Supramolecular helix of an amphiphilic pyrene derivative induced by chiral tryptophan through electrostatic interactions

Article abstract of DOI:10.1021/ol7030087

An amphophilic pyrene derivative (PyDNH₃) bearing positively charged ammonium cations has been synthesized and characterized. Self-assembly of PyDNH₃ in the presence of chiral tryptophan derivatives was investigated in ethanol/water by optical and chiroptical spectra, indicating the formation of helical aggregates. Scanning electron microscope (SEM) images showed the formation of ring-shape structures.

Full text of DOI:10.1021/ol7030087

ORGANIC
LETTERS
2008
Vol. 10, No. 4
645-648
Supramolecular Helix of an Amphiphilic
Pyrene Derivative Induced by Chiral
Tryptophan through Electrostatic
Interactions
Jinchong Xiao, Jialiang Xu, Shuang Cui, Huibiao Liu, Shu Wang, and Yuliang Li*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, People’s Republic of China
ylli@iccas.ac.cn
Received December 13, 2007
ABSTRACT
An amphiphilic pyrene derivative (PyDNH3) bearing positively charged ammonium cations has been synthesized and characterized. Self-
assembly of PyDNH3 in the presence of chiral tryptophan derivatives was investigated in ethanol/water by optical and chiroptical spectra,
indicating the formation of helical aggregates. Scanning electron microscope (SEM) images showed the formation of ring-shape structures.
Molecular recognition-driven self-assembly has been widely
studied for more than two decades.1 Especially, in recent
years some dye aggregates that could form helical assemblies
have led to the development of biomaterials, gels, nanotube
materials, chiroptical materials, and highly versatile liquid
crystalline materials.2 Electrostatic interaction, hydrogen
bonding, and π-π stacking interactions play critical roles
in successfully controlling the structures and shapes of the
aggregations such as columnar, ribbon, and cylindrical
architectures.3 Self-assembly of linear π-conjugated systems
has attracted much attention. For example, Ajayaghosh and
collaborators reported helicity induction from chiral to achiral
molecules in H-bonded supramolecular assemblies in recent
years.4
Pyrene and its derivatives are the subjects of tremendous
investigations because of their interesting optical and elec-
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10.1021/ol7030087 CCC: $40.75
© 2008 American Chemical Society
Published on Web 01/25/2008

trochemical properties. These materials have high quantum
yield, and can be modified by various substituents at the 1-,
6-, and 8-position of the pyrene core, which might tune their
absorption and emission spectra.5 The self-assembly of
pyrene and its derivatives into aggregates with controlled
nanostructures has attracted significant interest due to the
potential applications on optoelectronic devices, sensors, as
well as models for photosynthesis.6 Chiral acids are the
common and important structural units in many natural
products and drug molecules.7 In particular, chiral amino
acids are not only biologically important but also useful
substances for chiral auxiliaries and building blocks in
organic synthesis.8 Integration of these biologically important
small molecules and functional organic molecules provides
access to supramolecular nanoarchitectures that are capable
of converting biomolecular structural information to phys-
icochemical signals. For example, in the group of David K.
Smith, excellent low molecular weight gelators were devel-
oped that possess a toluene core extended with dendritic
amino acid derivatives.9 Yashima and co-workers also
reported hierarchical amplification of chirality information
from amino acid derivatives to the dynamic helical poly-
acetylene.10 To date, many studies on self-assembly of
biological molecules have been focused on biopolymers such
as proteins, oligopeptides, and nucleic acids.11 However,
biologically small molecules that graft into chromophores
through noncovalent interaction to create functional supra-
molecular systems also show great potential.
Herein, we describe self-assembly and thermochromic
supramolecular behavior of a positively charged pyrene
derivative (PyDNH3) in the presence of the negatively
charged tryptophan derivatives (TrpCO2) (Scheme 1). The
Scheme 1. Structure of Compound PyDNH3 and Chiral
Tryptophan TrpCO2
electrostatic repulsions between the positively charged
terminal units of compound PyDNH3 and the ammonium
ion of the amino acid may disturb the attractive interaction
because amino acids exist in the zwitterionic form. Therefore,
N-protected amino acids with Boc groups were used. For
L-TrpCO2 a negative Cotton effect was observed at the
longest wavelength, indicating an M-helical arrangement of
the chromophores and the D-TrpCO2 induced P-helicity
under acidic conditions.
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Amphiphilic PyDNH3 consists of an aromatic ethynyl-
pyrene moiety and ammonium group which are linked at
both ends of the aromatic core. The synthesis of PyDNH3
is given in the Supporting Information.12 TrpCO2 was
obtained according to known literature methods.13 The
resulting molecules were characterized by 1H NMR and 13
C
NMR and MALDI-TOF mass spectroscopies, which were
shown to be in full agreement with the structures presented.
The positively charged PyDNH3 exhibits good solubility
in polar solvents such as ethanol, acetonitrile, and DMSO.
PyDNH3 in ethanol (5 × 10-5 M) shows the monomeric
absorption peak with λmax at 394 and 413 nm (for the ethynyl-
pyrene core). It exhibits the emission maxima at 429 and
451 nm with a shoulder at 479 nm upon excitation at 390
nm (Figure S3, Supporting Information). The absorption of
PyDNH3 in ethanol/water (v/v, 8:92) shows similar maxima
at 394 and 413 nm. In comparison, the fluorescence was
obviously quenched in mixed solvents. The quantum yields
(Φf) of PyDNH3 in ethanol and water were measured as Φf
) 0.47 and 0.04, respectively, with quinine sulfate as
standard at 25 °C. These observations support intermolecular
chromophore interaction in water and the consequent exciton
coupling of the confined chromophores.
Aggregation was observed upon addition of chiral tryp-
tophan to PyDNH3 solution, which was strongly affected
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Org. Lett., Vol. 10, No. 4, 2008

