A bimolecular micelle constructed from amphiphilic pillar[5]arene molecules

Article abstract of DOI:10.1039/c3cc41186a

An amphiphilic macrocycle based on pillar[5]arene with polar lysine head groups spontaneously self-assembles into a bimolecular micelle in water. This self-assembled structure was characterized by small angle X-ray scattering (SAXS), field flow fluctuation coupled with multi-angle light scattering (FFF-MALS) and atomic force microscopy (AFM). The self-assembly of amphiphilic pillar[5]arene into dimeric spherical micelles represents a new molecular architecture for micelle formation. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc41186a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
A bimolecular micelle constructed from amphiphilic
pillar[5]arene molecules†
Tomoki Nishimura,a Yusuke Sanada,a Takuma Matsuo,a Tadashi Okobira,b
Efstratios Mylonas,c Naoto Yagic and Kazuo Sakurai*a
Cite this: Chem. Commun., 2013,
49, 3052
Received 13th February 2013,
Accepted 27th February 2013
DOI: 10.1039/c3cc41186a
www.rsc.org/chemcomm
An amphiphilic macrocycle based on pillar[5]arene with polar lysine blocks, the macrocycle units are particularly useful as the rigid
head groups spontaneously self-assembles into a bimolecular micelle in segments.4 For example, Hirsch et al. reported the formation of
water. This self-assembled structure was characterized by small angle a uniform and structurally precise micelle from seven dendritic
X-ray scattering (SAXS), field flow fluctuation coupled with multi-angle calixarene molecules in aqueous solution.3b–d Our group has
light scattering (FFF-MALS) and atomic force microscopy (AFM). The self- also reported the generation of monodisperse cubic-shaped
assembly of amphiphilic pillar[5]arene into dimeric spherical micelles micelles from cationic calixarene lipids.3e
represents a new molecular architecture for micelle formation.
Recently, a new class of macrocyclic compounds, i.e., pillar[5]-
arene, was synthesized by Ogoshi et al.5 Pillar[5]arene consists of five
Amphiphilic molecules dispersed in water undergo supramolecular methylene-linked hydroquinones with cylindrical D5-symmetry. The
self-assembly to produce a variety of micellar structures. These lack of difference between the upper and lower rims of pillar[5]arene
molecules have been attracting interest from the research community is a major dissimilarity between calix[5]arenes and pillar[5]arenes.
because of their theoretical and practical applications,1 which have These features suggest that pillar[5]arene should be a good candidate
extended from use in daily-use products such as soap and detergent for the synthesis of micelles. In this communication, we report the
to futuristic applications in nanomedicine.2 The micellar structures synthesis of pillar[5]arene-based amphiphile 1 (Fig. 1) with novel
entirely depend on their solution conditions such as concentration, features that facilitate aggregation behavior in water.
ionic strength, pH, and temperature. This is because the micellar
Compound 1 was prepared as outlined in Scheme S1 (ESI†).
structures are determined by a combination and balance of subtle Commercial 4-butoxyphenol was etherified using propargyl
intermolecular interactions, including hydrophobic, van der Waals, bromide, and cyclized in the presence of BF3ÁOEt2 to yield four
electrostatic, and p–p interactions, and a small perturbation in the constitutional isomers of pillar[5]arene.
balance may lead to a drastic change in their morphology. Micelles
are generally composed of 10–100 monomers and undergo rapid graphy followed by copper(I)-catalyzed azide–alkyne [3+2]
interconversion between the aggregate and monomer states.1a cycloaddition.6 The target compound 1 was then obtained by
The D5 isomer was isolated using silica gel column chromato-
In certain instances where the amphiphilic molecules comprise deprotecting the tert-butoxycarbonyl group. The final product 1
rigid skeletons and their chemical structures satisfy certain condi- was characterized by NMR and ESI mass spectroscopy and
tions, the resultant micelles assume a specific aggregation number
and thus a well-defined shape. Such structures are termed ‘‘shape-
persistent micelles’’.3 To generate such nanostructures, the corres-
ponding building blocks need to be elaborately designed because
the information determining their specific assembly should be
encoded in their molecular architecture. Among the many building
a Department of Chemistry and Biochemistry, University of Kitakyushu,
1-1 Hibikino, Wakamatsu, Kitakyushu, Fukuoka, Japan.
