A dendron based on natural amino acids: Synthesis and behavior as an organogelator and lyotropic liquid crystal

Article abstract of DOI:10.1002/anie.200500359

From linear to dendritic: Glycine is part of the predominant sequence in collagen proteins, and aspartate is similar to the repeating unit of poly(benzyl glutamate)s (PBG)s. Collagen and PBGs are linear peptides that self-organize into fibers and form liq

Full text of DOI:10.1002/anie.200500359

Angewandte
Chemie
Amino Acid Dendrons
present the synthesis of a dendron composed of two natural
amino acids, glycine and aspartic acid, and its self-assembly
which leads not only to organogels but also to lyotropic liquid
crystals. Recently, a type of amphiphilic dendritic dipeptide
was described by Percec et al.[9] as self-assembling in helical
pores. In contrast to their design of an aromatic dendron and a
peptide core, our NAADs have peptidic dendrons with a
benzyl periphery.
DOI: 10.1002/anie.200500359
A Dendron Based on Natural Amino Acids:
Synthesis and Behavior as an Organogelator and
Lyotropic Liquid Crystal**
The poly(Gly-Asp) dendrons were convergently synthe-
sized as depicted in Scheme 1. It is well-known that glycine
appears at every third position in the predominant sequences
of collagen,[2] and aspartic acid is similar to the building unit
of a well-investigated linear model peptide, poly(benzyl l-
glutamate) (PBLG), which behaves as a gelator and liquid
crystal.[10] The dendrons, which resemble the repetitive
patterns always found in the sequences of natural fibrous
proteins,[2d] were branched iteratively with Gly-Asp dipep-
tides, instead of with one amino acid as in other NAADs,[11]
and without any help from a nonnatural amino acid linkage.
Standard DCC coupling of Boc-glycine and benzyl-
protected aspartic acid, which was readily prepared by the
benzyl esterification of aspartic acid, gave the branching unit
in 60% yield. This is the first generation (G1) of the dendron.
The second (G2) and third generations (G3) were synthesized
convergently in 35 and 17% yields, respectively, by repeating
a two-reaction cycle, that is, by removing the Boc group of the
lower-generation dendron with TFA, and then coupling the
resultant N-deprotected intermediate to the C-deprotected
G1 (Boc-G1-COOH), prepared by hydrogenative debenzyl-
ation of the dipeptide. Currently, one of the major obstacles
that hindered the research and applications of NAADs is the
tedious process for the purification of the dendritic prod-
ucts.[10] Notably, G3 is relatively easy to obtain by simple
precipitation in water and MeOH consecutively (see Sup-
Yan Ji, Yu-Fei Luo, Xin-Ru Jia,* Er-Qiang Chen,
Yun Huang, Chun Ye, Bing-Bing Wang, Qi-Feng Zhou,
and Yen Wei*
Biomolecules are prone to self-assemble in vivo and in vitro.[1]
For example, collagen proteins can self-organize initially into
triple helices and then into ordered fibers, gels, and liquid-
crystalline (LC) phases through noncovalent forces.[2a,b]
Elucidation of the underlying mechanisms is important but
tremendously challenging. An alternative is to decipher
natureꢀs bewildering tricks with the help of synthetic systems
made from natural starting materials.[2] In this context, the
natural amino acid based dendrons or dendrimers (NAADs)
are of great significance because of their similarity to proteins
in composition and topology,[3] as well as their architectural
difference from currently prevalent linear model peptides. In
addition, potentially chiral and biocompatible NAADs pro-
vide exciting possibilities in the creation of new materials for
nano- and biotechnology.[4]
However, little attention has been paid to the self-
assembly of NAADs,[5] and especially to their gelation and
LC properties. Only polylysine dendrimers have been studied
as gelators by Smith et al.[6] and gelation is mentioned for
dendrimers built from glutamates by Ranganathan et al.[3c,7]
Research on NAADs as liquid crystals is even scarcer.
