Morphological transformation between nanofibers and vesicles in a controllable bipyridine-tripeptide self-assembly

Article abstract of DOI:10.1002/anie.201006897

(Chemical Equation Presented) Tuning structures: Stimulus-responsive peptide self-assembly requires a balance of conformational change and structural continuity of stable β sheets. In an amphiphilic bipyridine-tripeptide model, temperature, and ultrasound

Full text of DOI:10.1002/anie.201006897

DOI: 10.1002/anie.201006897
Peptide Self-Assembly
Morphological Transformation between Nanofibers and Vesicles in a
Controllable Bipyridine–Tripeptide Self-Assembly**
Damei Ke, Chuanlang Zhan,* Alexander D. Q. Li, and Jiannian Yao*
One of the main challenges in self-assembly is to emulate
biological systems and to design building blocks that have
stimulus-responsive properties for programmed self-organ-
ization. In the past few decades, various building blocks
arranged from b-sheet-forming peptides have been developed
for synthetic well-defined architectures having potential
applications in biomedicine, biomaterials, and optical and
electronic materials.[1–3] Peptides offer distinct features
including biocompatibility, highly ordered hydrogen bonds,
ease of decoration with functional elements, and high stability
of the b sheets.[4] In general, stimulus-responsive self-assem-
blies at the molecular level require a conformational change
that leads to different secondary interactions and further
molecular packing.[5] Considering the high stability of
b sheets,[4] we aim to balance their conformational change
and stability in stimulus-responsive self-assembly based on
building blocks containing b-sheet-forming peptides (here-
Scheme 1. a) Chemical structures of 1–4. b) Hydrogen bonds and
b-sheet-like structures formed from 1. c,d) Proposed models for
after termed peptide building blocks). Accordingly, such a
self-assembly is controlled at the b-sheet level[6] and morpho-
logical transformation is carried out on the basis of a
“structural continuity of the stable b sheets”. For example,
transitions between plate nanoribbons and nanotubes rely on
the capability of the twisting and unwinding of the plate
nanoribbons.[6a] Transition between nanotubes and vesicles
interrelates to the layered b sheets and the inner hollow
nature.[6b] In addition, photoresponsive transition from quad-
ruple helix to individual fibers is based on the fibril
structures.[6c]
As a proof of principle, we designed an amphiphilic
tripeptide–bipyridine conjugate 1 (Scheme 1). Two b-sheet-
forming Lys-Phe-Ala peptides[7] are covalently linked at the
5,5’-positions of a hydrophilic bipyridine unit.[8] The sequence
is designed as bipyridine–linker–Ala-Phe-Lys-Boc (Boc =
tert-butoxycarbonyl) to produce possible U-shaped b-sheet-
like structures in which the N-Cbz lysine (Cbz = benzyloxy-
molecular packing and conformations of 1 inside vesicles (U-shaped I)
and nanofibers (U-shaped II).
carbonyl) side chains act as the hydrophobic U-shaped tails.
U-shaped molecules have recently been reported to form
vesicles.[9] The unprecedented cooperation between these
functions leads to a controllable self-assembly forming
vesicles and nanofibers. Surprisingly, external stimuli of
temperature and ultrasound can further switch a reversible
morphological transformation between vesicles and nano-
fibers.
From the structural point of view, nanofibers and vesicles
are distinctly different. Nanofibers are 1D nanostructures in
which molecules tightly pack along the direction of 1D
intermolecular interactions forming end-opened self-assem-
blies, whereas vesicles form a bilayer structure in which
amphiphilic molecules have a hollow and spherical morphol-
ogy, generally much larger than that of nanofibers. To the best
of our knowledge, there are no reports regarding the
morphological transition between these two structures
based on regulating self-assembly by using peptide building
blocks.
Compound 1 was successfully synthesized by coupling a
tripeptide sequence of Boc-{(N-Cbz)-Lys}-Phe-Ala-COOH
with 5,5’-dibromomethyl-2,2’-bipyridine in a yield of 70–80%.
