Tunable Nanosupramolecular Aggregates Mediated by Host–Guest Complexation

Article abstract of DOI:10.1002/anie.201605420

Nanosupramolecular assemblies with controlled topological features have inventive applications in fundamental studies and material manufacturing. Herein, a variety of morphologically interesting aggregates have been constructed using the supramolecular modulation with bipyridinium-modified diphenylalanine derivative (BP-FF). Benefiting from the high binding affinity of bipyridinium group with four different macrocyclic receptors, namely cucurbit[7]uril, cucurbit[8]uril, pillar[5]arene, and tetrasulfonated crown ether, we have succeeded in tuning the topological aggregates of BP-FF from fine nanofibers to nanorods, octahedron-like nanostructure, helical nanowires, and rectangular nanosheets without any tedious chemical modification. This supramolecular approach may provide us a powerful method to construct well-defined nanostructures with different morphologies that can be conveniently controlled by facile host–guest interactions.

Full text of DOI:10.1002/anie.201605420

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605420
German Edition: DOI: 10.1002/ange.201605420
Supramolecular Assembly
Tunable Nanosupramolecular Aggregates Mediated by Host–Guest
Complexation
Wen Zhang, Ying-Ming Zhang, Sheng-Hua Li, Yong-Liang Cui, Jie Yu, and Yu Liu*
[
5]
Abstract: Nanosupramolecular assemblies with controlled
topological features have inventive applications in fundamen-
tal studies and material manufacturing. Herein, a variety of
morphologically interesting aggregates have been constructed
using the supramolecular modulation with bipyridinium-
modified diphenylalanine derivative (BP-FF). Benefiting
from the high binding affinity of bipyridinium group with
four different macrocyclic receptors, namely cucurbit[7]uril,
cucurbit[8]uril, pillar[5]arene, and tetrasulfonated crown
ether, we have succeeded in tuning the topological aggregates
of BP-FF from fine nanofibers to nanorods, octahedron-like
nanostructure, helical nanowires, and rectangular nanosheets
without any tedious chemical modification. This supramolec-
ular approach may provide us a powerful method to construct
well-defined nanostructures with different morphologies that
can be conveniently controlled by facile host–guest interac-
tions.
state. Second, the structural regulation can be also achieved
by chemical modification of FF backbone. Many functional
groups, such as naphthalene, fluorene, and ferrocene, have
been covalently connected to FF core, thus exhibiting
improved physicochemical performance and novel functions
[
6]
as compared to parent FF. However, it should be noted that
although supramolecular approach is widely accepted as
a convenient and powerful method to regulate the physico-
chemical properties of individual subunits, FF-involved nano-
systems with host–guest recognition have not been reported
yet, which is probably due to the assumption that two isolated
phenyl rings and the relatively short peptide chain in natural
FF cannot make efficient noncovalent interactions with most
classic macrocyclic hosts in aqueous media.
Motivated by our ongoing interest concerning the molec-
ular binding behaviors of water-soluble macrocycles, herein
we present a complexation-controlled self-assembling process
of bipyridinium-modified FF (BP-FF) through host–guest
interactions in water. The introduction of bipyridinium salts
can not only enhance the water solubility of FF but also
provide abundant anchoring points to communicate with
different macrocyclic receptors, that is, cucurbit[7]uril (CB-
[7]), cucurbit[8]uril (CB[8]), water-soluble pillar[5]arene
(WP5A), and tetrasulfonated 1,5-dinaphtho-32-crown-8
(DNC; Scheme 1). Spectroscopic and microscopic investiga-
W
ell-defined nanostructures constructed from biologically
relevant molecules (for example, peptides, proteins, and
[
1]
DNA) have aroused wide attention in recent decades.
Among these frequently used bioactive building blocks,
peptides have been recognized as a promising candidate for
creating self-assembled nanostructures for biological appli-
cations, owing to their inherent biocompatibility, biodegrad-
[2]
ability, simple chemical modification, and easy availability.