Moreover, to the best of our knowledge, there is no report
on amino acid based dendrimers that function as gelators and
form liquid crystals. We have been interested in the synthesis
and self-assembly of dendrimers for many years.[8] Herein, we
1
porting Information). H, 13C, two-dimensional, and DEPT
NMR techniques, MALDI-TOF mass spectrometry, and
elemental analyses were used to verify the structure and
purity of the G3 dendron (see Supporting Information).
We found that G3 could act as an efficient low-molecular-
mass organogelator (LMOG). LMOGs[12c] are a family of low-
molecular-mass organic molecules (Mw ꢀ 3000) that can gel
organic solvents at very low concentrations through non-
covalent forces. LMOGs have attracted considerable renewed
interest in recent years, not only because of their interesting
roles in materials science and supramolecular chemistry but
also because gelation remains poorly understood. Most
organogels are achieved through the ordered arrangement
of gelators during the cooling of the mixture of LMOGs and
organic solvents after heating to form a homogeneous
solution.[12] However, a solvent–solvent gel preparation
procedure distinguishes G3 from other LMOGs, including
the previously reported dendritic ones.[13] In a typical
example, G3 (0.04 g) was first dissolved in CHCl3/CH3OH
(9:1 v/v; 6 mL) by sonication. Then, with the addition of ethyl
acetate (4 mL), the solution immediately became viscous and
gelled within 5 seconds. Meanwhile, a 10-mL gel as dilute as
0.2 wt% (0.98 mm, one G3 molecule entraps about 9000
solvent molecules) in a 1-cm-diameter test tube could be
turned upside down without downward flow of the gel. A thin
[*] Y. Ji, Y.-F. Luo, Prof. X.-R. Jia, Prof. E.-Q. Chen, Y. Huang, C. Ye,
B.-B. Wang, Prof. Q.-F. Zhou
Department of Polymer Science & Engineering and
The Key Laboratory of Polymer Chemistry and Physics of the
Ministry of Education
College of Chemistry & Molecular Engineering
Peking University, Beijing 100871 (China)
Fax: (+86)10-62751708
E-mail: xrjia@pku.edu.cn
Prof. Y. Wei
Department of Chemistry, Drexel University
Philadelphia, PA 19104 (USA)
Fax: (+1)215-895-1265
E-mail: weiyen@drexel.edu
[**] This work was supported by the National Natural Science
Foundation of China (NSFC Grant Nos. 20374002 and
20340420002) and by the National Institutes of Health (NIH Grant
No. DE09848). We are most grateful to Prof. Dr. Kenji Hanabusa of
Shinshu University in Japan for discussions on gelators and to all
four reviewers of this article for their comments and suggestions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 6025 –6029
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6025

Communications
atomic force microscopy (AFM) image
recorded in the tapping mode (Fig-
ure 1c) revealed that the smaller fibers
were about 30–70 nm in width, many
micrometers in length, but less than
20 nm in height. Considering the con-
volution effect arising from the finite
size of the AFM tips, the real width of
the fibers should be even less.
The gel preparation details have a
great effect on gelation. If ethyl acetate
was added before G3 was totally dis-
solved in CH3OH/CHCl3, an opaque
suspension was formed instead of a gel.
TEM shows a rather different morphol-
ogy of the aggregates in such a suspen-
sion (Figure 1b). In addition to the
above case, G3 dissolved in and gelled
a mixture of CH3OH and THF in
several hours. However, the resultant
gels were less stable than the CH3Cl/
CH3OH/ethyl acetate gels at the same
concentrations. Neither G1 nor G2
formed a gel in the above-mentioned
mixed solvents under similar experi-
mental conditions. CH3OH, CHCl3,
CH2Cl2, THF, acetone, ethyl acetate,
diethyl ether, ethanol, petroleum ether,
hexane, cyclohexane, dioxane, acetoni-
Scheme 1. Convergent synthesis of the first three generations of poly(Gly-Asp) dendrons Gn (n=1–3).