Analogues 2–4 were synthesized in a similar way. All
conjugates were fully characterized by 1H and 13C NMR
spectroscopy and mass spectrometry.
Although insoluble in water, 1 is soluble in tetrahydro-
furan (THF). Addition of water to a THF solution of 1, for
example, up to a concentration of 2.0 mgmLꢀ1 in 1:1 THF/
water (v/v) led to the formation of vesicles (Figure 1a). The
[*] D. M. Ke, Dr. C. L. Zhan, Prof. Dr. J. N. Yao
Beijing National Laboratory for Molecular Sciences
Laboratory of Photochemistry, Institute of Chemistry, CAS
Beijing 100190 (P.R. China)
Fax: (+86)10-8261-6517
E-mail: clzhan@iccas.ac.cn
jnyao@iccas.ac.cn
Prof. Dr. A. D. Q. Li
Department of Chemistry, Washington State University (USA)
[**] This work was financially supported by NSFC (Nos. 20973182,
20872145, and 20733006), the Chinese Academy of Sciences, 973
Projects (2006CB806200 and 2007CB936401), and NSF (CHE-
0805547).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006897.
Angew. Chem. Int. Ed. 2011, 50, 3715 –3719
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3715

Communications
Figure 1. SEM and TEM images of a) vesicles and b,c) nanofibers
obtained after ultrasound treatment of the transparent vesicle solution
for 10 min ([1]=2.0 mgmLꢀ1). Scale bars: 1 mm.
Figure 2. a) SEM observations of the conversion process from vesicles
into nanofibers. A homogeneous solution of the vesicles was subjected to
ultrasound for various periods. b) SEM observations of the conversion
process from nanofibers into vesicles. The opaque gel was incubated at
608C in a water bath for various periods ([1]=2.0 mgmLꢀ1). Scale bars:
1 mm.
hollow feature of the vesicles was clearly evidenced by
transmission electron microscopy (TEM; inset of Figure 1a).
The wall thickness was estimated to be about 5 nm, consistent
with a loose bilayer of the b-sheet-like structures in which 1
was proposed to adopt a conformation called U-shaped I
(Scheme 1c and d). In contrast, 2 and 3 without the lysine
residue did not form vesicles under similar conditions. In fact,
2 formed rigid 1D nanoplates and 3 self-assembled into
leaflike structures (Figures S1 and S2 in the Supporting
Information). Moreover, the observation of vesicles from 4
pointed towards the vital role of the lysine residue in the
formation of vesicles in our conjugates (Figure S3 in the
Supporting Information).
nanofiber networks and “dissolve” into the solution. This
“dissolving” process may further facilitate the transition from
nanofibers to vesicles.
What governs such a reversible transformation between
nanofibers and vesicles? What are the differences in the
conformations of the molecules inside the nanostructures,
which may be sensitive to heat and ultrasound treatment and
finally lead to the morphological change? To gain insight into
the structural information inside the nanofibers and vesicles, a
series of spectroscopic experiments was performed.
The X-ray diffraction (XRD) data (Figure 3a) clearly
reveal the formation of b-sheet-like structures. For the
nanofibers, a very intense signal at 2q = 19.08 gives a d spacing
of 4.7 ꢀ, which is related to the spacing between peptide
backbones.[10] In addition, several reflections were detected in
the 2q range of 5–128, which originate from the ordered
stacking periodicity of the b sheets (Scheme 1c). The d value
of 16.2 ꢀ corresponds to the intersheet spacing between the
Upon ultrasound treatment (0.40 Wcmꢀ2 and 40 kHz) of
this vesicle solution for 10–30 min, an opaque gel formed.