by external conditions such as solvent, chiraltryptophan
concentration, and pH. We have investigated the concentra-
tion effect of L-TrpCO2 on the aggregation in ethanol/water.
In UV-vis titration experiments, the concentration of
PyDNH3 was kept constant upon addition of L-TrpCO2 at
different ratios. As shown in Figure 1A, the addition of
Figure 2. (A) CD spectra titration of L-TrpCO2 to PyDNH3 in
ethanol/water (v/v, 8:92): [PyDNH3] ) 5 × 10-5 M, [L-TrpCO2]
) (a) 0, (b) 2.5 × 10-4 M, (c) 5.0 × 10-4 M, (d) 1.0 × 10-3 M,
(e) 1.25 × 10-3 M, (f) 1.5 × 10-3 M; pH 2.5; Figure S4 (Supporting
Information) shows the titration curves of PyDNH3 ([θ]452) with
L-TrpCO2. (B) CD spectra of complex TrpCO2/PyDNH3 in
ethanol/water (v/v, 8:92): [PyDNH3] ) 5 × 10-5 M, [L- or
D-TrpCO2] ) 5.0 × 10-4 M; pH 2.5.
Figure 1. (A) UV-vis absorption and (B) fluorescence spectra
(excitation at 390 nm) titration of L-TrpCO2 to PyDNH3 in ethanol/
water (v/v, 8:92): [PyDNH3] ) 5 × 10-5 M, (a) 0, (b) 2.5 × 10-4
M, (c) 5.0 × 10-4 M, (d) 7.5 × 10-4 M, (e) 1.0 × 10-3 M, (f) 1.25
× 10-3 M, (g) 1.5 × 10-3 M; pH 2.5.
configurations of chiral amino acid derivatives. These
observations suggest not only that the aggregation of
PyDNH3 in the presence of chiral tryptophan derivatives is
strongly affected by amino acid concentration, but also that
the complex could form the left- or right-handed helical
arrangement based on the configurations of tryptophan
derivatives. Under the typical sergeant and soldiers condi-
tions, the complex (L-[TrpCO2] ) 1.0-5.0 × 10-5 M;
[PyDNH3] ) 5.0 × 10-5 M) did not exhibit any CD siganal
(data not shown). Titration of L-TrpCO2 (5.0 × 10-4 M)
with PyDNH3 (1.0-5.8 × 10-5 M) resulted in a nonlinear
increase of ICD (Figure S5, Supporting Information). These
observations support the fact that the complexes exhibited
their chirality in the aggregation while all of the positively
charged ammonium cations interacted with the negatively
charged ions.15
L-TrpCO2 (0-1.5 × 10-3 M) to PyDNH3 solution (5.0 ×
10-5 M) led to a decrease in the intensities of the initial
absorption bands at 394 and 413 nm with the concomitant
growth of an intense red-shifted band at 451 nm through an
isosbestic point at 426 nm. These results suggest that J-type
assemblies are formed in the solution, which are promoted
by the electrostatic and π-π stacking interactions among
the complex.14 The concentration dependence of the aggre-
gation was also reflected in the fluorescence and induced
CD (ICD) spectra. Figure 1B showed the fluorescence spectra
for a solution of the complex in ethanol/water at different
ratios. When the concentration of PyDNH3 was kept
constant, the fluorescence of the complex showed obvious
quenching with increasing concentration of L-TrpCO2. CD
spectroscopy was helpful for monitoring the definitive
interaction between PyDNH3 and amino acid deirvatives.
PyDNH3 itself was optically inactive, and no CD signal was
detected in ethanol/water (Figure 2A). However, upon the
addition of chiral tryptophan derivative, the exciton-coupled
CD signals with negative (482, 418, 390 nm) and positive
(455, 432, 367 nm) Cotton effects were observed in the
region of the π-π* transition of ethynyl-pyrene. The split-
type ICD intensity of the ethynyl-pyrene region increased
gradually with increasing concentration of chiral tryptophan
and reached a constant value at 30 equiv of L-TrpCO2 at
room temperature. Figure 2B shows the ICD and the
absorption spectra of PyDNH3 in the presence of L- and
D-tryptophan in ethanol/water (v/v, 8:92) at [PyDNH3]/
[TrpCO2] ) 1:10. The complex exhibited mirror images of
the split-type intense ICDs in the π-conjugated ethynyl-
pyrene region (300-550 nm), which showed a good rela-
tionship between the Cotton effect signals and the absolute
Figure 3A,B shows the pH dependence of absorption and
fluorescence spectra of aggregation. Increasing the pH of
Figure 3. (A) UV-vis absorption, (B) fluorescence spectra, and
(C) CD spectra of complex PyDNH3/L-TrpCO2 in ethanol/water
(v/v, 8:92): [PyDNH3] ) 5 × 10-5 M, [L-TrpCO2] ) 5.0 × 10-4
M. (D) Normalized FL spectra.
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Org. Lett., Vol. 10, No. 4, 2008
647