Reagents and conditions: a) p-toluenesulfonic acid, benzene, benzyl alcohol, reflux; b) CH3OH, KOH;
c) Boc-glycine, DCC, À108C; d) TFA, CH2Cl2; e) Pd-C, H2, ethanol; and f) DCC, Boc-G1-COOH, N-methyl-
morpholine, NHS, À108C. Boc=tert-butoxycarbonyl, DCC=dicyclohexylcarbodiimide, TFA=trifluoroacetic
acid, NHS=N-hydroxysuccinimide.
trile,
N,N-dimethylformamide,
layer of liquid exuded as time went by, but the main part kept
its gel state for more than half a year. The transmission
electron microscopy (TEM) image of the G3 gel showed that
the G3 dendrons self-assembled into a ramified network of
intertwined fibers. The width of the fibers ranged from several
dozen to more than a hundred nanometers (Figure 1a). The
dimethyl sulfoxide, styrene, water, and benzyl alcohol were
tested for the preparation of G3 gels by the conventional
heating and cooling method. Except for benzyl alcohol, none
of the single solvents could be gelled by G3.
Noncovalent forces, mainly hydrogen-bonding and aro-
matic-stacking interactions, were proposed to be the driving
force for the self-assembly of G3 dendron. The role of
hydrogen bonding in gelation is supported by the fact that the
gels changed into clear solutions after the addition of LiCl,
which is known to interact strongly with amides and break
hydrogen bonds (see Supporting Information). The FT-IR
spectrum of the dry gel also revealed characteristic stretching
vibrations as a result of hydrogen-bonding interactions in the
À1
N H (3309 cm ) and C O (amide I: 1660 cmÀ1) moieties.
However, further proof is needed to verify the presence of
hydrogen bonds in the gel state. Other weak interactions may
also play an important role in the self-assembly. Fluorescence
spectroscopy[14] with pyrene as a probe was employed to
examine the effect of benzyl rings. The intensity ratio I1/I3 of
pyrene emissions is an established index for the polarity of the
environment. This ratio decreased as the concentration of G3
was increased in THF/CH3OH (9:1; see Supporting Informa-
tion), which suggests that nonpolar domains were formed as a
consequence of the aromatic stacking of peripheral benzyl
rings.
À
=
What is the relationship between the structure of a native
gel and the organization of gelators in the solid state? This is
an important question that has puzzled researchers for many
years. To address this question in the case of G3, as well as to
Figure 1. Morphology of G3: a) TEM image of a G3/CH3Cl/CH3OH/
ethyl acetate gel; b) TEM image of a G3/CH3Cl/CH3OH/ethyl acetate
suspension; c) AFM image of the G3/CH3Cl/CH3OH/ethyl acetate gel
([G3]=0.02 wt%).
6026 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6025 –6029

Angewandte
Chemie
obtain some hints on its packing pattern in native gels, X-ray
diffraction (XRD) was carried out on the native gel, xerogel,
and dendrite. No reflection could be detected in the CH3OH/
CHCl3/ethyl acetate gel, presumably as a consequence of
strong scattering from the electron-rich CHCl3 solvent. The
one-dimensional (1D) XRD pattern of the xerogel is shown in
Figure 2. The large scattering halo with a maximum in 2q at
axis, and that the diffraction lines that correspond to a
d spacing of 1.162, 0.581, 0.522, and 0.451 nm are the (100),
(200), (141), and (006) planes, respectively (Figure 2), we
might tentatively propose an orthorhombic lattice with a =
1.162, b = 2.395, and c = 2.706 nm. As shown in the right-hand
side of Figure 3, all the diffraction lines observed can be
indexed. The calculated d spacings fitted the measured ones
quite well (see Supporting Information). Meanwhile, we
noted that the (100) diffraction with a d spacing of 1.162 nm in
the dendrite could also be identified in the xerogel (see the
inset of Figure 2). Although the intensity is relatively weaker,
it indicates that the crystal in the xerogel could have the same
structure as the dendrite. However, the coexistence of three
peaks with very close d spacings proves that the G3 crystals
are in fact polymorphic. Still assuming an orthorhombic
lattice, we found that the strongest diffraction with a d spacing
of 1.126 nm and following peaks in the higher 2q region in
Figure 2 could be indexed with a = 1.126, b = 2.331, and c =
2.582 nm, values that deviate only slightly from those
proposed for the dendrite. Reexamination of the dendrite
testified that polymorphism also occurred. Occasionally, two
sets of diffractions could be detected (see Supporting
Information) in one fiber. This polymorphism may arise
from the rich intra- and intermolecular hydrogen-bonding
interactions, which lead to a G3 molecular packing trapped in
different local Gibbs energy minima during crystallization by
solvent evaporation.