Scanning electron microscopy (SEM) and TEM images of the
xerogels (Figure 1b and c) revealed formation of micrometer-
long fibril structures with sizes of 50–100 nm in width, which
suggests an ultrasound-induced morphological transition
from vesicles to nanofibers. No vesicles were observed,
which indicates complete conversion of the vesicles to
nanofibers. Interestingly, after the opaque gel was incubated
at 608C in a water bath for about an hour, the nanofibers were
fully transformed into vesicles and thus the gel was com-
pletely converted into the original transparent solution. This
morphological transition was reversible and repeatable with-
out any obvious fatigue effects, thus switching the macro-
scopic properties between transparent solution and opaque
gel. It should be pointed out that compounds 2–4 exhibited no
morphological changes upon heating and ultrasound treat-
ment.
Ultrasound treatment led to collapse of the vesicles and
subsequent formation of short nanoribbons, which further
twisted under direction of chirality of the peptide segments
and elongated into short nanofibers (Figure 2a, 0–5 min).
With the increase of treatment time, the short nanofibers
became longer, and this finally led to the formation of
nanofiber networks and consequential gelation (Figure 2a, 7–
10 min). Conversely, the reverse transition from nanofibers to
vesicles most likely resulted from a process of heat-induced
blooming of disklike microplates fused from the nanofiber
networks (Figure 2a, 10 min), accompanied by blooming of
vesicles from the open end of nanofibers (Figure 2a, 15–
50 min). The mother microplates split into daughter vesicles
with different sizes (Figure 2a, 50 min).[1c] Because the nano-
fibers are insoluble, while the vesicles are soluble, the newly
formed vesicles and microplates can disengage from the
Figure 3. a) XRD patterns, b) solid-state CP-MAS 13C NMR spectra,
c) FTIR spectra, and d) CD spectra of the as-synthesized nanofibers (black
lines) and vesicles (red lines; [1]=2.0 mgmLꢀ1). The assigning of the CP-
MAS 13C NMR spectra is provided in detail in the Supporting Information
(Figures S13 and S14).
3716
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3715 –3719

lysine side chains, and values of 10.2, 8.8, and 7.7 ꢀ are
assigned to the intersheet periodicity between the phenyl-
alanine side-chain packing at localized positions. Such an
ordered stacking finally led to the formation of tight bilayers
of the b-sheet-like structures in which 1 was considered to
take a U-shaped II conformation. With regard to the vesicles,
only one broad peak at 2q = 18.78 with a d spacing of 4.8 ꢀ
was detected, which had been assigned to the spacing between
peptide backbones.[10]
nanofibers most of them select similar conformations. This
conformational change resulting from ultrasound treatment is
deeply related to the flexible nature of the linkers, as
observed in ultrasound-induced gelation.[14] The flexibility of
the linkers is further supported by FTIR spectra in which the
C O stretching band occurs at 1742 cmꢀ1 (Figure 3c), which is
=
attributed to unbound ester.[12,13a]
The ultrasound-induced conformational changes of the
linkers may accompany different orientations of the bipyr-
idine group in both nanostructures, as confirmed by circular
dichroism (CD) experiments. Figure 3d shows that the CD
signals of the nanofibers are totally different from those of the
vesicles. Typical CD spectra of the nanofibers display two
broad negative bands in the aromatic range. The broad
negative band around 250 nm is attributed to a combination
of the n!p* transitions of bipyridine and p!p* transitions
of benzene groups.[15] The relatively strong CD signals around
295 (shoulder) and 306 nm originate from the characteristic
bipyridine p!p* transitions.[15a,b] In the vesicles, the bipyr-
idine p!p* transitions show a positive and negative Cotton
effect at 278 and 307 nm, respectively, with a zero crossover at
291 nm. This reveals a quite different orientation of the
bipyridine groups in the vesicles and nanofibers. Similarly, the
benzene functions in the range of 250–285 nm show a positive
Cotton effect in the vesicles, while in the nanofibers a
negative Cotton effect arises at this band,[15c,d] further
supporting the different packing of the peptide side chains
in both nanostructures. The different packing of the side
chains is consistent with the different positions of the aliphatic
carbon atoms of the side chains occurring in the range of 45–
10 ppm, for example, the b carbon atoms of the alanine (16.1
versus 18.2 ppm) and g carbon atoms of the lysine residues
(23.7 versus 25.9 ppm; Figures S13 and S14 in the Supporting
Information).
