays a
slightly broadened peak, whereas 3T-VE and 3T-LE display
increasingly stronger absorption bands that are hypsochromi-
cally shifted by 37 nm and 44 nm, respectively. However, the
UV-Vis spectrum of 3T-IE shows a hypsochromically shifted
shoulder that is weaker than that observed for 3T-VE. Thus, in
this system, the excitonic coupling between terthiophenes is
strongest in the nanotubes of 3T-LE, and appears to weaken in
assemblies of 3T-VE, 3T-IE, and 3T-GE. This phenomenon
is clearly illustrated by the observation of a stronger hypso-
chromic shift in lmax relative to unassembling conditions
(pH = 10), both in the magnitude of the shift and the
absorbance intensity. Furthermore, variable concentration
UV-Vis spectra suggest that the p–p stacking interactions in
3T-GE assemblies are weaker than those in 3T-VE, 3T-IE, and
3T-LE (Fig. S2, ESIw).
Fourier transform infrared (FTIR) spectroscopy (Fig. S4,
ESIw) was used to characterize the nature of hydrogen
bonding in each of the assemblies. Analysis of the FTIR
spectra shows that the N–H stretch for each of the molecules
is B3300 cmÀ1, indicating as expected, the presence of inter-
molecular hydrogen bonding in all four assemblies. Relative
to 3T-GE, the frequency of the N–H stretch is increasingly
red-shifted for 3T-LE, 3T-IE, and farthest red-shifted for
3T-VE (Table 2). It is commonly accepted that the magnitude
of the red shift in N–H stretch is proportional to the hydrogen
bond strength.12 Thus, intermolecular hydrogen bonding is
strongest in the spiral sheet assemblies of 3T-VE, and gradually
weakens for assemblies of 3T-IE, 3T-LE, and 3T-GE. The
amide I frequencies observed are inconclusive for the presence
of any peptide secondary structure.
Cryogenic transmission electron microscopy of the resulting
nanostructures revealed that the amino acid side chain has
a dramatic influence on the overall morphology of the
assembled nanostructures (Fig. 1). 3T-GE assembles into
aggregates of flat structures that are 70–120 nm wide. How-
ever, 3T-VE, which incorporates the amino acid valine,
assembles into spiral sheets with diameters ranging from
200–400 nm. 3T-LE, which incorporates the bulkier amino
acid leucine, assembles into surprisingly monodisperse nano-
tubes that are 210–240 nm in diameter. Comparison of the
nanostructures formed by 3T-GE, 3T-VE, and 3T-LE initially
led us to the hypothesis that the formation of more highly
curved supramolecular morphologies was a direct result of
steric hindrance between amino acids with increasingly larger
side chains. However, 3T-IE incorporates isoleucine, with the
same van der Waals volume as leucine, and assembles into
giant, flat sheets with dimensions of several micrometers.
Thus, the assembled morphologies observed by TEM do not
arise solely from steric interactions of amino acid side chains
in neighboring molecules.9 Furthermore, since isoleucine and
valine share similar b-branched structures, comparison of
3T-IE and 3T-VE effectively serves as a control for any steric
effect that arises from b-branching of the amino acid side
chain.10 Solution-phase small angle X-ray scattering (SAXS)
(Fig. S1, ESIw) of 3T-IE displays a slope of À2 in the low q
range, indicating that the flat sheet structures consist of a
single 1.8 nm thick bilayer. The scattering curves for 3T-GE,
3T-VE, and 3T-LE are further complicated by form factors
that are difficult to fit using conventional models.
Taken together, the UV-Vis and FTIR spectra indicate that
p–p stacking and hydrogen-bonding interactions both contri-
bute to the self-assembly, but also compete with each other.
This is evident as we see opposite trends in the UV-Vis and
FTIR spectra—i.e., as the p–p stacking becomes weaker, the
hydrogen bonding becomes stronger. In this self-assembling
system, the balancing of p–p stacking and hydrogen bonding
manifests itself in the assembly of different nanostructures.
One explanation for the variety of nanostructures observed
The assemblies were further investigated spectroscopically
to gain insight into molecular packing arrangements in the
Fig. 1 Cryogenic transmission electron microscopy of the nanostructures assembled in water at a concentration of 0.2% (w/v). (a) 3T-GE
assembles into aggregates of flat nanostructures, (b) 3T-VE assembles into spiral sheets, (c) 3T-LE assembles into nanotubes, and (d) 3T-IE
assembles into giant, flat sheets (scale bar = 500 nm).
c
9712 Chem. Commun., 2012, 48, 9711–9713
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clear that control of the relative strengths of p–p stacking and
hydrogen bonding in these molecules is accessed by mutation
of hydrophobic amino acids.
The authors thank the NSF (DMR-0605427) for financial
support. This work made use of instruments in the Biological
Imaging Facility, Keck Biophysics Facility, Institute for Bio-
Nanotechnology in Medicine, Integrated Molecular Structure
Education and Research Center, and Keck-II facility of the
Northwestern University Atomic and Nanoscale Character-
ization Experimental Center (NUANCE). NUANCE is
supported by the NSF-NSEC, NSF-MRSEC, Keck Founda-
tion, the State of Illinois, and Northwestern University.
Portions of this work were performed at the DuPont-North-
western-Dow Collaborative Access Team (DND-CAT) located
at Sector 5 of the Advanced Photon Source (APS). DND-CAT
is supported by E.I. DuPont de Nemours & Co., The Dow
Chemical Company and Northwestern University. Use of the
APS, an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne
National Laboratory, was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357. We also thank Liam
Palmer and Ronit Bitton for helpful discussions.
Fig. 2 UV-Vis absorption spectra. 3T-GE shows
a broadened
absorption peak, 3T-LE shows a peak at 333 nm, 3T-VE shows a
shoulder at 340 nm, and 3T-IE shows a broadened absorption peak.
All spectra were compared to that obtained under conditions that do
not promote self-assembly by dissolving the sample in excess (10 mM)
NaOH, which shows a single absorption maximum at 377 nm. The
black curve shown above is a representative example using 3T-GE, but
all molecules gave identical traces.
Table 2 FTIR spectroscopy of hydrogen-bonded assemblies
Notes and references
Peptide stretch (cmÀ1
)
3T-GE
3T-LE
3T-IE
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3T-VE
N–H
Amide I
3345
1650
3330
1622
3310
1619
3300
1619
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c
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