Figure 2. 1D XRD pattern of the G3/CH3OH/CHCl3/ethyl acetate xero-
gel. The inset shows reflections around 2q=7.868.
around 208 (corresponding to a d spacing of ꢁ 0.45 nm)
indicates that the major portion of G3 in the xerogel was
amorphous. However, the existence of a series of sharp
diffractions reveals that the G3 of the xerogel could partially
crystallize. Efforts were made to grow single crystals, but
failed. However, G3 dendrites up to centimeter dimensions
were obtained in DMF through evaporation at room temper-
ature for a month. Figure 3 shows a two-dimensional (2D)
The gel became a turbid solution and would not regel after
it was destroyed by mechanical agitation. Therefore, the
morph responsible for gelation is probably not of the most
thermodynamically stable phase. Recently, it was proposed[15]
that one prerequisite of gel formation is the 1D alignment of
gelator molecules. If so, the morph in the gel state should be
different from that in the 3D orthorhombic crystals. A
structure change, such as crystallization, is quite possible
during the formation of a xerogel from a gel by the
evaporation of solvents.
G3 can self-organize into lyotropic LC phases in benzyl
alcohol. When the concentration of G3 exceeds a certain limit
(about 6 wt%), spherulites and oily streaks (Figure 4a)
appeared, as viewed by polarizing optical microscopy
(POM). With an increase of the concentration, more spher-
ulites formed and eventually the whole field of view became
dominated by a polygonal texture (Figure 4b). The textures
can be deformed with pressure and are self-healing. No
evidence of microcrystallites was observed. The lyotropic LC
Figure 3. 2D WAXD pattern (left) and its schematic drawing (right) for
one fiber-like branch in a G3 dendrite.
wide-angle XRD (WAXD) pattern obtained
from one fiber-like branch of a dendrite,
which was collected in the transmission
mode with the X-ray incident beam perpen-
dicular to the long axis of the fiber. The
sample was rotated through 3608 around the
long axis during exposure. It was difficult to
deduce the precise crystal structure of G3
Figure 4. Representative microphotographic textures of G3 in benzyl alcohol under crossed polariz-
ers. a) Spherulites and oily streaks in 14-wt% samples at room temperature (RT); b) polygonal tex-
ture (or focal–conic-like texture) in 30-wt% samples at RT; c) filament texture of 40-wt% samples
heated at 10 KminÀ1 and held for 5 min at 1008C.
because of the scarcity of diffraction lines.
However, by assuming that the crystallo-
graphic c axis is parallel to the fiber long
Angew. Chem. Int. Ed. 2005, 44, 6025 –6029
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6027

Communications
behavior of the G3/benzyl alcohol phase at several represen-
at low temperature, the samples shared the same lyotropic LC
phase where the lamellae coexisted with the solvent. We
expected that the lamellae should be a bilayer structure.