The differences in diffraction peak numbers and relative
intensities between these two sets of XRD data imply that the
side chains of amino acid residues in fibril structures are in a
more orderly and compact arrangement than those in the
vesicles (Scheme 1c). The tight and loose bilayers are further
supported by small-angle X-ray diffraction (SAXRD) data
(Figure S5 in the Supporting Information): The peak with d =
49.0 ꢀ is consistent with the loose bilayers of the b-sheet-like
structures in the vesicles and the peak at 2q = 3.18 is assigned
to the orderly bilayered packing of the b-sheet-like structures
in the nanofibers (Figure S6 in the Supporting Information).
Solid-state cross-polarization magic angle spinning (CP-
MAS) 13C NMR spectra confirm essentially the same patterns
for the dipeptide segment of the Boc-{(N-Cbz)-Lys}-Phe
sequence, which is bound by the backboneꢁs hydrogen bonds
inside both the nanofibers and vesicles (Figure 3b). The two
amide-carbonyl peaks overlapped at approximately 172 ppm,
which indicates that the dipeptide backbones adopt b-sheet-
like rather than helixlike structures for which the resonances
were expected at about 176 ppm.[11] The magnetic resonances
of a carbon atoms of the lysine and phenylalanine residues
occur respectively at 55.5 and 54.5 ppm for vesicles, and 55.8
and 54.4 ppm for nanofibers. Cbz and carbamate carbonyl
peaks were observed at about 157.3 and 156.5 ppm for
vesicles, and 157.7 and 156.7 ppm for nanofibers (Figures S13
and S14 in the Supporting Information), respectively.
Considering the large difference in the CD signals
between both structures, variable-temperature CD spectros-
copy was conducted to gain deeper insight into the conforma-
tional variety of the aromatics during the reversible morpho-
logical transition. Figure 4a and b show the recorded CD
spectra during heating and cooling, respectively. Initial
heating from 25 to 458C resulted in a slight increase in CD
intensity, possibly as a result of the heat-induced dissolution
of the nanofibers. When the temperature was above 558C, an
obvious decrease of [q]307 occurred. The sample was then kept
at 608C for 40 min until no further changes in the CD spectra
were observed. At this point, a spectral trajectory totally
different from that of the nanofibers but similar to that of the
vesicles appeared, which indicated full transition from nano-
fibers to vesicles. With the decrease of temperature from 60 to
258C, the [q]307 value initially remained nearly constant and
then increased when the temperature was below 358C,
because of ordered packing of the molecules at lower
temperature.
These results suggest that the dipeptide segment adopts
similar conformations inside both nanostructures. The nature
of hydrogen bonding of the dipeptide segment is further
confirmed by FTIR spectrometry (Figure 3c). The NH bands
centered at 3309 cmꢀ1 for nanofibers and 3306 cmꢀ1 for
vesicles are typical of NH functions involved in hydrogen
bonding.[12] The amide I band at 1647 cmꢀ1 for nanofibers but
1649 cmꢀ1 for vesicles is also evidence for the b-sheet-like
ꢀ1
conformations.[13] The C O stretching band at 1695 cm is
=
assigned to hydrogen-bonded carbamate and Cbz func-
tions.[12] Thus, the overall set of data suggests 1) the formation
of b-sheet-like structures in both nanostructures via intermo-
lecular hydrogen-bonding networks of the Boc-{(N-Cbz)}-
Lys-Phe segment (Scheme 1b) and 2) the stability of the
b-sheet-like structures upon heat and ultrasound treatment.