Consequently, various natural and artificial peptides, includ-
ing amphiphilic, cyclic, dendritic, and aromatic peptides, have
been designed and developed for bottom-up nanofabrica-
[
3]
tion. In particular, as an extensively studied short and simple
peptide, diphenylalanine (l-Phe-l-Phe, FF) has been selected
as a hotspot for self-assembling research, because it could
spontaneously self-assembly into stable nanotubes in solu-
[
4]
tion. In general, there are two main different ways for
morphological regulation of FF-based nanostructures. First,
the self-assembling morphology of FF can be controlled by
changing external environments, for example, pH, temper-
ature, solvent, concentration, counterions, and interfacial
Scheme 1. Chemical structures and proton designation of host and
guest compounds.
[
*] W. Zhang, Dr. Y.-M. Zhang, Dr. S.-H. Li, Y.-L. Cui, J. Yu, Prof. Dr. Y. Liu
Department of Chemistry
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P.R. China)
E-mail: yuliu@nankai.edu.cn
tions jointly reveal that the macrocyclic encapsulation of
bipyridinium moiety in BP-FF can dramatically affect the
spatial orientation of chiral FF core, thus leading to a broad
range of morphological variation from nanofibers to nano-
rods, octahedron-like structure, helical nanowires, and rec-
tangular nanosheets. Moreover, benefiting from the photo-
induced isomerization of azobenzene, a photoresponsive
morphological transformation could be achieved in BP-
FFꢀCB[8] complex under UV light irradiation. This unex-
pected morphological transformation of FF derivatives by
Prof. Dr. Y. Liu
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin)
Nankai University, Tianjin 300071 (P.R. China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605420.
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host–guest interactions in the present research will serve us to
consistently understand the structure–activity relationship in
peptide-based molecular assemblies.
The synthetic route, compound characterization, and
proton assignments for guest molecule BP-FF are shown in
the Supporting Information (Scheme S1, Figures S1–S4).
Possessing two positively charged pyridinium sites and
a hydrophobic aromatic skeleton, BP-FF can be readily
encapsulated in the cavity of various macrocyclic receptors
through multiple host–guest interactions: 1) BP can form
stable inclusion complexes with CBs, in which the main
driving force was ascribable to ion-dipole interaction between
the positive charges of the bipyridinium salts and the portal
[7]
oxygen atoms of CBs; 2) WP5A was found to form 1:1
binary inclusion complex with BP mainly through electro-
[
8]
static interaction; and 3) more remarkably, tetrasulfonated
,5-dinaphtho-32-crown-8 (DNC), a new type of water-
1
soluble crown ether developed in our group, can form
a highly stable charge-transfer complex with BP through
[
9]
favorable electrostatic and p-stacking interactions. In our
case, after validating the 1:1 complex stoichiometry by Job
plots and mass spectrometry (Supporting Information, Figur-
es S5–S12), the binding constants (K ) in the complexation of
S
BP-FF with CB[7], CB[8], WP5A, and DNC were deter-
4
4
4
6
À1
mined as 5.33 ꢀ 10 , 8.08 ꢀ 10 , 8.20 ꢀ 10 , and 7.94 ꢀ 10 m ,
respectively (Supporting Information, Figures S13–S16).
The binding modes between BP-FF and four macrocycles
1
were further investigated by H NMR and UV/Vis spectros-
copy experiments. In BP-FFꢀCB[7] complex, the protons
(
H ) of the pyridinium moiety in BP-FF exhibited a large
b,c
upfield shift in the presence of CB[7], while those of the
phenyl rings were essentially unchanged (Figure 1b,c). There-
fore, we can speculate that pyridinium moiety was tightly
bound to CB[7], leaving the diphenylalanine moiety outside
the cavity. In BP-FFꢀCB[8] complex, both pyridinium (H
a,b,c
in Figure 1d) and partial phenyl protons shifted to higher field
upon addition of equivalent amount of CB[8], indicating that
the pyridinium moiety and phenyl ring of diphenylalanine
were concurrently included in the cavity of CB[8]. Further-
1
1
more, the H– H COSY spectrum showed that the pyridinium
next to N-terminus (H ) underwent a pronounced upfield
a,b
1
Figure 1. H NMR spectra (400 MHz, D O) of a) free CB[7], b) BP-
2
shifts by CB[8] (Supporting Information, Figure S17). Con-
FFꢀCB[7] complex, c) free BP-FF, d) BP-FFꢀCB[8] complex, e) free
WP5A, f) BP-FFꢀWP5A complex, g) free BP-FF, h) BP-FFꢀDNC com-
plex, and i) free DNC at 258C ([BP-FF]=[CB[7]]=[CB-
[8]]=[WP5A]=[DNC]=2.0 mm).