According to our preliminary computer modeling of the G3
molecule, the distance from the focal point to the periphery
was estimated to be about 2.8 nm if the G3 molecules were
fully extended. Hence, each lamella might consist of two
layers of G3 molecules that are partially interdigitated. In the
inner part of the lamellae, the G3 molecules might associate
with their neighbors through hydrogen bonds, while the
benzyl rings of the G3 molecules are packed on the lamellar
top and bottom surfaces. More systematic structural and
thermodynamic transition property analyses, as well as more
morphological studies, are necessary to finally identify the
textures and the corresponding phases.
In summary, we have successfully synthesized a novel
dipeptide-branched poly(Gly-Asp) dendron (G3) capable of
self-organizing into organogels and lyotropic LC phases. With
a solvent–solvent gel preparation process, the G3 dendrons
self-assembled into a network of intertwined fibers. Both
hydrogen-bonding and aromatic-stacking interactions might
be responsible for the aggregation. The ordered structure in
the xerogel has a close relationship with the orthorhombic
packing of G3 in dendritic crystals. G3 exhibits lyotropic LC
behavior in benzyl alcohol, and a low-temperature lamellar
phase with a long period of 4.59 nm can be identified. Fibrous
proteins are ubiquitous in nature. Silk fibroin, amyloid, and
collagens are but a few remarkable examples.[18] The systems
described herein may give us more chance to understand the
basic principles governing their supramolecular organization
from a new angle different to that offered by linear model
peptides. Further investigation is in progress to gain deeper
insight into the mechanisms of gelation and mesophase
formation, and to unravel the precise molecular-packing
patterns of G3 in the gels, crystals, and liquid crystals. In
addition, the applications of such nanostructured materials
could be explored as potentially biocompatible systems for
drug delivery, molecular recognition, and other diagnostic or
therapeutic functions.
tative concentrations was also examined by differential
scanning calorimetry (DSC; see Supporting Information). A
single endothermal peak was observed on heating 8-, 14-, and
20-wt% samples. The peak temperature increased monoto-
nously from 608C for 8-wt% samples to 808C for 20-wt%
samples, which was in accordance with the trend of transition
of birefringence from anisotropic to isotropic observed by
POM.
In the cases of 25-, 30-, 40-, and 50-wt% samples, the DSC
heating traces showed a broad endothermic process with two
peaks, which indicates two possible transitions. For example,
the 40-wt% sample exhibited a lower peak at 888C and a
higher peak at 958C before it completely entered the isotropic
state after 1058C. A new POM texture different to that in
Figure 4b developed above the lower transition temperature
(Figure 4c). This was also the case for the other samples in the
range of 25–50 wt%. The combination of the POM and DSC
results suggests that the G3/benzyl alcohol mixture may have
one lyotropic LC phase below the isotropic temperature at a
concentration of 5–20 wt%. As the concentration ranged
from 25 to 50 wt%, the samples may have one low- and one
high-temperature LC phase prior to the LC-to-isotropic
transition.
In contrast to most other LC dendrimers,[16] G3 is
lyotropic and intrinsically mesogenic with relatively flexible
chains. It may serve as a good testing bench for the study of
the structure–mesophase relationship. The lyotropic LC
phase structure of the G3/benzyl alcohol mixture at low
temperature (below the isotropic temperature for 5- to 20-
wt% samples and below the lower transition temperature for
25- to 50-wt% samples) was identified by XRD. In the low 2q
region (Figure 5), a series of scatterings up to the fourth order
could be observed for all the concentrations investigated, and
the corresponding scattering vector ratios exactly followed
1:2:3:4 which clearly indicated a lamellar structure. This
finding agrees well with the spherulites, oily streaks, and
polygonal textures observed with POM, which were believed
to be the characteristic features of the lyotropic lamellar
phase.[17]
Received: January 31, 2005
Revised: June 17, 2005
Published online: August 17, 2005
Despite varying the concentration, the long period of the
lamellae was measured as 4.59 nm. This result indicated that,
Keywords: amino acids · dendrimers · gelators · liquid crystals ·
.
self-assembly
[1] a) I. S. Choi, N. Bowden, G. M. Whitesides, Angew. Chem. 1999,
111, 3265 – 3268; Angew. Chem. Int. Ed. 1999, 38, 3078 – 3081;
b) A. M. Smith, S. F. A. Acquah, N. Bone, H. W. Kroto, M. G.