Surprisingly, the CP-MAS 13C NMR spectra show that the
a carbon atoms of the alanine residues in both nanostructures
are split into two bands at approximately 48.0 and 50.6 ppm,
which arise from b-sheet and non-b-sheet forms, respec-
tively.[11] Integrals indicated that only around 50% of the
alanine residues in the vesicles and up to about 90% in the
nanofibers were engaged in the b-sheet-like structures. This
splitting reveals that up to 50% of the alanine residues inside
the vesicles adopt different conformations, whereas in the
As inferred from the spectral data, the b-sheet-like
structures formed from the terminal dipeptide segments are
stable enough upon external stimulus. However, the flexible
central linkers are free of hydrogen bonding, and thus possess
high mobility and capability of conformational changes when
subjected to heat and ultrasound. The intermolecular hydro-
Angew. Chem. Int. Ed. 2011, 50, 3715 –3719
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3717

Communications
In summary, we have reported a smooth and reversible
transition between nanofibers and vesicles from a control-
lable self-assembly of an amphiphilic tripeptide–bipyridine
conjugate. Such a morphological reversibility originates from
a balance between the conformational change of the flexible
central linkers and the stable b-sheet-like structures formed
between the terminal peptide segments together with the
mobile bipyridine units. Therefore, we have provided an
elegant case to demonstrate how a small conformational
change imparts morphological reversibility based on struc-
tural continuity of stable b sheets. This not only adds value in
the understanding of stimulus-responsive self-assembly and
morphological control from peptide building blocks, but also
will be integrated into the design concept toward new
artificial structures that mimic biological systems.
Received: November 3, 2010
Published online: February 15, 2011
Keywords: morphological transformation · nanofibers ·
.
peptides · self-assembly · vesicles
[1] a) F. Zhao, M. Ma, B. Xu, Chem. Soc. Rev. 2009, 38, 883 – 891;
b) X. Yan, Q. He, K. Wang, L. Duan, Y. Cui, J. Li, Angew. Chem.
2007, 119, 2483 – 2486; Angew. Chem. Int. Ed. 2007, 46, 2431 –
2434; c) X. Yan, Y. Cui, Q. He, K. Wang, J. Li, W. Mu, B. Wang,
Z. Ou-yang, Chem. Eur. J. 2008, 14, 5974 – 5980; d) B. Bulic, M.
Pickhardt, B. Schmidt, E.-M. Mandelkow, H. Waldmann, E.
Mandelkow, Angew. Chem. 2009, 121, 1772 – 1785; Angew.
Chem. Int. Ed. 2009, 48, 1740 – 1752.
Figure 4. a,b) Temperature-dependent CD spectra of the morphologi-
cal transition from nanofibers to vesicles in THF/water (1:1, v/v;
[1]=2.0 mgmLꢀ1). c) CD intensity at 307 nm as a function of temper-
ature.
[2] a) K. J. Channon, G. L. Devlin, C. E. MacPhee, J. Am. Chem.
Soc. 2009, 131, 12520 – 12521; b) K. J. Channon, G. L. Devlin,
S. W. Magennis, C. E. Finlayson, A. K. Tickler, C. Silva, C. E.
MacPhee, J. Am. Chem. Soc. 2008, 130, 5487 – 5491.
[3] a) S. R. Diegelmann, J. M. Gorham, J. D. Tovar, J. Am. Chem.
Soc. 2008, 130, 13840 – 13841; b) L. Hsu, G. L. Cvetanovich, S. I.
Stupp, J. Am. Chem. Soc. 2008, 130, 3892 – 3899; c) H. Shao, T.
Nguyen, N. C. Romano, D. A. Modarelli, J. R. Parquette, J. Am.
Chem. Soc. 2009, 131, 16374 – 16376; d) N. Ashkenasy, W. S.
Horne, M. R. Ghadiri, Small 2006, 2, 99 – 102.
gen-bonding distance in the peptide segments is approxi-
mately 4.7 ꢀ and the directionality of the b-sheet-like hydro-
gen-bonding network that coincides with packing of the
flexible central linkers and hydrophilic bipyridine groups
yields a much longer distance between the bipyridine units
than the stable p···p stacking distance (ca. 3.4 ꢀ).[16] There-
fore, the bipyridine groups are mobile enough upon external
triggering.