[10]
sidering the peptide sequence recognition with CBs, the
benzene ring next to N-terminus should be priority to be
included in the cavity of CB[8] because of the unfavorable
repulsion between carboxyl group at C-terminus and the
portal oxygen atoms of CBs. The coexistence of phenyl and
bipyridinium moieties in the cavity of CB[8] was further
indicated by the broad absorption at long-wavelength region
in UV/Vis spectra, indicative of the formation of charge-
transfer interaction in BP-FFꢀCB[8] complex (Supporting
Information, Figure S18, red line).
between WP5A and BP-FF in water. Meanwhile, considering
the numerous alkoxybenzene moieties in WP5A as strong
electron-rich groups, the broad absorption band centered at
450 nm could be contributed to the charge-transfer interac-
tion with the electron-deficient BP-FF in water (Supporting
Information, Figure S18, green line). Finally, as can be seen
from Figure 1h, the chemical shifts of aromatic protons in
DNC and all protons in bipyridinium moiety of BP-FF
exhibited an upfield shift upon complexation with each other,
which are mainly attributed to mutual shielding effect
between naphthalene and pyridinium rings. Similarly, a new
As shown in Figure 1 f, when an equimolar amount of
WP5A was added, the proton peaks of the pyridinium ring of
BP-FF underwent an upfield shift. A ROESY spectrum of
BP-FFꢀWP5A complex also showed the obvious correlation
peaks between pyridinium protons and the aromatic protons
of WP5A (Supporting Information, Figure S19). These phe-
nomena jointly demonstrate the host–guest association
2
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absorption band centered at 460 nm was observed upon
assembled nanostructures were obtained by mixing BP-FF
mixing DNC and BP-FF in aqueous solution (Supporting
Information, Figure S18, purple line), here again indicating
that bipyridinium part was encapsulated between two naph-
thyl rings of DNC through p-stacking interaction (Supporting
with equimolar macrocyclic hosts in aqueous solution. In the
case of BP-FFꢀCB[7] complex, a number of nanorods were
observed with the length and width of about 500 and 200 nm,
respectively (Figure 2c,d). Meanwhile, DLS data showed that
the assembly of BP-FFꢀCB[7] complex possessed an average
hydrodynamic diameter of 672 nm in solution (Supporting
Information, Figure S21). Moreover, the octahedron-like
structures at the micrometer scale were observed for the
BP-FFꢀCB[8] complex, which is rarely reported in the FF-
related nanosystems (Figure 2e,f). TEM images of self-
assembled BP-FFꢀWP5A complex showed right-handed
helical nanowires with the diameters around 600 nm (Fig-
ure 2h). More notably, left-handed helical nanowires were
exclusively found in SEM images (Figure 2g). This opposite
helicity in BP-FFꢀWP5A complex is probably attributed to
the different binding mode and affinity of extensive carboxyl
groups in WP5A with the sample matrix (that is, carbon film
in TEM and silicon wafer in SEM experiments, respectively).
Furthermore, rectangular nanosheets were found in the BP-
FFꢀDNC complex, with the length ranging from 200 to
700 nm (Figure 2i,j). These microscopic investigation results
demonstrate that the addition of macrocyclic compounds can
dramatically influence the spatial alignment of FF and then
induce a broad range of morphological variation from
nanofibers to nanorods, octahedron-like nanostructure, hel-
ical nanowires, and rectangular nanosheets. Interestingly, the
addition of CB[8] or DNC as competitive host to the BP-
FFꢀCB[7] and BP-FFꢀWP5A assemblies could also trigger
the supramolecular structural changes (Supporting Informa-
tion, Figures S22). These microscopic investigation results
1
Information, Figure S20). Overall, H NMR and UV/Vis
spectra jointly demonstrate that in our case, despite the
different molecular shape, cavity size, and charge number,
four macrocycles can exclusively entrap the bipyridinium
moiety of BP-FF through multiple noncovalent forces,
namely, the ion-dipole interconnection with CB[7] and CB[8],
the charge-transfer interaction in CB[8], WP5A, and DNC,
and the electrostatic attraction with WP5A and DNC. Thus, it
is believed that the highly affiliative host–guest complexation
may have a great impact on the topological morphology of FF
self-aggregates, and this hypothesis would be validated by the
microscopic investigation, as described below.