E-mail: sakurai@kitakyu-u.ac.jp; Tel: +81 93 695 3298
b Department of Chemical Science and Engineering, Ariake National College of
Technology, 150 Higashihagio, Omuta, Fukuoka, Japan
c Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Koto,
Sayo, Hyogo, Japan
Fig. 1 The chemical and energy-minimized structures of amphiphile 1 obtained
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc41186a
using MOPAC.
c
3052 Chem. Commun., 2013, 49, 3052--3054
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
Fig. 3 (a) Absolute SAXS intensity plotted against q for 1 (10 mM) in water
(50 mM NaCl, pH = 3.0). Inset: Guinier plot at c - 0 used to evaluate I(0)/c by
extrapolating q - 0. (b) Field flow fractionation (FFF) chromatogram of 1 in
water (50 mM NaCl, pH = 3.0). Open circle, 901 light-scattering signal; solid line,
molar mass; dashed line, the deconvolution of the light-scattering signal.
Fig. 2 (a) A typical AFM tapping mode image of 1 on a mica surface. (b) Height
profiles along the dashed lines indicated in each image.
shown to be in complete agreement with the structures pre-
sented herein (see ESI†).
Compound 1 was dissolved in water and the pH was adjusted to FFF, we reconstructed the SAXS profiles of 1 and found that the
3.0 where the polar head groups of amines are expected to be fully sharp intensity minima observed in Fig. 3a were absent from the
protonated.7 The formation of micelles of 1 in aqueous solution was resultant profile. It is thus proposed that the focusing process prior
examined in the presence of pyrene (Fig. S1, ESI†), and the critical to the fluctuation of the FFF may disturb the aggregate, thereby
micelle concentration (CMC) was determined to be 0.1 mM. Further generating artificial polydispersity.9
characterizations were performed at concentrations above its CMC.
Direct reconstruction of the scattering object from the
The micellar morphology of 1 was examined using AFM and experimental SAXS data was performed using the ab initio
transmission electron microscopy (TEM). AFM indicated a spherical shape determination program DAMMIN10 by employing the
structure with a uniform height in the 4.2–4.5 nm range (Fig. 2a scattering profile at infinite dilution in the q-range of up to
and b). The TEM image was consistent with that of AFM (Fig. S2, 5 nmÀ1. There was no difference in the final result obtained
ESI†). The dynamic light scattering (DLS) technique indicates that using the initial conditions of either no symmetry or C2
the micelles are monodisperse with a mean hydrodynamic diameter symmetry. Fig. 4a presents the final iteration model after
of 4.3 Æ 1.1 nm, which is in excellent agreement with AFM and TEM obtaining a reasonable agreement with the experimental data
observations (Fig. S3, ESI†).
(Fig. S5, ESI†). The resultant model had an ellipsoidal shape
The detailed structural features of the micelles were evaluated with dimensions 5.3 Â 4.1 nm and was hollow at the center.
using SAXS measurements. The scattering curve suggested the Fitting the data to spherical models did not provide as good a
presence of isolated scattering objects, as evidenced by the slope fit as that obtained with the ellipsoidal models. Because of the
of zero in the low q region (Fig. 3a). In addition, three minima were limitation of the computer power, we had to assume, for the
observed at q = 1.9, 4.0, and 7.5 nmÀ1, indicating that the aggregate purpose of calculation, that the scattering object comprises
has a well-defined shape with a narrow size distribution and high many beads with the same electron density. The presence of the
symmetry.3e,8 The Guinier plot in Fig. 3a was obtained by extra- hollow at the center implies that there is less electron density at
polating the scattering intensities to infinite dilution (Fig. S4, ESI†). the central region, presumably representing the alkyl tails.11
A weight-averaged molar mass (Mw) of 1 was determined to be 5.11
Because both the aromatic pillar[5]arene ring and the alkyl
(Æ0.10) Â 103 g molÀ1 from the intercept by taking the electron tails are hydrophobic moieties, by taking into account that the
density as 416 e nmÀ3 and the specific volume as 0.229 mL gÀ1 (see aggregation number is two, there are two possible ways for such
ESI†). Based on the molar mass (M) of 1 (2587.46 g molÀ1), the a dimer to assemble: one is the double-decker model in which
average aggregation number N = Mw/M was calculated to be the alkyl tails face each other to form the inner domain in the
2.0. However, there is some ambiguity in the determination of same manner as other normal micelles, and the other is the
the molar mass from SAXS because the calculated electron density binocular model in which the aromatic surfaces of the two
does not take into account the ion condensation effect that occurs pillararenes are stacked so that the pillararene axes align in
for charged molecules in water. Thus, the aggregation number of parallel. It is proposed that the presence of the low-electron-
the micelle was verified using the FFF-MALS technique.3e The FFF density region at the center is suggestive of the double-decker
chromatogram of 1 shows three distinguishable species separated model. In the case of the binocular model, the micellar core
by the cross-flow (Fig. 3b). The molar mass of the main peak should consist of the aromatic rings and thus the electron
(peak b) is B5.0 Â 103 g molÀ1, corresponding to the dimer. The density should be higher in the central region, which is in
first (peak a) and third species (peak c) have Mw of ca. 2.5 Â 103 and contrast to the observation using the dummy model. Following
1.0 Â 104 g molÀ1, which correspond to the monomer and energy minimization using MOPAC, a rigid body model was
tetramer, respectively. Following the deconvolution of these peaks, generated for the double-decker arrangement, and the SAXS
the area ratio of the three components was determined to be data were calculated based on the model (Fig. 4b–d).12 The
0.19 : 0.70 : 0.11, indicating that the dimer is the major species. calculated SAXS profiles beautifully reproduced the experi-
Based on the FFF data, it can be argued that the micelle is not mental data, indicating the validity of the double-decker model.