[4] a) S. Zhang, D. Marini, W. Hwang, S. Santoso, Curr. Opin. Chem.
Biol. 2002, 6, 865 – 871; b) Y.-B. Lim, K.-S. Moon, M. Lee, Chem.
Soc. Rev. 2009, 38, 925 – 934; c) I. Cherny, E. Gazit, Angew.
Chem. 2008, 120, 4128 – 4136; Angew. Chem. Int. Ed. 2008, 47,
4062 – 4069; d) T.-B. Yu, J. Bai, Z. Guan, Angew. Chem. 2009,
121, 1117 – 1121; Angew. Chem. Int. Ed. 2009, 48, 1097 – 1101.
[5] a) S. Yagai, A. Kitamuraa, Chem. Soc. Rev. 2008, 37, 1520 – 1529;
b) J. Wu, T. Yi, T. Shu, M. Yu, Z. Zhou, M. Xu, Y. Zhou, H.
Zhang, J. Han, F. Li, C. Huang, Angew. Chem. 2008, 120, 1079 –
1083; Angew. Chem. Int. Ed. 2008, 47, 1063 – 1067; c) C. Wang,
Q. Chen, F. Sun, D. Zhang, G. Zhang, Y. Huang, R. Zhao, D.
Zhu, J. Am. Chem. Soc. 2010, 132, 3092 – 3096.
[6] a) H. Shao, J. R. Parquette, Angew. Chem. 2009, 121, 2563 –
2566; Angew. Chem. Int. Ed. 2009, 48, 2525 – 2528; b) P. P.
Bose, A. K. Das, R. P. Hegde, N. Shamala, A. Banerjee, Chem.
Mater. 2007, 19, 6150 – 6157; c) T. Muraoka, H. G. Cui, S. I.
Stupp, J. Am. Chem. Soc. 2008, 130, 2946 – 2947.
We propose that the reversible transition between the
vesicles and nanofibers is related to a conformational variety
of the flexible central linkers and the bipyridine groups
adopting either loose or tight packing to accommodate
structural changes in b-sheet-like bilayers when triggered by
heat or ultrasound (Scheme 1c). Ultrasound treatment leads
to collapse of the vesicles and formation of dense bilayer
structures (Scheme 1c). This transformation is underpinned
by the ultrasound-induced rotation of the bipyridine groups
and a consequential conformational change of the flexible
linkers (Scheme 1d). The directionality and the sticky nature
of peptide hydrogen bonds are the driving forces in the
dynamic self-assembly and elongation of the dense bilayers,
which lead to the formation of nanofibers. As a result of the
high stability of the bilayered b-sheet-like structures, the
ultrasound-induced conformational changes of unbound
flexible linkers and different orientations of the mobile
bipyridine groups can be fully recovered when heating the
nanofibers and this leads to the formation of vesicles.
[7] a) O. M. A. El-Agnaf, J. M. Sheridan, C. Sidera, G. Siligardi, R.
Hussain, P. I. Haris, B. M. Austen, Biochemistry 2001, 40, 3449 –
3457; b) R. Vidal, B. Frangione, A. Rostagno, S. Mead, T.
Rꢂvꢂsz, G. Plant, J. Ghiso, Nature 1999, 399, 776 – 781.
[8] a) D. E. Przybyla, J. Chmielewski, J. Am. Chem. Soc. 2008, 130,
12610 – 12611; b) D. E. Przybyla, J. Chmielewski, J. Am. Chem.
3718
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3715 –3719

ˇ
Soc. 2010, 132, 7866 – 7867; c) A. Richard, V. Marchi-Artzner,
M.-N. Lalloz, M.-J. Brienne, F. Artzner, T. Gulik-Krzywicki, M.-
A. Guedeau-Boudeville, J.-M. Lehn, Proc. Natl. Acad. Sci. USA
2004, 101, 15279 – 15284.
3313 – 3315; b) E. Zagar, J. Grdadolnik, J. Mol. Struct. 2003, 658,
143 – 152.