the solution from 2.5 to 6.3, the absorption maximum was
slightly blue-shifted from 394 to 389 nm. With further
addition of NaOH solution to the mixture (pH 9.2, 11.2),
the absorption maximum was at 358 nm, accompanied by a
slight peak broadening. The largest fluorescence emission
was also at a pH around 2.5. Taking the absorption and FL
spectra into consideration, we might propose that the complex
could precipitate at high pH. From the CD spectra as shown
in Figure 3C, the magnitude of the ICD disappeared in basic
solution. The experimental results illustrate the maximum
J-band absorption at pH around 2.5, which corresponds to
the isoelectric point of tryptophan.
important role in controlling the aggregation morphologies
of the complex.
A structural model for the spatial arrangement of the
ethynyl-pyrene unit in the complex is proposed in Figure 4.
To obtain further evidence of the electrostatic interactions
between PyDNH3 and chiral amino acid derivtives, CD
spectra were studied as a function of NaCl concentration in
solution. The magnitude of the ICD gradually decreased upon
successively adding NaCl as shown in Figure S6 (Supporting
Information), which corroborated the hypothesis on the
electrostatic nature of the interactions between PyDNH3 and
L-TrpCO2 in aqueous solution.10
Figure 4. Schematic representation of self-assembly of complex
PyDNH3/TrpCO2.
We studied the morphologies of the complex by SEM.
Samples were prepared by casting thin films of the self-
assembly complex from ethanol/water solution onto silica
slices. Figure S7A (Supporting Information) showed that a
complex of ethanol/water ([PyDNH3] ) 5 × 10-5 M,
[L-TrpCO2] ) 5 × 10-4 M) could form ring-shape aggrega-
tions with diameters of several microns. Figure S7B (Sup-
porting Information) displays different morphologies of ring
structures and Figure S7C (Supporting Information) shows
an isolated ring, whereas compound PyDNH3 itself from
ethanol or ethanol/water (v/v, 8:92) resulted in nanoparticles
similar to that previously described6f and did not gave any
ring structures (Figure S8, Supporting Information). The
formation of ring-shape structures might result from the
presence of a gas bubble in the solution. The bubble-induced
aggregation was the result of a complex process determined
by hydrodynamic and surface effects.16 Dynamic light
scattering (DLS) analysis provides direct information about
the size of the superstructure in solution. PyDNH3 (in
ethanol), PyDNH3 (in ethanol/water, v/v, 8:92), and PyDNH3/
L-TrpCO2 (in ethanol/water, v/v, 8:92; pH 2.5) showed
broad peaks corresponding to an average hydrodynamic
radius (Rh) of approximately 215, 350, and 980 nm,
respectively, as shown in Figure S9 (Supporting Information).
Combining the SEM and DLS results, one might probably
obtain that the chiral amino acid derivative played an
The UV-vis, FL, and CD spectra strongly suggest that the
configuration of chiral amino acid may hold the key to
determine the handedness of the resultant supramolecular
assemblies. In ethanol/water, PyDNH3 was first complexed
with chiral amino acid derivatives. Self-assembly of the
complex was then initiated by π-π stacking interaction of
ethynyl-pyrene units and hydrophilic and hydrophobic inter-
actions. Mutual rotation in the same direction may reduce
steric hindrance between amino acid moieties. Through
homochiral recognition of molecules, the complex could twist
into a helical arrangement during the self-assembly process.
In summary, we have synthesized an amphiphilic pyrene
molecule (PyDNH3) and demonstrated the self-assembly and
hierarchical helical formation of PyDNH3 in the presence
of chiral amino acid in aqueous solution. By employing the
noncovalent combinatorial approach, the biologically small
molecules might be the building blocks for the self-assembly.
The present study will provide new valuable information for
creating a nanoscale chiral architecture that could be
developed for designing chiral recognition and nanoscale
functional materials.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20531060,
20418001, 20473102) and the Major State Basic Research
development Program (2006CB806200, 2005CB623602).
Supporting Information Available: Synthetic and ex-
perimental details, spectral characterization, and additional
spectroscopy. This material is available free of charge via
the Internet at http://pubs.acs.org.
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