Transmission electron microscopy (TEM) and scanning
electron microscopy (SEM) were used to investigate the
influence of hosts on the self-assembled structures of BP-FF.
As shown in Figure 2a,b, fine nanofibers with 20 nm diameter
were formed by BP-FF alone. Interestingly, diverse self-
further corroborated the K -dependent molecular assembling
S
behaviors.
Circular dichroism (CD) and Fourier transform IR
(FTIR) spectroscopy were used to investigate the spatial
orientation of BP-FF and its host–guest complexes (Support-
ing Information, Figures S23 and S24). The CD signature of
BP-FF nanofiber was characterized by a strong positive band
at 198 nm (p–p* transition), corresponding to the assumed b-
[
11]
turns of FF in solution. Despite the signal intensity was
a little weakened as compared to the parent BP-FF, similar
CD spectral features were observed for the BP-FFꢀCB[7]
and BP-FFꢀWP5A complexes, implying that the introduction
of exogenous CB[7] and WP5A could not affect the main
assembling mode of FF moiety. This speculation was further
supported by FTIR spectroscopy, that is, two characteristic
À1
peaks were clearly observed at 1640 and 1680 cm in both
free BP-FF and BP-FFꢀCB[7] assembly, which could be
[
11]
assigned to the b-turn conformation of FF backbone. In the
BP-FFꢀDNC complex, besides the peak assignment on b-
turn conformation at 195 nm, the negative band at 233 nm
and positive band at 249 nm were contributed to the induced
CD signals from DNC’s naphthyl rings. Meanwhile, the peaks
À1
at 1639 and 1683 cm in FTIR spectrum further verified the
b-turn conformation, which was consistent with the CD
results.
In contrast, an obvious spectral change was observed in
the CD spectrum of BP-FFꢀCB[8] complex, in which the
negative peaks at 193 nm could be assigned to the formation
Figure 2. a),c),e),g),i) SEM and b),d),f),h),j) TEM images of the self-
assembly of a),b) free BP-FF, c),d) BP-FFꢀCB[7] complex, e),f) BP-
FFꢀCB[8] complex, g),h) BP-FFꢀWP5A complex, and i),j) BP-FFꢀDNC
complex ([BP-FF]=[CB[7]]=[CB[8]]=[WP5A]=[DNC]=0.02 mm,
pH 6.0, 258C).
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[
12]
of b-sheet structure in the molecular assembling process.
ure S28). Along with the spectroscopic investigation in
aqueous solution, there was an obvious morphological
change in the solid state; that is, well-defined rhombic
dodecahedrons were observed upon complexation of CB[8]
with BP-FFand trans-Azoim (Figure 3), while it was gradually
transformed to nanofibers under sufficient UV light irradi-
ation. Comparatively, in the control experiment, AzoimꢀCB-
[8] complex only formed nanoparticles upon irradiation with
UV light (Supporting Information, Figure S29).
1
Combining the aforementioned H NMR spectroscopic
results, we can infer that the unique CD spectroscopic
behaviors were mainly attributed to the coexistence of
bipyridinium and phenyl rings in the cavity of CB[8]. Overall,
these spectroscopic results jointly demonstrate that FF
residue can predominantly maintain its b-turn conformation
in the BP-FFꢀCB[7], BP-FFꢀWP5A, and BP-FFꢀDNC
complexes, while the charge-transfer interaction between
bipyridinium and phenyl moieties upon complexation with
CB[8] can profoundly change the assembling mode of BP-FF,
thus leading to a b-sheet conformation in solution and an
octahedron-like structures in the solid state.