[13] a) D. Ke, C. Zhan, X. Li, X. Wang, Y. Zeng, J. Yao, J. Colloid
Interface Sci. 2009, 337, 54 – 60; b) T. Miyazawa, E. R. Blout,
J. Am. Chem. Soc. 1961, 83, 712 – 719.
[9] a) C. Huang, L. Wen, H. Liu, Y. Li, X. Liu, M. Yuan, J. Zhai, L.
Jiang, D. Zhu, Adv. Mater. 2009, 21, 1721 – 1725; b) Y. Li, X. Li,
Y. Li, H. Liu, S. Wang, H. Gan, J. Li, N. Wang, X. He, D. Zhu,
Angew. Chem. 2006, 118, 3721 – 3725; Angew. Chem. Int. Ed.
2006, 45, 3639 – 3643.
[10] a) O. Makin, L. Serpell, Fiber Diffr. Rev. 2004, 12, 29 – 35; b) P.
Sikorski, E. Atkins, L. C. Serpell, Structure 2003, 11, 915 – 926;
c) D. Ke, C. Zhan, X. Li, Y. Wang, A. D. Q. Li, J. Yao,
Tetrahedron 2009, 65, 8269 – 8276.
[11] a) A. Shoji, T. Ozaki, H. Saito, R. Tabeta, I. Ando, Macro-
molecules 1984, 17, 1472 – 1479; b) H. R. Kricheldorf, D. Muller,
Macromolecules 1983, 16, 615 – 623; c) H. Saito, R. Tabeta, A.
Shoji, T. Ozaki, I. Ando, Macromolecules 1983, 16, 1050 – 1057;
d) D. K. Lee, A. Ramamoorthy, J. Phys. Chem. B 1999, 103, 271 –
275.
[12] a) D. Bardelang, F. Camerel, J. C. Margeson, D. M. Leek, M.
Schmutz, M. Badruz Zaman, K. Yu, D. V. Soldatov, R. Ziessel,
C. I. Ratcliffe, J. A. Ripmeester, J. Am. Chem. Soc. 2008, 130,
[14] a) Y. Wang, C. Zhan, H. Fu, X. Li, X. Sheng, Y. Zhao, D. Xiao, Y.
Ma, J. Ma, J. Yao, Langmuir 2008, 24, 7635 – 7638; b) C.
Baddeley, Z. Q. Yan, G. King, P. M. Woodward, J. D. Badjic,
J. Org. Chem. 2007, 72, 7270 – 7278; c) T. Naota, H. Koori, J. Am.
Chem. Soc. 2005, 127, 9324 – 9325; d) X. D. Yu, Q. Liu, J. Wu, M.
Zhang, X. Cao, S. Zhang, Q. Wang, L. M. Chen, T. Yi, Chem.
Eur. J. 2010, 16, 9099 – 9106; e) K. Isozaki, H. Takaya, T. Naota,
Angew. Chem. 2007, 119, 2913 – 2915; Angew. Chem. Int. Ed.
2007, 46, 2855 – 2857.
[15] a) Y. Liu, X. Li, Y. Chen, X. Guan, J. Phys. Chem. B 2004, 108,
19541 – 19549; b) P. S. Braterman, B. C. Noble, R. D. Peacock,
J. Phys. Chem. 1986, 90, 4913 – 4915; c) S. M. Kelly, T. J. Jess,
N. C. Price, Biochim. Biophys. Acta Proteins Proteomics 2005,
1751, 119 – 139; d) C. Krittanai, W. C. Johnson, Jr., Anal. Bio-
chem. 1997, 253, 57 – 64.
[16] a) H. W. Roesky, M. Andruh, Coord. Chem. Rev. 2003, 236, 91 –
119; b) X. Li, C. Zhan, Y. Wang, J. Yao, Chem. Commun. 2008,
2444 – 2446.
Angew. Chem. Int. Ed. 2011, 50, 3715 –3719
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3719