Ryadnov, M. S. P. Stevens, D. R. M. Walton, D. N. Woolfson,
Angew. Chem. 2005, 117, 329 – 332; Angew. Chem. Int. Ed. 2005,
44, 325 – 328.
[2] a) R. Martin, L. Waldmann, D. L. Kaplan, Biopolymers 2003, 70,
435 – 444; b) D. G. Lynn, S. C. Meredith, J. Struct. Biol. 2000, 130,
153 – 173; c) H. A. Lashuel, S. R. LaBrenz, L. Woo, L. C. Serpell,
J. W. Kelly, J. Am. Chem. Soc. 2000, 122, 5262 – 5277; d) D. M.
Marini, W. Hwang, D. A. Lauffenburger, S. G. Zhang, R. D.
Kamm, Nano Lett. 2002, 2, 295 – 299.
Figure 5. 1D XRD patterns of G3 in benzyl alcohol. Left: diffraction
patterns from 2q=2.58 to 2q=128 with a prolonged exposure time.
Right: overview diffraction patterns at different concentrations.
6028 www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6025 –6029

Angewandte
Chemie
[3] a) D. A. Tomalia, B. Huang, D. R. Swanson, H. M. Brothers,
J. W. Klimash, Tetrahedron 2003, 59, 3799 – 3813; b) P. J. Dand-
liker, F. Diederich, J. P. Gisselbrecht, A. Louati, M. Gross,
Angew. Chem. 1995, 107, 2906 – 2909; Angew. Chem. Int. Ed.
Engl. 1995, 34, 2725 – 2728; c) D. Ranganathan, S. Kurur, R.
Gilardi, I. L. Karle, Biopolymers 2000, 54, 289 – 295.
[4] a) U. Boas, P. M. H. Heegaard, Chem. Soc. Rev. 2004, 33, 43 – 63;
b) F. Aulenta, W. Hayes, S. Rannard, Eur. Polym. J. 2003, 39,
1741 – 1771.
Newkome, G. Baker, J. Polym. Sci. B 1997, 35, 2787 – 2793; f) M.
Jorgensen, K. Bechgaard, T. Bjornholm, P. Sommer-Larse, L. G.
Hansen, K. Schaunberg, J. Org. Chem. 1994, 59, 5877 – 5882;
g) W. D. Jang, D. L. Jiang, T. Aida, J. Am. Chem. Soc. 2000, 122,
3232 – 3233; h) C. Kim, K. T. Kim, Y. Chang, H. H. Song, T. Y.
Cho, H. J. Jeon, J. Am. Chem. Soc. 2001, 123, 5586 – 5587;
i) W. D. Jang, T. Aida, Macromolecules 2003, 36, 8461 – 8469;
j) W. Zhang, S. O. Gonzalez, E. E. Simanek, Macromolecules
2002, 35, 9015 – 9021.
[14] M. Yumoto, M. Kimura, H. Shirai, K. Hanabusa, Chem. Eur. J.
2003, 9, 348 – 354.
[5] J. S. Choi, D. K. Joo, C. H. Kim, K. Kim, J. S. Park, J. Am. Chem.
Soc. 2000, 122, 474 – 480.
[15] a) O. Gronwald, S. Shinkai, Chem. Eur. J. 2001, 7, 4329 – 4334;
b) K. Sugiyasu, N. Fujita, S. Shinkai, Angew. Chem. 2004, 116,
1249 – 1253; Angew. Chem. Int. Ed. 2004, 43, 1229 – 1233, and
references therein.
[16] For selected publications on LC dendrimers, see: a) V. Percec,
M. Kawasumi, Macromolecules 1992, 25, 3843 – 3850; b) V.
Percec, M. Glodde, G. Johansson, V. S. K. Balagurusamy, P. A.