monodisperse. Thus, by assuming the composition suggested by In fact, on superimposing the dummy-bead on the rigid body
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3052--3054 3053

View Article Online
ChemComm
Communication
bimolecular micelle in which the alkyl tails face each other
and the hydrophobic portions are entirely covered by the long
hydrophilic groups.
This work is financially supported by the JST CREST
program; all SAXS measurements were carried out at SPring-8
BL40B2 (2012A1177).
Notes and references
1 (a) M. J. Rosen and J. T. Kunjappu, Surfactants and Interfacial
Phenomena, Wiley & Sons, Inc., 2012; (b) I. W. Hamley, Introduction
to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid Crystals,
Wiley & Sons, Inc., 2000.
2 (a) V. P. Torchilin, Pharm. Res., 2007, 24, 1–16; (b) M. P. Pileni, Nat.
Mater., 2003, 2, 145–150; (c) Z. Dong, W. Yongguo, Y. Yin and J. Liu,
Curr. Opin. Colloid Interface Sci., 2011, 16, 451–458.
3 (a) B. Trappmann, K. Ludwig, M. R. Radowski, A. Shukla, A. Mohr,
H. Rehage, C. Bottcher and R. Haag, J. Am. Chem. Soc., 2010, 132,
¨
11119–11124; (b) C. M. Jager, A. Hirsch, B. Schade, C. Bottcher and
T. Clark, Chem.–Eur. J., 2009, 15, 8586–8592; (c) C. M. Jager,
A. Hirsch, B. Schade, K. Ludwig, C. Bottcher and T. Clark, Langmuir,
2009, 26, 10460–10466; (d) M. Kellermann, W. Bauer, A. Hirsch,
B. Schade, K. Ludwig and C. Bottcher, Angew. Chem., Int. Ed., 2004,
43, 2959–2962; (e) S. Fujii, Y. Sanada, T. Nishimura, I. Akiba,
K. Sakurai, N. Yagi and E. Mylonas, Langmuir, 2012, 28, 3092–3101.
4 (a) O. Hayashida, K. Mizuki, K. Akagi, A. Matsuo, T. Kanamori,
T. Nakai, S. Sando and Y. Aoyama, J. Am. Chem. Soc., 2003, 125, 594–
601; (b) K. Helttunen and P. Shahgaldian, New J. Chem., 2010, 34,
Fig. 4 (a) A dummy atom model derived from the SAXS profile: left, side view;
right, cross section. (b) Rigid body model: left, side view; right, top view. (c) The
dummy atom model (transparent cyan surface representation) is superimposed
on the rigid body model. (d) Comparison of the calculated (red line) and
experimental data (black circle).
¨
2704–2714; (c) S. Hoger, Chem.–Eur. J., 2004, 10, 1320–1329;
(d) C. Larpent, A. Laplace and T. Zemb, Angew. Chem., Int. Ed.,
2004, 43, 3163–3167; (e) M. Lee, S. J. Lee and L. H. Jiang, J. Am. Chem.
Soc., 2004, 126, 12724–12725; ( f ) J.-H. Ryu, N.-K. Oh and M. Lee,
Chem. Commun., 2005, 1770–1772; (g) S. H. Seo, J. Y. Chang and
G. N. Tew, Angew. Chem., Int. Ed., 2006, 45, 7526–7530; (h) C. Suksai,
S. Figueiras Gomez, A. Chhabra, J. Liu, J. N. Skepper, T. Tuntulani
and S. Otto, Langmuir, 2006, 22, 5994–5997; (i) D. Wang, J. F. Hsu,
M. Bagui, V. Dusevich, Y. Wang, Y. Liu, A. J. Holder and Z. Peng,
Tetrahedron Lett., 2009, 50, 2147–2149; ( j) W. Zhang and J. S. Moore,
Angew. Chem., Int. Ed., 2006, 45, 4416–4439; (k) J. K. Kim, E. Lee,
M. C. Kim, E. Sim and M. Lee, J. Am. Chem. Soc., 2009, 131,
17768–17770.