Furthermore, the formation of a charge-transfer complex
between bipyridinium and phenyl moieties in the BP-
FFꢀCB[8] complex inspired us to hypothesize that a rever-
sible binding process may be achieved by introducing an
appropriate guest. Therefore, azophenyl imidazolium salt
(
Azoim) was chosen as the competitive guest to expel the
phenyl group of FF from the cavity of CB[8], taking the good
electron-donating properties and photoinduced isomerization
ability of azobenzene units into account. H NMR experi-
Figure 3. a) SEM and b) TEM images of the rhombohedral dodecahe-
drons formed by Azoim·BP-FFꢀCB[8] complex ([Azoim]=[BP-
FF]=[CB[8]]=0.02 mm, pH 6.0, 258C).
1
ments were carried out to investigate the molecular binding
behaviors in the ternary Azoim·BP-FFꢀCB[8] complex
(
Supporting Information, Figures S25 and S26). As shown in
Figure S25, the phenyl protons at 6.25 ppm in FF exhibited no
obvious change upon addition of Azoim into BP-FFꢀCB[8]
complex, suggesting that the phenyl ring could not be
expelled by Azoim. However, considering that both of the
In conclusion, we put forward a supramolecular modu-
lation method to efficiently control the assembling morphol-
ogy of FF backbone simply through host-guest interactions.
Superior to the previous FF-based nanostructures, the com-
plexation of BP-FF with different water-soluble macrocyclic
receptors (namely, CB[7], CB[8], WP5A, and DNC) could
conveniently fabricate diverse morphologically interesting
aggregates, including nanorods, octahedron-like nanostruc-
ture, helical nanowires, and rectangular nanosheets, without
necessitating any chemical modification. The noncovalent
association of bioactive molecules with artificial macrocycles
in the present work further stress the distinct advantage of
supramolecular cooperativity in the design and engineering of
biomimetic and multistimuli-responsive hybrid molecular
assemblies, and thus we expect that the obtained supramolec-
ular aggregates can provide us with a facile modular strategy
for the creation of more advanced biocompatible materials
with new functionality.
pyridinium (H ) and Azoim protons gave a sizable upfield
c,d
shift, we can infer that part of azophenyl group in Azoim was
readily accommodated by CB[8] to form a ternary complex,
and the imidazolium moiety was located outside the cavity to
avoid unfavorable electrostatic repulsion with bipyridinium
group. Meanwhile, the negative bands at 199 and 210 nm in
[12]
CD spectra indicated a b-sheet structure in solution, which
was similar to the assembling mode in BP-FFꢀCB[8] com-
plex. Moreover, the enhanced CD signal intensity of Azoim at
about 340 nm further corroborated the partial inclusion of
Azoim in the BP-FFꢀCB[8] complex (Supporting Informa-
tion, Figure S27, blue line), as illustrated in Scheme 2.
Interestingly, when the Azoim·BP-FFꢀCB[8] complex
was exposed to UV irradiation, only BP-FF could be kept in
solution, accompanied by the precipitation of AzoimꢀCB[8]
complex from the solution (Supporting Information, Fig-
Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 21432004, 21472100, and 91527301) for financial
support.
Keywords: diphenylalanine · host–guest complexes ·
macrocyclic receptors · molecular recognition ·
supramolecular assembly
Scheme 2. Photoregulation process of the BP-FFꢀCB[8] complex in the
presence of Azoim.
4
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Received: June 3, 2016
Published online: && &&, &&&&
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Communications
Supramolecular Assembly
Be my guest: A variety of morphologically
interesting aggregates have been con-
structed. The method uses supramolec-
ular modulation of a bipyridinium-modi-
fied diphenylalanine guest with four dif-
ferent macrocyclic hosts. Azoim=azo-
phenyl imidazolium salt.
W. Zhang, Y.-M. Zhang, S.-H. Li, Y.-L. Cui,
J. Yu, Y. Liu*
&&&& — &&&&
Tunable Nanosupramolecular Aggregates
Mediated by Host–Guest Complexation
6
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