Heiney, Angew. Chem. 2003, 115, 4474 – 4478; Angew. Chem. Int.
Ed. 2003, 42, 4338 – 4342; c) M. Lehmann, I. Fischbach, H. W.
Spiess, H. Meier, J. Am. Chem. Soc. 2004, 126, 772 – 784; d) Y. Li,
S. T. Lin, W. A. Goddard, J. Am. Chem. Soc. 2004, 126, 1872 –
1885; e) G. Ungar, Y. S. Liu, X. B. Zeng, V. Percec, W. D. Cho,
Science 2003, 299, 1208 – 1211; f) R. Elsäßer, G. H. Mehl, J. W.
[6] For articles on polylysine dendritic gelators, see: a) A. R. Hirst,
D. K. Smith, M. C. Feiters, H. P. M. Geurts, Langmuir 2004, 20,
7070 – 7077; b) C. S. Love, A. R. Hirst, V. Chechik, D. K. Smith,
I. Ashworth, C. Brennan, Langmuir 2004, 20, 6580 – 6585;
c) K. S. Partridge, D. K. Smith, G. M. Dykes, P. T. McGrail,
Chem. Commun. 2001, 319 – 320; d) A. R. Hirst, D. K. Smith,
M. C. Feiters, H. P. M. Geurts, A. C. Wright, J. Am. Chem. Soc.
2003, 125, 9010 – 9011; e) A. R. Hirst, D. K. Smith, M. C. Feiters,
H. P. M. Geurts Chem. Eur. J. 2004, 10, 5901 – 5910.
[7] D. Ranganathan, S. Kurur, K. P. Madhusudanan, R. Roy, I. L.
Karle, J. Pept. Res. 1998, 50, 297 – 302.
[8] a) B. B. Wang, X. Zhang, X. R. Jia, Z. C. Li, Y. Ji, L. Yang, Y.
Wei, J. Am. Chem. Soc. 2004, 126, 15180 – 15194; b) L. Yang,
Y. F. Luo, X. R. Jia, Y. Ji, L. P. You, Q. F. Zhou, Y. Wei, J. Phys.
Chem. B 2004, 108, 1176 – 1178; c) J. F. Wang, X. R. Jia, H.
Zhong, H. Z. Wu, Y. Y. Li, X. J. Xu, M. Q. Li, Y. Wei, J. Polym.
Sci. A 2000, 38, 4147 – 4153; d) J. F. Wang, X. R. Jia, H. Zhong, Y.
Qin, J. Y. Chen, X. S. Zhao, W. X. Cao, M. Q. Li, Y. Wei, Chem.
Mater. 2002, 14, 2854 – 2858.
[9] V. Percec, A. E. Dulcey, V. S. K. Balagurusamy, Y. Miura1, J.
Smidrkal, M. Peterca1, S. Nummelin, U. Edlund, S. D. Hudson,
P. A. Heiney, H. Duan, S. N. Magonov, S. A. Vinogradov, Nature
2004, 430, 764 – 768.
[10] Y. Cohen, A. Dagan, Macromolecules 1995, 28, 7638 – 7644.
[11] For selected publications on the synthesis of natural amino acid
based dendrimers, see: a) L. Crespo, G. Sanclimens, B. Mon-
taner, R. Perez-Tomas, M. Royo, M. Pons, F. Albericio, E. Giralt,
J. Am. Chem. Soc. 2002, 124, 8876 – 8883; b) M. Driffield, D. M.
Goodall, A. S. Klute, D. K. Smith, K. Wilson, Langmuir 2002, 18,
8660 – 8665, and references therein; c) L. Twyman, A. E. Beezer,
J. C. Mitchell, Tetrahedron Lett. 1994, 35, 4423 – 4424; d) D.
Ranganathan, S. Kurur, Tetrahedron Lett. 1997, 38, 1265 – 1268;
e) I. B. Rietveld, E. Kim, S. A. Vinogradov, Tetrahedron 2003,
59, 3821 – 3831, and references therein; f) J. Kress, A. Rosner, A.