5 (a) T. Ogoshi, S. Kanai, S. Fujinami, T. Yamagishi and Y. Nakamoto,
J. Am. Chem. Soc., 2008, 130, 5022–5023; (b) T. Ogoshi, J. Inclusion
Phenom. Macrocyclic Chem., 2012, 72, 247–262; (c) M. Xue, Y. Yang,
X. Chi, Z. Zhang and F. Huang, Acc. Chem. Res., 2012, 45, 1294–1308.
6 (a) T. Ogoshi, R. Shiga, M. Hashizume and T. Yamagishi, Chem.
Commun., 2011, 47, 6927–6929; (b) N. L. Strutt, R. S. Forgan,
J. M. Spruell, Y. Y. Botros and J. F. Stoddart, J. Am. Chem. Soc.,
2011, 133, 5668–5671; (c) T. Ogoshi, D. Yamafuji, D. Kotera, T. Aoki,
S. Fujinami and T. Yamagishi, J. Org. Chem., 2012, 77, 11146–11152;
(d) G. Yu, Z. Zhang, C. Han, M. Xue, Q. Zhou and F. Huang, Chem.
Commun., 2012, 48, 2958–2960.
7 (a) W.-Y. Yang, B. Breiner, S. V. Kovalenko, C. Ben, M. Singh,
S. N. LeGrand, Q.-X. A. Sang, G. F. Strouse, J. A. Copland and
I. V. Alabugin, J. Am. Chem. Soc., 2009, 131, 11458–11470;
(b) D. Costa, M. G. A. Miguel and B. R. Lindman, J. Phys. Chem. B,
2007, 111, 8444–8452.
8 O. Glatter and O. Kratky, Small Angle X-ray Scattering, Academic
Press, 1982.
9 T. Cho and V. Hackley, Anal. Bioanal. Chem., 2010, 398, 2003–2018.
10 (a) D. Svergun, J. Appl. Crystallogr., 1992, 25, 495–503; (b) D. I. Svergun,
Biophys. J., 1999, 76, 2879–2886.
model, we found that their structures agreed with each other.
In this model, the hydrophilic groups spread out and are
staggered to reduce the electrostatic repulsion and also com-
pletely cover the hydrophobic portions. A careful comparison of
the rigid body and dummy models shows that the latter (5.3 Â
4.1 nm) is slightly larger than the former (4.8 Â 3.1 nm). This
may be because of the fact that the hydrophilic surface is
surrounded by bound water and the electron density of the
surrounding species (352–408 e nmÀ3) is large compared to that
of the bulk water (344 e nmÀ3).13
According to the generally accepted definition,1a the aggregation
number of micelles lies in the range of 10–100, and the aggregation
number as well as the aggregate shape dynamically fluctuate. In
this respect, our finding is quite unique, and the present case
might be beyond the scope of this definition of micelles. However,
one other dimeric aggregate in aqueous solution is recognized for
cholic acid derivatives14 that assemble into a dimer in which the
hydrophobic steroid backbones are stacked with each other via
strong hydrophobic interactions. These derivatives tend to form
secondary and even larger aggregates in concentrated solutions. In
the present case, the dimer was quite stable in the high concen-
tration range of 0.5–10 mg mlÀ1, plausibly because the hydrophilic
groups cover the hydrophobic surface of the micelle, indicating that
the large size of the hydrophilic groups is essential for producing
such a dimer. Two derivatives of 1 (compounds 9 and 12) with a 11 K. Naruse, K. Eguchi, I. Akiba, K. Sakurai, H. Masunaga, H. Ogawa
and J. S. Fossey, J. Phys. Chem. B, 2009, 113, 10222–10229.
12 M. V. Petoukhov and D. I. Svergun, Biophys. J., 2005, 89, 1237–1250.
13 H. Durchschlag and P. Zipper, Eur. Biophys. J., 2003, 32, 487–502.
shorter hydrophilic group than 1 were synthesized, and it was
found that these compounds did not persist with the same
aggregation number and shape (Fig. S6 and S7, ESI†).
14 (a) N. Funasaki, M. Fukuba, T. Kitagawa, M. Nomura, S. Ishikawa,
S. Hirota and S. Neya, J. Phys. Chem. B, 2003, 108, 438–443;
(b) J. Gyimesi and L. Barcza, J. Inclusion Phenom. Macrocyclic Chem.,
1993, 15, 153–158; (c) K. Kano, S. Tatemoto and S. Hashimoto,
J. Phys. Chem., 1991, 95, 966–970.
In summary, we synthesized a novel pillararene amphiphile
1 and characterized its self-assembly behavior in water. SAXS
and MALS measurements revealed that 1 forms a stable
c
3054 Chem. Commun., 2013, 49, 3052--3054
This journal is The Royal Society of Chemistry 2013