Hirsch, Chem. Eur. J. 2000, 6, 247 – 257; g) B. Buschhaus, W.
Bauer, A. Hirsch, Tetrahedron 2003, 59, 3899 – 3915; h) Y. Kim,
F. W. Zeng, S. C. Zimmerman, Chem. Eur. J. 1999, 5, 2133 – 2138;
i) L. J. Twyman, A. E. Beezer, R. Esfand, B. T. Mathews, J. C.
Mitchell, J. Chem. Res. Synop. 1998, 12, 758 – 759.
Goodby, M. Veith, Angew. Chem. 2001, 113, 2760 – 2762;
A
ngew. Chem. Int. Ed. 2001, 40, 2688 – 2690; g) P. Busson, H. Ihre,
A. Hult, J. Am. Chem. Soc. 1998, 120, 9070 – 9071; h) D. J. Pesak,
J. S. Moore, Angew. Chem. 1997, 109, 1709 – 1712; Angew. Chem.
Int. Ed. Engl. 1997, 36, 1636 – 1639, and references therein; i) M.
Nishii, T. Matsuoka, Y. Kamikawa, T. Kato, Org. Biomol. Chem.
2005, 3, 875.
[17] a) G. Wang, A. D. Hamilton, Chem. Eur. J. 2002, 8, 1954 – 1961;
b) Y. Rangon, J. Charvolin, J. Phys. Chem. 1988, 92, 6339 – 6344;
c) J. Yang, G. Wegner, Macromolecules 1992, 25, 1786 – 1790.
[18] a) N. L. Goeden-Wood, J. D. Keasling, S. J. Muller, Macromole-
cules 2003, 36, 2932 – 2938; b) K. Lu, J. Jacob, P. Thiyagarajan,
V. P. Conticello, D. G. Lynn, J. Am. Chem. Soc. 2003, 125, 6391 –
6393; c) A. Aggeli, M. Bell, N. Boden, J. N. Keen, P. F. Knowles,
T. C. B. McLeish, M. Pitkeathly, S. E. Radford, Nature 1997, 386,
259 – 262.
[12] a) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3159; b) O.
Gronwald, E. Snip, S. Shinkai, Curr. Opin. Colloid Interface Sci.
2002, 7, 148 – 156; c) D. J. Abdallah, R. G. Weiss, Adv. Mater.
2000, 12, 1237 – 1246.
[13] For publications on dendritic gelators other than polylysine
dendrimers, see: a) G. R. Newkome, G. R. Baker, M. J. Saun-
ders, P. S. Russo, V. K. Gupta, Z. Q. Yao, J. E. Miller, K.
Bouillion, J. Chem. Soc. Chem. Commun. 1986, 10, 752 – 753;
b) G. R. Newkome, G. R. Baker, S. Arai, M. J. Saunders, P. S.
Russo, K. J. Theriot, C. N. Moorefield, L. E. Rogers, J. E. Miller,
T. R. Lieux, M. E. Murray, B. Phillips, L. Pascal, J. Am. Chem.
Soc. 1990, 112, 8459 – 8465; c) G. R. Newkome, C. N. Moorefield,
G. R. Baker, R. K. Behera, G. H. Escamilla, M. J. Saunders,
Angew. Chem. 1992, 104, 901 – 903; Angew. Chem. Int. Ed. Engl.
1992, 31, 917 – 919; d) G. R. Newkome, X. F. Lin, C. Yaxiong,
G. H. Escamilla, J. Org. Chem. 1993, 58, 3123 – 3129; e) K. H.
Yu, P. S. Russo, L. Younger, W. G. Henk, D. W. Hua, G. R.
Angew. Chem. Int. Ed. 2005, 44, 6025 –6029
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6029